![2031 Factors in Cardiac Output](https://upload.wikimedia.org/wikipedia/commons/1/15/2031_Factors_in_Cardiac_Output.jpg)
In
cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols
,
, or
,
[ edited by Catherine E. Williamson, Phillip Bennett] is the
volumetric flow rate of the
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...
's pumping output: that is, the volume of
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
being pumped by both
ventricles of the heart, per unit time (usually measured per minute). Cardiac output (CO) is the product of the
heart rate
Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
(HR), i.e. the number of heartbeats per minute (bpm), and the
stroke volume (SV), which is the volume of blood pumped from the left ventricle per beat; thus giving the formula:
:
Values for cardiac output are usually denoted as L/min. For a healthy individual weighing 70 kg, the cardiac output at rest averages about 5 L/min; assuming a heart rate of 70 beats/min, the stroke volume would be approximately 70 mL.
Because cardiac output is related to the quantity of blood delivered to various parts of the body, it is an important component of how efficiently the heart can meet the body's demands for the maintenance of adequate tissue
perfusion.
Body tissues require continuous oxygen delivery which requires the sustained transport of oxygen to the tissues by
systemic circulation of oxygenated blood at an adequate pressure from the left ventricle of the heart via the aorta and arteries. Oxygen delivery (DO
2 mL/min) is the resultant of blood flow (cardiac output CO) times the blood oxygen content (CaO
2). Mathematically this is calculated as follows: oxygen delivery = cardiac output × arterial oxygen content, giving the formula:
:
With a resting cardiac output of 5 L/min, a 'normal' oxygen delivery is around 1 L/min. The amount/percentage of the circulated oxygen consumed (VO
2) per minute through metabolism varies depending on the activity level but at rest is circa 25% of the DO
2. Physical exercise requires a higher than resting-level of oxygen consumption to support increased muscle activity. In the case of
heart failure, actual CO may be insufficient to support even simple activities of daily living; nor can it increase sufficiently to meet the higher metabolic demands stemming from even moderate exercise.
Cardiac output is a global blood flow parameter of interest in
hemodynamics
Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously m ...
, the study of the flow of blood. The factors affecting stroke volume and heart rate also affect cardiac output. The figure at the right margin illustrates this dependency and lists some of these factors. A detailed hierarchical illustration is provided in
a subsequent figure.
There are many methods of measuring CO, both invasively and non-invasively; each has advantages and drawbacks as described below.
Definition
The function of the heart is to drive blood through the
circulatory system
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
in a cycle that delivers oxygen, nutrients and chemicals to the body's cells and removes cellular waste. Because it pumps out whatever blood comes back into it from the
venous system
Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...
, the quantity of blood returning to the heart effectively determines the quantity of blood the heart pumps out – its cardiac output, ''Q''. Cardiac output is classically defined alongside
stroke volume (SV) and the
heart rate
Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
(HR) as:
In standardizing what CO values are considered to be within normal range independent of the size of the subject's body, the accepted convention is to further index equation () using
body surface area
In physiology and medicine, the body surface area (BSA) is the measured or calculated surface area of a human body. For many clinical purposes, BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal adi ...
(BSA), giving rise to the
Cardiac index Cardiac index (CI) is a haemodynamic parameter that relates the cardiac output (CO) from left ventricle in one minute to body surface area (BSA), thus relating heart performance to the size of the individual. The unit of measurement is litres per ...
(CI). This is detailed in equation () below.
Measurement
There are a number of clinical methods to measure cardiac output, ranging from direct intracardiac catheterization to non-invasive measurement of the arterial pulse. Each method has advantages and drawbacks. Relative comparison is limited by the absence of a widely accepted "gold standard" measurement. Cardiac output can also be affected significantly by the phase of respiration – intra-thoracic pressure changes influence diastolic filling and therefore cardiac output. This is especially important during mechanical ventilation, in which cardiac output can vary by up to 50% across a single respiratory cycle. Cardiac output should therefore be measured at evenly spaced points over a single cycle or averaged over several cycles.
Invasive methods are well accepted, but there is increasing evidence that these methods are neither accurate nor effective in guiding therapy. Consequently, the focus on development of non-invasive methods is growing.
Doppler ultrasound
![VTI LVOT](https://upload.wikimedia.org/wikipedia/commons/a/a4/VTI_LVOT.png)
This method uses
ultrasound
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies ...
and the
Doppler effect to measure cardiac output. The blood velocity through the heart causes a Doppler shift in the frequency of the returning ultrasound waves. This shift can then be used to calculate flow velocity and volume, and effectively cardiac output, using the following equations:
*
*
*
where:
* CSA is the valve orifice cross sectional area,
* r is the valve radius, and,
* VTI is the velocity time integral of the trace of the Doppler flow profile.
Being non-invasive, accurate and inexpensive, Doppler ultrasound is a routine part of clinical ultrasound; it has high levels of reliability and reproducibility, and has been in clinical use since the 1960s.
Echocardiography
Echocardiography is a non-invasive method of quantifying cardiac output using ultrasound. Two-dimensional (2D) ultrasound and Doppler measurements are used together to calculate cardiac output. 2D measurement of the diameter (d) of the aortic annulus allows calculation of the flow cross-sectional area (CSA), which is then multiplied by the VTI of the Doppler flow profile across the aortic valve to determine the flow volume per beat (
stroke volume, SV). The result is then multiplied by the heart rate (HR) to obtain cardiac output. Although used in clinical medicine, it has a wide test-retest variability. It is said to require extensive training and skill, but the exact steps needed to achieve clinically adequate precision have never been disclosed. 2D measurement of the aortic valve diameter is one source of noise; others are beat-to-beat variation in stroke volume and subtle differences in probe position. An alternative that is not necessarily more reproducible is the measurement of the pulmonary valve to calculate right-sided CO. Although it is in wide general use, the technique is time-consuming and is limited by the reproducibility of its component elements. In the manner used in clinical practice, precision of SV and CO is of the order of ±20%.
Transcutaneous
Ultrasonic Cardiac Output Monitor (USCOM) uses
continuous wave Doppler
Doppler ultrasonography is medical ultrasonography that employs the Doppler effect to perform imaging of the movement of tissues and body fluids (usually blood), and their relative velocity to the probe. By calculating the frequency shift of ...
to measure the Doppler flow profile VTI. It uses
anthropometry
Anthropometry () refers to the measurement of the human individual. An early tool of physical anthropology, it has been used for identification, for the purposes of understanding human physical variation, in paleoanthropology and in various at ...
to calculate aortic and pulmonary valve diameters and CSAs, allowing right-sided and left-sided ''Q'' measurements. In comparison to the echocardiographic method, USCOM significantly improves reproducibility and increases sensitivity of the detection of changes in flow. Real-time, automatic tracing of the Doppler flow profile allows beat-to-beat right-sided and left-sided ''Q'' measurements, simplifying operation and reducing the time of acquisition compared to conventional echocardiography. USCOM has been validated from 0.12 L/min to 18.7 L/min
in new-born babies,
children
and adults.
The method can be applied with equal accuracy to patients of all ages for the development of physiologically rational haemodynamic protocols. USCOM is the only method of cardiac output measurement to have achieved equivalent accuracy to the implantable flow probe.
This accuracy has ensured high levels of clinical use in conditions including sepsis, heart failure and hypertension.
Transoesophageal
![TEE-Sonde](https://upload.wikimedia.org/wikipedia/commons/5/5a/TEE-Sonde.png)
The Transoesophageal Doppler includes two main technologies;
transoesophageal echocardiogram—which is primarily used for diagnostic purposes, and
oesophageal Doppler monitoring—which is primarily used for the clinical monitoring of cardiac output. The latter uses continuous wave Doppler to measure blood velocity in the
descending thoracic aorta. An ultrasound probe is inserted either orally or nasally into the oesophagus to mid-thoracic level, at which point the oesophagus lies alongside the descending
thoracic aorta
The descending thoracic aorta is a part of the aorta located in the thorax. It is a continuation of the aortic arch. It is located within the posterior mediastinal cavity, but frequently bulges into the left pleural cavity. The descending thoracic ...
. Because the transducer is close to the blood flow, the signal is clear. The probe may require re-focussing to ensure an optimal signal. This method has good validation, is widely used for fluid management during surgery with evidence for improved patient outcome, and has been recommended by the UK's National Institute for Health and Clinical Excellence (
NICE
Nice ( , ; Niçard dialect, Niçard: , classical norm, or , nonstandard, ; it, Nizza ; lij, Nissa; grc, Νίκαια; la, Nicaea) is the prefecture of the Alpes-Maritimes departments of France, department in France. The Nice urban unit, agg ...
). Oesophageal Doppler monitoring measures the velocity of blood and not true ''Q'', therefore relies on a nomogram based on patient age, height and weight to convert the measured velocity into stroke volume and cardiac output. This method generally requires patient sedation and is accepted for use in both adults and children.
Pulse pressure methods
Pulse pressure
Pulse pressure is the difference between systolic and diastolic blood pressure. It is measured in millimeters of mercury (mmHg). It represents the force that the heart generates each time it contracts. Resting blood pressure is normally appro ...
(PP) methods measure the pressure in an artery over time to derive a waveform and use this information to calculate cardiac performance. However, any measure from the artery includes changes in pressure associated with changes in arterial function, for example compliance and impedance. Physiological or therapeutic changes in vessel diameter are assumed to reflect changes in ''Q''. PP methods measure the combined performance of the heart and the blood vessels, thus limiting their application for measurement of ''Q''. This can be partially compensated for by intermittent calibration of the waveform to another ''Q'' measurement method then monitoring the PP waveform. Ideally, the PP waveform should be calibrated on a beat-to-beat basis. There are invasive and non-invasive methods of measuring PP.
Finapres methodology
In 1967, the Czech physiologist Jan Peňáz invented and patented the
volume clamp method of measuring continuous blood pressure. The principle of the volume clamp method is to dynamically provide equal pressures, on either side of an artery wall. By clamping the artery to a certain volume, inside pressure—intra-arterial pressure—balances outside pressure—finger cuff pressure. Peñáz decided the finger was the optimal site to apply this volume clamp method. The use of finger cuffs excludes the device from application in patients without vasoconstriction, such as in sepsis or in patients on vasopressors.
In 1978, scientists at BMI-TNO, the research unit of
Netherlands Organisation for Applied Scientific Research
Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek (TNO; en, Netherlands Organisation for Applied Scientific Research) is an independent research organisation in the Netherlands that focuses on applied science.
The organisat ...
at the
University of Amsterdam
The University of Amsterdam (abbreviated as UvA, nl, Universiteit van Amsterdam) is a public research university located in Amsterdam, Netherlands. The UvA is one of two large, publicly funded research universities in the city, the other being ...
, invented and patented a series of additional key elements that make the volume clamp work in clinical practice. These methods include the use of modulated infrared light in the optical system inside the sensor, the lightweight, easy-to-wrap finger cuff with
velcro
Velcro, officially known as Velcro IP Holdings LLC and trading as Velcro Companies, is a British privately held company, founded by Swiss electrical engineer George de Mestral in the 1950s. It is the original manufacturer of hook-and-loop fast ...
fixation, a new pneumatic proportional control valve principle, and a set point strategy for the determining and tracking the correct volume at which to clamp the finger arteries—the Physiocal system. An acronym for physiological calibration of the finger arteries, this Physiocal tracker was found to be accurate, robust and reliable.
The Finapres methodology was developed to use this information to calculate arterial pressure from finger cuff pressure data. A generalised algorithm to correct for the pressure level difference between the finger and brachial sites in patients was developed. This correction worked under all of the circumstances it was tested in—even when it was not designed for it—because it applied general physiological principles. This innovative brachial pressure waveform reconstruction method was first implemented in the Finometer, the successor of Finapres that BMI-TNO introduced to the market in 2000.
The availability of a continuous, high-fidelity, calibrated blood pressure waveform opened up the perspective of beat-to-beat computation of integrated haemodynamics, based on two notions: pressure and flow are inter-related at each site in the arterial system by their so-called characteristic impedance. At the proximal aortic site, the 3-element
Windkessel model of this impedance can be modelled with sufficient accuracy in an individual patient with known age, gender, height and weight. According to comparisons of non-invasive peripheral vascular monitors, modest clinical utility is restricted to patients with normal and invariant circulation.
Invasive
Invasive PP monitoring involves inserting a
manometer
Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressu ...
pressure sensor into an artery—usually the
radial
Radial is a geometric term of location which may refer to:
Mathematics and Direction
* Vector (geometric), a line
* Radius, adjective form of
* Radial distance, a directional coordinate in a polar coordinate system
* Radial set
* A bearing f ...
or
femoral artery
The femoral artery is a large artery in the thigh and the main arterial supply to the thigh and leg. The femoral artery gives off the deep femoral artery or profunda femoris artery and descends along the anteromedial part of the thigh in the f ...
—and continuously measuring the PP waveform. This is generally done by connecting the catheter to a signal processing device with a display. The PP waveform can then be analysed to provide measurements of cardiovascular performance. Changes in vascular function, the position of the catheter tip or damping of the pressure waveform signal will affect the accuracy of the readings. Invasive PP measurements can be calibrated or uncalibrated.
=Calibrated PP – PiCCO, LiDCO
=
(
PULSION Medical Systems AG, Munich, Germany) and PulseCO (LiDCO Ltd, London, England) generate continuous ''Q'' by analysing the arterial PP waveform. In both cases, an independent technique is required to provide calibration of continuous ''Q'' analysis because arterial PP analysis cannot account for unmeasured variables such as the changing compliance of the vascular bed. Recalibration is recommended after changes in patient position, therapy or condition.
In PiCCO, transpulmonary thermodilution, which uses the Stewart-Hamilton principle but measures temperatures changes from central venous line to a central arterial line, i.e., the femoral or axillary arterial line, is used as the calibrating technique. The ''Q'' value derived from cold-saline thermodilution is used to calibrate the arterial PP contour, which can then provide continuous ''Q'' monitoring. The PiCCO algorithm is dependent on blood pressure waveform morphology (mathematical analysis of the PP waveform), and it calculates continuous ''Q'' as described by Wesseling and colleagues.
Transpulmonary thermodilution spans right heart, pulmonary circulation and left heart, allowing further mathematical analysis of the thermodilution curve and giving measurements of cardiac filling volumes (
), intrathoracic blood volume and extravascular lung water. Transpulmonary thermodilution allows for less invasive ''Q'' calibration but is less accurate than PA thermodilution and requires a central venous and arterial line with the accompanied infection risks.
In LiDCO, the independent calibration technique is
lithium chloride
Lithium chloride is a chemical compound with the formula Li Cl. The salt is a typical ionic compound (with certain covalent characteristics), although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorid ...
dilution using the Stewart-Hamilton principle. Lithium chloride dilution uses a peripheral vein and a peripheral arterial line. Like PiCCO, frequent calibration is recommended when there is a change in Q.
Calibration events are limited in frequency because they involve the injection of lithium chloride and can be subject to errors in the presence of certain muscle relaxants. The PulseCO algorithm used by LiDCO is based on pulse power derivation and is not dependent on waveform morphology.
=Statistical analysis of arterial pressure – FloTrac/Vigileo
=
![Frank Starling’s curve](https://upload.wikimedia.org/wikipedia/commons/4/42/Frank_Starling%E2%80%99s_curve.png)
FloTrac/Vigileo (
Edwards Lifesciences
Edwards Lifesciences is an American medical technology company headquartered in Irvine, California, specializing in artificial heart valves and hemodynamic monitoring. It developed the SAPIEN transcatheter aortic heart valve made of cow tissue wi ...
) is an uncalibrated, haemodynamic monitor based on pulse contour analysis. It estimates cardiac output (''Q'') using a standard arterial catheter with a manometer located in the femoral or radial artery. The device consists of a high-fidelity pressure transducer, which, when used with a supporting monitor (Vigileo or EV1000 monitor), derives left-sided cardiac output (''Q'') from a sample of arterial pulsations. The device uses an algorithm based on the
Frank–Starling law of the heart, which states pulse pressure (PP) is proportional to stroke volume (SV). The algorithm calculates the product of the standard deviation of the arterial pressure (AP) wave over a sampled period of 20 seconds and a vascular tone factor (Khi, or χ) to generate stroke volume. The equation in simplified form is:
, or,
. Khi is designed to reflect arterial resistance; compliance is a multivariate polynomial equation that continuously quantifies arterial compliance and vascular resistance. Khi does this by analyzing the morphological changes of arterial pressure waveforms on a bit-by-bit basis, based on the principle that changes in compliance or resistance affect the shape of the arterial pressure waveform. By analyzing the shape of said waveforms, the effect of vascular tone is assessed, allowing the calculation of SV. ''Q'' is then derived using equation (). Only perfused beats that generate an arterial waveform are counted for in HR.
This system estimates Q using an existing arterial catheter with variable accuracy. These arterial monitors do not require intracardiac catheterisation from a pulmonary artery catheter. They require an arterial line and are therefore invasive. As with other arterial waveform systems, the short set-up and data acquisition times are benefits of this technology. Disadvantages include its inability to provide data regarding right-sided heart pressures or mixed venous oxygen saturation. The measurement of Stroke Volume Variation (SVV), which predicts volume responsiveness is intrinsic to all arterial waveform technologies. It is used for managing fluid optimisation in high-risk surgical or critically ill patients. A physiologic optimization program based on haemodynamic principles that incorporates the data pairs SV and SVV has been published.
Arterial monitoring systems are unable to predict changes in vascular tone; they estimate changes in vascular compliance. The measurement of pressure in the artery to calculate the flow in the heart is physiologically irrational and of questionable accuracy,
and of unproven benefit. Arterial pressure monitoring is limited in patients off-ventilation, in atrial fibrillation, in patients on vasopressors, and in those with a dynamic autonomic system such as those with sepsis.
= Uncalibrated, pre-estimated demographic data-free – PRAM
=
Pressure Recording Analytical Method (PRAM), estimates ''Q'' from the analysis of the pressure wave profile obtained from an arterial catheter—radial or femoral access. This PP waveform can then be used to determine ''Q''. As the waveform is sampled at 1000 Hz, the detected pressure curve can be measured to calculate the actual beat-to-beat stroke volume. Unlike FloTrac, neither constant values of impedance from external calibration, nor form pre-estimated
in vivo
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
or
in vitro
''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...
data, are needed.
PRAM has been validated against the considered gold standard methods in stable condition and in various haemodynamic states. It can be used to monitor pediatric and mechanically supported patients.
Generally monitored haemodynamic values, fluid responsiveness parameters and an exclusive reference are provided by PRAM: Cardiac Cycle Efficiency (CCE). It is expressed by a pure number ranging from 1 (best) to -1 (worst) and it indicates the overall heart-vascular response coupling. The ratio between heart performance and consumed energy, represented as CCE "stress index", can be of paramount importance in understanding the patient's present and future courses.
Impedance cardiography
Impedance cardiography
Impedance cardiography (ICG) is a non-invasive technology measuring total electrical conductivity of the thorax and its changes in time to process continuously a number of cardiodynamic parameters, such as stroke volume (SV), heart rate (HR), car ...
(often abbreviated as ICG, or Thoracic Electrical Bioimpedance (TEB)) measures changes in
electrical impedance
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit.
Quantitatively, the impedance of a two-terminal circuit element is the ratio of the com ...
across the thoracic region over the cardiac cycle. Lower impedance indicates greater intrathoracic fluid volume and blood flow. By synchronizing fluid volume changes with the heartbeat, the change in impedance can be used to calculate stroke volume, cardiac output and systemic vascular resistance.
Both invasive and non-invasive approaches are used. The reliability and validity of the non-invasive approach has gained some acceptance, although there is not complete agreement on this point. The clinical use of this approach in the diagnosis, prognosis and therapy of a variety of diseases continues.
Non-invasive ICG equipment includes the Bio-Z Dx, the Niccomo, and TEBCO products by BoMed.
Ultrasound dilution
Ultrasound dilution (UD) uses body-temperature normal saline (NS) as an indicator introduced into an extracorporeal loop to create an atrioventricular (AV) circulation with an ultrasound sensor, which is used to measure the dilution then to calculate cardiac output using a proprietary algorithm. A number of other haemodynamic variables, such as total end-diastole volume (TEDV), central blood volume (CBV) and active circulation volume (ACVI) can be calculated using this method.
The UD method was firstly introduced in 1995. It was extensively used to measure flow and volumes with extracorporeal circuit conditions, such as
ECMO
Extracorporeal membrane oxygenation (ECMO), also known as extracorporeal life support (ECLS), is an extracorporeal technique of providing prolonged cardiac and respiratory support to persons whose heart and lungs are unable to provide an adequat ...
and
Haemodialysis
Hemodialysis, also spelled haemodialysis, or simply dialysis, is a process of purifying the blood of a person whose kidneys are not working normally. This type of dialysis achieves the extracorporeal removal of waste products such as creatini ...
, leading more than 150 peer reviewed publications. UD has now been adapted to
intensive care unit
220px, Intensive care unit
An intensive care unit (ICU), also known as an intensive therapy unit or intensive treatment unit (ITU) or critical care unit (CCU), is a special department of a hospital or health care facility that provides intensi ...
s (ICU) as the COstatus device.
The UD method is based on ultrasound indicator dilution. Blood ultrasound velocity (1560–1585 m/s) is a function of total blood protein concentration—sums of proteins in plasma and in red blood red cells—and temperature. Injection of body-temperature normal saline (ultrasound velocity of saline is 1533 m/s) into a unique AV loop decreases blood ultrasound velocity, and produces dilution curves.
UD requires the establishment of an extracorporeal circulation through its unique AV loop with two pre-existing arterial and central venous lines in ICU patients. When the saline indicator is injected into the AV loop, it is detected by the venous clamp-on sensor on the loop before it enters the patient's heart's right atrium. After the indicator traverses the heart and lung, the concentration curve in the arterial line is recorded and displayed on the COstatus HCM101 Monitor. Cardiac output is calculated from the area of the concentration curve using the Stewart-Hamilton equation. UD is a non-invasive procedure, requiring only a connection to the AV loop and two lines from a patient. UD has been specialised for application in pediatric ICU patients and has been demonstrated to be relatively safe although invasive and reproducible.
Electrical cardiometry
Electrical cardiometry is a non-invasive method similar to Impedance cardiography; both methods measure thoracic electrical bioimpedance (TEB). The underlying model differs between the two methods; Electrical cardiometry attributes the steep increase of TEB beat-to-beat to the change in orientation of red blood cells. Four standard ECG electrodes are required for measurement of cardiac output. Electrical Cardiometry is a method trademarked by Cardiotronic, Inc., and shows promising results in a wide range of patients. It is currently approved in the US for use in adults, children and babies. Electrical cardiometry monitors have shown promise in postoperative cardiac surgical patients, in both haemodynamicially stable and unstable cases.
Magnetic resonance imaging
Velocity-encoded phase contrast Magnetic resonance imaging (MRI) is the most accurate technique for measuring flow in large vessels in mammals. MRI flow measurements have been shown to be highly accurate compared to measurements made with a beaker and timer, and less variable than the Fick principle and thermodilution.
Velocity-encoded MRI is based on the detection of changes in the phase of proton
precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...
. These changes are proportional to the velocity of the protons' movement through a magnetic field with a known gradient. When using velocity-encoded MRI, the result is two sets of images, one for each time point in the cardiac cycle. One is an anatomical image and the other is an image in which the signal intensity in each
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device.
In most digital display devices, pixels are the ...
is directly proportional to the through-plane velocity. The average velocity in a vessel, i.e., the
aorta
The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...
or the
pulmonary artery
A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and ...
, is quantified by measuring the average signal intensity of the pixels in the cross-section of the vessel then multiplying by a known constant. The flow is calculated by multiplying the mean velocity by the cross-sectional area of the vessel. This flow data can be used in a flow-versus-time graph. The area under the flow-versus-time curve for one
cardiac cycle
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, following ...
is the stroke volume. The length of the cardiac cycle is known and determines heart rate; ''Q'' can be calculated using equation (). MRI is typically used to quantify the flow over one cardiac cycle as the average of several heart beats. It is also possible to quantify the stroke volume in real-time on a beat-for-beat basis.
While MRI is an important research tool for accurately measuring ''Q'', it is currently not clinically used for haemodynamic monitoring in emergency or intensive care settings. , cardiac output measurement by MRI is routinely used in clinical cardiac MRI examinations.
Dye dilution method
The dye dilution method is done by rapidly injecting a dye,
indocyanine green, into the right atrium of the heart. The dye flows with the blood into the aorta. A probe is inserted into the aorta to measure the concentration of the dye leaving the heart at equal time intervals
, ''T''until the dye has cleared. Let ''c''(''t)'' be the concentration of the dye at time ''t''. By dividing the time intervals from
, ''T''into subintervals Δ''t'', the amount of dye that flows past the measuring point during the subinterval from
to
is:
where
is the rate of flow that is being calculated. The total amount of dye is:
and, letting
, the amount of dye is:
Thus, the cardiac output is given by:
where the amount of dye injected
is known, and the integral can be determined using the concentration readings.
The dye dilution method is one of the most accurate methods of determining cardiac output during exercise. The error of a single calculation of cardiac output values at rest and during exercise is less than 5%. This method does not allow measurement of 'beat to beat' changes, and requires a cardiac output that is stable for approximately 10 s during exercise and 30 s at rest.
Factors influencing cardiac output
![2036 Summary of Factors in Cardiac Output](https://upload.wikimedia.org/wikipedia/commons/3/3f/2036_Summary_of_Factors_in_Cardiac_Output.jpg)
Cardiac output is primarily controlled by the oxygen requirement of tissues in the body. In contrast to
other pump systems, the heart is a demand pump that does not regulate its own output.
When the body has a high metabolic oxygen demand, the metabolically controlled flow through the tissues is increased, leading to a greater flow of blood back to the heart, leading to higher cardiac output.
The capacitance, also known as compliance, of the arterio-vascular channels that carry the blood also controls cardiac output. As the body's blood vessels actively expand and contract, the resistance to blood flow decreases and increases respectively. Thin-walled veins have about eighteen times the capacitance of thick-walled arteries because they are able to carry more blood by virtue of being more distensible.
From this formula, it is clear the factors affecting stroke volume and heart rate also affect cardiac output. The figure to the right illustrates this dependency and lists a few of these factors. A more detailed hierarchical illustration is provided in
a subsequent figure.
Equation () reveals HR and SV to be the primary determinants of cardiac output Q. A detailed representation of these factors is illustrated in the figure to the right. The primary factors that influence HR are autonomic
innervation
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
plus
endocrine control. Environmental factors, such as electrolytes, metabolic products, and temperature are not shown. The determinants of SV during the cardiac cycle are the contractility of the heart muscle, the degree of preload of myocardial distention prior to shortening and the afterload during ejection.
Other factors such as electrolytes may be classified as either positive or negative inotropic agents.
Cardiac response
Clinical significance
When ''Q'' increases in a healthy but untrained individual, most of the increase can be attributed to an increase in heart rate (HR). Change of posture, increased
sympathetic nervous system activity, and decreased
parasympathetic nervous system activity can also increase cardiac output. HR can vary by a factor of approximately 3—between 60 and 180 beats per minute—while stroke volume (SV) can vary between , a factor of only 1.7.
Diseases of the cardiovascular system are often associated with changes in ''Q'', particularly the pandemic diseases
hypertension and
heart failure. Increased ''Q'' can be associated with cardiovascular disease that can occur during infection and sepsis. Decreased ''Q'' can be associated with
cardiomyopathy and heart failure.
Sometimes, in the presence of ventricular disease associated with
dilatation, EDV may vary. An increase in EDV could counterbalance LV dilatation and impaired contraction. From equation (), the resulting cardiac output Q may remain constant. The ability to accurately measure ''Q'' is important in clinical medicine because it provides for improved diagnosis of abnormalities and can be used to guide appropriate management.
Example values
Related measurements
Ejection fraction
Ejection fraction
An ejection fraction (EF) is the volumetric fraction (or portion of the total) of fluid (usually blood) ejected from a chamber (usually the heart) with each contraction (or heartbeat). It can refer to the cardiac atrium, ventricle, gall bladder, ...
(EF) is a parameter related to SV. EF is the fraction of blood ejected by the left ventricle (LV) during the contraction or ejection phase of the cardiac cycle or
systole. Prior to the start of systole, during the filling phase (
diastole
Diastole ( ) is the relaxed phase of the cardiac cycle when the chambers of the heart are re-filling with blood. The contrasting phase is systole when the heart chambers are contracting. Atrial diastole is the relaxing of the atria, and ventri ...
), the LV is filled with blood to the capacity known as end diastolic volume (EDV). During systole, the LV contracts and ejects blood until it reaches its minimum capacity known as end systolic volume (ESV). It does not completely empty. The following equations help translate the effect of EF and EDV on cardiac output Q, via SV.
Cardiac input
Cardiac input (CI) is the inverse operation of cardiac output. As cardiac output implies the volumetric expression of ejection fraction, cardiac input implies the volumetric
injection fraction
An ejection fraction (EF) is the volumetric fraction (mathematics), fraction (or portion of the total) of fluid (usually blood) ejected from a chamber (usually the heart) with each contraction (or cardiac cycle, heartbeat). It can refer to the card ...
(IF).
IF = end diastolic volume (EDV) / end systolic volume (ESV)
Cardiac index
In all resting mammals of normal mass, CO value is a linear function of body mass with a slope of 0.1 L/(min kg).
[WR Milnor: Hemodynamics, Williams & Wilkins, 1982][BB Sramek: Systemic Hemodynamics and Hemodynamic Management, 2002, ] Fat has about 65% of oxygen demand per mass in comparison to other lean body tissues. As a result, the calculation of normal CO value in an obese subject is more complex; a single, common "normal" value of SV and CO for adults cannot exist. All blood flow parameters have to be indexed. It is accepted convention to index them by the body surface area, BSA
2">2 by DuBois & DuBois Formula, a function of height and weight:
The resulting indexed parameters are stroke index (SI) and cardiac index (CI). Stroke index, measured in mL/beat/m
2, is defined as
Cardiac index, measured in L/(min m
2), is defined as
The CO equation () for indexed parameters then changes to the following.The normal range for these indexed blood flow parameters are between 35 and 65 mL/beat/m
2 for SI and between 2.5 and 4 L/(min m
2) for CI.
Combined cardiac output
Combined cardiac output is the sum of the outputs of the right and left sides of the heart. It is a useful measurement in
fetal circulation In humans, the circulatory system is different before and after birth. The fetal circulation is composed of the placenta, umbilical blood vessels encapsulated by the umbilical cord, heart and systemic blood vessels. A major difference between t ...
, where cardiac outputs from both sides of the heart work partly in parallel by the
foramen ovale and
ductus arteriosus
The ''ductus arteriosus'', also called the ''ductus Botalli'', named after the Italian physiologist Leonardo Botallo, is a blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta. It a ...
, which directly supply the
systemic circulation.
Historical methods
Fick principle
![Spirometry NIH](https://upload.wikimedia.org/wikipedia/commons/0/05/Spirometry_NIH.jpg)
The Fick principle, first described by
Adolf Eugen Fick
Adolf Eugen Fick (3 September 1829 – 21 August 1901) was a German-born physician and physiologist.
Early life and education
Fick began his work in the formal study of mathematics and physics before realising an aptitude for medicine. He ...
in 1870, assumes the rate of oxygen consumption is a function of the rate of blood flow and the rate of oxygen picked up by the red blood cells. Application of the Fick principle involves calculating the oxygen consumed over time by measuring the oxygen concentration of venous blood and arterial blood. ''Q'' is calculated from these measurements as follows:
* ''V''
O2 consumption per minute using a
spirometer
A spirometer is an apparatus for measuring the volume of air inspired and expired by the lungs. A spirometer measures ventilation, the movement of air into and out of the lungs. The spirogram will identify two different types of abnormal ventilat ...
(with the subject re-breathing air) and a
CO2 absorber
* the oxygen content of blood taken from the pulmonary artery (representing mixed venous blood)
* the oxygen content of blood from a cannula in a peripheral artery (representing arterial blood)
From these values, we know that:
:
where
* ''C''
A is the oxygen content of arterial blood, and,
* ''C''
V is the oxygen content of venous blood.
This allows us to say
:
and therefore calculate ''Q''. (''C
A'' – ''C
V'') is also known as the
arteriovenous oxygen difference
The arteriovenous oxygen difference, or a-vO2 diff, is the difference in the oxygen content of the blood between the arterial blood and the venous blood. It is an indication of how much oxygen is removed from the blood in capillaries as the blood ...
.
While considered to be the most accurate method of measuring ''Q'', the Fick method is invasive and requires time for sample analysis, and accurate oxygen consumption samples are difficult to acquire. There have been modifications to the Fick method where respiratory oxygen content is measured as part of a closed system and the consumed oxygen is calculated using an assumed oxygen consumption index, which is then used to calculate ''Q''. Other variations use
inert gas
An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...
es as tracers and measure the change in inspired and expired gas concentrations to calculate ''Q'' (Innocor, Innovision A/S, Denmark).
The calculation of the arterial and venous oxygen content of the blood is a straightforward process. Almost all oxygen in the blood is bound to
hæmoglobin molecules in the red blood cells. Measuring the content of hæmoglobin in the blood and the percentage of saturation of hæmoglobin—the oxygen saturation of the blood—is a simple process and is readily available to physicians. Each
gram
The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...
of haemoglobin can carry 1.34 mL of
O2; the oxygen content of the blood—either arterial or venous—can be estimated using the following formula:
:
Pulmonary artery thermodilution (trans-right-heart thermodilution)
![Pulmonary artery catheter english](https://upload.wikimedia.org/wikipedia/commons/e/ea/Pulmonary_artery_catheter_english.JPG)
The indicator method was further developed by replacing the indicator dye with heated or cooled fluid. Temperature changes rather than dye concentration are measured at sites in the circulation; this method is known as thermodilution. The
pulmonary artery catheter
A pulmonary artery catheter (PAC), also known as a Swan-Ganz catheter or right heart catheter, is a balloon-tipped catheter that is inserted into a pulmonary artery in a procedure known as pulmonary artery catheterization or right heart cathet ...
(PAC) introduced to clinical practice in 1970, also known as the
Swan-Ganz catheter, provides direct access to the right heart for thermodilution measurements. Continuous, invasive, cardiac monitoring in intensive care units has been mostly phased out. The PAC remains useful in right-heart study done in cardiac catheterisation laboratories.
The PAC is balloon tipped and is inflated, which helps "sail" the catheter balloon through the right ventricle to occlude a small branch of the pulmonary artery system. The balloon is then deflated. The PAC thermodilution method involves the injection of a small amount (10 mL) of cold glucose at a known temperature into the pulmonary artery and measuring the temperature a known distance away using the same catheter with temperature sensors set apart at a known distance.
The historically significant Swan-Ganz multi-lumen catheter allows reproducible calculation of cardiac output from a measured time-temperature curve, also known as the thermodilution curve.
Thermistor
A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''.
Thermistors are divided based on their conduction ...
technology enabled the observations that low CO registers temperature change slowly and high CO registers temperature change rapidly. The degree of temperature change is directly proportional to the cardiac output. In this unique method, three or four repeated measurements or passes are usually averaged to improve accuracy. Modern catheters are fitted with heating filaments that intermittently heat up and measure the thermodilution curve, providing serial ''Q'' measurements. These instruments average measurements over 2–9 minutes depending on the stability of the circulation, and thus do not provide continuous monitoring.
PAC use can be complicated by arrhythmias, infection, pulmonary artery rupture and damage to the right heart valve. Recent studies in patients with critical illnesses, sepsis, acute respiratory failure and heart failure suggest that use of the PAC does not improve patient outcomes.
This clinical ineffectiveness may relate to its poor accuracy and sensitivity, which have been demonstrated by comparison with flow probes across a sixfold range of ''Q'' values.
Use of PAC is in decline as clinicians move to less invasive and more accurate technologies for monitoring hæmodynamics.
References
External links
Hemodynamics training for Junior Medical StaffThe Gross Physiology of the Cardiovascular SystemThe Determinants of Cardiac Output (online video)
{{DEFAULTSORT:Cardiac Output
Cardiovascular physiology