Cardiac output is the volume of blood being pumped by the heart in a minute. It is equal to the heart rate multiplied by the stroke volume.

So if there are 70 beats per minute, and 70 ml blood is ejected with each beat of the heart, the cardiac output is 4900 ml/minute. This value is typical for an average adult at rest, although cardiac output may reach up to 30 liters/minute during extreme exercise.

When cardiac output increases in a healthy but untrained individual, most of the increase can be attributed to increase in heart rate. Change of posture, increased sympathetic nervous system activity, and decreased parasympathetic nervous system activity can also increase cardiac output. Heart rate can vary by a factor of approximately 3, between 60 and 180 beats per minute, whilst stroke volume can vary between 70 and 120 ml, a factor of only 1.7.

Measuring Cardiac Output

---

There are many invasive and several non-invasive methods for measuring cardiac output in mammals.

A non-invasive method, often used in teaching students of physiology, reasons as follows:

• The pressure in the heart rises as blood is forced into the aorta
• The more stretched the aorta, the greater the pulse pressure
• In healthy young subjects, each additional 2ml of blood results in a 1 mmHg rise in pressure
• Therefore Stroke volume = 2ml x Pulse pressure
• Cardiac Output is therefore 2ml x Pulse Pressure x Heart Rate

The Fick Principle

Developed by Adolf Eugen Fick (1829 - 1901), this involves measuring:

• VO₂ consumption per minute using a spirometer (with the subject re-breathing air) and a CO₂ absorber
• the oxygen content of blood taken from the pulmonary artery (representing venous blood)
• the oxygen content of blood from a cannula in a peripheral artery (representing arterial blood)

From these values, we know that:

$VO\_2\; =\; (CO\; \backslash times\backslash \; C\_A)\; -\; (CO\; \backslash times\backslash \; C\_V)$

where CO = Cardiac Output, C_A = Oxygen concentration of arterial blood and C_V = Oxygen concentration of venous blood.

This allows us to say

$CO\; =\; \backslash frac\{VO\_2\}\{C\_A\; -\; C\_V\}$

and hence calculate cardiac output. In reality, this method is rarely used these days due to the difficulty of collecting and analysing the gas concentrations. However, by using an assumed value for oxygen consumption, cardiac output can be closely approximated without the cumbersome and time-consuming oxygen consumption measurement. This is sometimes called an assumed Fick determination. A commonly-used value for O₂ consumption at rest is 125 ml O₂ per minute per square meter of body surface area.

The Fick principle relies on the observation that the total uptake of (or release of) a substance by the peripheral tissues is equal to the product of the blood flow to the peripheral tissues and the arterial-venous concentration difference (gradient) of the substance. In the determination of cardiac output, the substance most commonly measured is the oxygen content of blood, and the flow calculated is the flow across the pulmonary system. This gives a simple way to calculate the cardiac output:

$Cardiac\backslash \; Output\; =\; \backslash frac\{Oxygen\backslash \; consumption\}\{ArterioVenous\backslash \; Oxygen\backslash \; difference\}$

Assuming there are no shunts across the pulmonary system, the pulmonary blood flow equals the systemic blood flow. Measurement of the arterial and venous oxygen content of blood involves the sampling of blood from the pulmonary artery (low oxygen content) and from the pulmonary vein (high oxygen content). In practice, sampling of peripheral arterial blood is a surrogate for pulmonary venous blood. Determination of the oxygen consumption of the peripheral tissues is more complex.

The calculation of the arterial and venous oxygen content of the blood is a simple process. Most oxygen in the blood is bound to hemoglobin molecules in the red blood cells. Measuring the content of hemoglobin in the blood and the percentage of saturation of hemoglobin (the oxygen saturation of the blood) is a simple process and is readily available to physicians. Using the fact that each gram of hemoglobin can carry 1.36 ml of O₂, the oxygen content of the blood (either arterial or venous) can be estimated by the following formula:

$Oxygen\backslash \; content\backslash \; of\backslash \; blood\; =\; \backslash left\backslash right\backslash left\; (\; g/dl\; \backslash right\; )\; \backslash \; \backslash times\backslash \; 1.36\; \backslash left\; (\; ml\backslash \; O\_2\; /g\backslash \; of\backslash \; hemoglobin\; \backslash right\; )\; \backslash times\backslash \; 10\backslash \; \backslash times\backslash \; percentage\backslash \; saturation\backslash \; of\backslash \; blood$

Dilution methods

This method measures how fast flowing blood can dilute an indicator substance introduced to the circulatory system, usually using a pulmonary artery catheter. Early methods used a dye, the cardiac output being inversely proportional to the concentration of dye sampled downstream. More specifically, the cardiac output is equal to the quantity of indicator dye injected divided by the area under the dilution curve measured downstream (the Stewart-Hamilton equation):

$Cardiac\backslash \; output\; =\; \backslash frac\{Quantity\backslash \; of\backslash \; Indicator\}\{\backslash int\_0^\backslash infty\; Concentration\backslash \; of\backslash \; Indicator.\backslash ,\; dt\}$

The trapezoid rule is often used as an approximation of this integral. A more modern technique is to use cold saline as the indicator, and then measure the change in temperature downstream. Cardiac output can be affected by the phase of respiration, especially under mechanical ventilation, and should therefore be measured at a defined phase of the respiratory cycle (typically end-expiratory).

Doppler method

This technique uses ultrasound and the Doppler effect to measure cardiac output. The blood velocity through the aorta cause a 'Doppler shift' in the frequency of the returning ultrasound waves. Echocardiographic measurement of the aortic root cross-sectional area (or, alternatively, the descending aorta area) multiplied by the measured velocity time integral of flow through that area and heart rate, yields the cardiac output.

Pulmonary Artery Thermodilution (Trans-right-heart Thermodilution)

The pulmonary artery catheter (PAC) also known as the Swan-Ganz thermodilution catheter provides right heart blood pressures. Using the PAC thermodilution cardiac output can be measured. Modern catheters are fitted with a distal heated filament, which allows automatic thermodilution measurement via heating the blood and measuring the resultant thermodilution trace. This provides near continuous cardiac output monitoring. The PAC is used in assessment of haemodynamic status and direct intracardiac and pulmonary artery pressures. The distal (pulmonary artery) port allows sampling of mixed venous blood for the assessment of oxygen transport and the calculation of derived parameters such as oxygen consumption, oxygen utilization coefficient, and intrapulmonary shunt fraction.

The PAC is balloon tipped which can be inflated to occlude the pulmonary artery, the subsequence back pressure is a reflection of the left atrial filling pressure and until recently was considered a good indicator of preload.

The pulmonary artery wedge pressure (PAWP) has been superseded by more reliable techniques such as intrathoracic blood volume or stroke volume variation as indicators of volume status. The PAC also allows sampling of mixed venous blood, the oxygen content of which can be used to indicate the adequacy of overall oxygen delivery. The PAC has fallen out of common use as clinicians favour less invasive, less hazardous technologies for monitoring haemodynamic status. Considerable controversy exists over whether the PAC increases mortality; recent studies suggest it neither increases nor improves mortality. Complications such as cardiac tamponade, pulmonary artery rupture and air emboli are a danger.

PulseCO and PiCCO Technology

PiCCO (PULSION Medical Systems AG, Munich, Germany) and PulseCO (LiDCO Ltd, London, England) generate continuous cardiac output by analysis of the arterial blood pressure waveform. In both cases, an independent technique is required to provide initial calibration of the continuous cardiac output analysis, as arterial waveform analysis cannot account for unmeasured variables such as compliance of the vascular tree.

In the case of PiCCO, transpulmonary thermodilution is used as the independent technique. This uses the Stewart-Hamilton principle outlined above, but measured from central venous line to a central (i.e. femoral or axillary) arterial line. The cardiac output derived from this cold-saline thermodilution is used to calibrate the arterial pulse contour analysis, which can then provide continuous cardiac output monitoring. The PiCCO algorithm is dependent on blood pressure waveform morphology (i.e. mathematical analysis of the pulse contour waveform) and calculates continuous cardiac output as described by Wesseling and co-workers. Transpulmonary thermodilution spans right heart, pulmonary circulation and left heart; this allows further mathematical analysis of the thermodilution curve, giving measurements of cardiac filling volumes (GEDV), intrathoracic blood volume, and extravascular lung water.

In the case of LiDCO, the independent calibration technique is lithium dilution, again using the Stewart-Hamilton principle. Lithium dilution has the advantage of being usable from a peripheral vein to a peripheral arterial line; however, it does not provide information on cardiac filling volumes and extravascular lung water. Dilution measurements cannot be performed too frequently, and can be subject to error 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.

FloTrac technology

A more recent development is the FloTrac system which can derive cardiac output from the arterial waveform without the need for an independent method of calibration. Hence continuous cardiac output can be measured directly from a conventional arterial line. This method has yet to be extensively evaluated, but early studies suggest that it is accurate. Another similar system that uses the arterial pulse is the pressure reording analytical method (PRAM). Neither the FloTrac nor the PRAM require external calibration.

Impedance plethysmography

This advanced technique was developed by NASA, it measures changing impedance in the chest as the heart beats to calculate cardiac output. This technique has progressed clinically (often called BioZ, i.e. biologic impedance, as promoted by the leading manufacturer in the US) and allows low cost, non-invasive estimations of cardiac output and total peripheral resistance, using only 4 skin electrodes, with minimal removal of clothing in physician offices having the needed equipment.

Equations

---

By simplifying D'arcy's Law, we get the equation that

$Flow\; =\; \backslash frac\{Pressure\}\; \{Resistance\}$

When applied to the circulatory system, we get:

$Cardiac\backslash \; Output\; =\; \backslash frac\{ABP\; -\; RAP\}\{TPR\}$

Where ABP = Aortic (or Arterial) Blood Pressure, RAP = Right Atrial Pressure and TPR = Total Peripheral Resistance.

However, as ABP>>RAP, and RAP is approximately 0, this can be simplified to:

$Cardiac\backslash \; Output\; \backslash approx\; \backslash frac\{Aortic\backslash \; Blood\backslash \; Pressure\}\{Total\backslash \; Peripheral\backslash \; Resistance\}$

Physiologists will often re-arrange this equation, making ABP the subject, to study the body's responses.

As has already been stated, Cardiac Output is also the product of the heart rate and the stroke volume, which allows us to say:

$Heart\backslash \; Rate\; \backslash times\; Stroke\backslash \; Volume\; \backslash approx\; \backslash frac\{ABP\}\{TPR\}$

External links

---

• The Gross Physiology of the Cardiovascular System
• The Determinants of Cardiac Output (online video)

Cardiology

Herzminutenvolumen central venous pressure (cvp)= right atrial pressure (0-+4mmhg) -charracterizes the pumping activity of the heart -sets the pressure gradient for venous return determination and stimation of cvp