Cardiac output
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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.
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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 - 1921), 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:
- <math>VO_2 = (CO \times\ C_A) - (CO \times\ C_V)</math>
where CO = Cardiac Output, CA = Oxygen concentration of arterial blood and CV = Oxygen concentration of venous blood.
This allows us to say
- <math>CO = \frac{VO_2}{C_A - C_V}</math>
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.
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:
- <math>Cardiac\ Output = \frac{Oxygen\ consumption}{ArterioVenous\ Oxygen\ difference}</math>
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:
- <math>Oxygen\ content\ of\ blood = \left [ Hemoglobin \right ] \left ( g/dl \right )\ \times\ 1.36 \left ( ml\ O_2 /g\ of\ hemoglobin \right ) \times\ 10\ \times\ percentage\ saturation\ of\ blood</math>
Dilution methods
This method measures how fast flowing blood can dilute a marker 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 being equal to the mass of dye injected divided by the integral of the dye concentration from zero to infinity. The trapezoid rule is often used as an approximation of this integral. A more modern technique is to introduce cold or room temperature water, and then measure the change in temperature downstream. This method can however be affected by the phase of respiration, especially under mechanical ventilation, and should therefore be performed at the same 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 (Transpulmonary 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. The PAC has been associated with a significant increase in 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. The PulseCO algorithm usually derives cardiac output from the radial arterial pressure waveform. The techniques differ considerably from one another; the PiCCO algorithm is blood pressure waveform morphology dependent (i.e. mathematical analysis of the pulse contour waveform) and calculates continuous cardiac output as described by Wesseling and co-workers. The PulseCO algorithm is based on pulse power derivation and is not dependent on waveform morphology.
Impedance plethysmography
This advanced technique was developed by NASA, it measures changing resistance in the chest as the heart beats to calculate cardiac output. This technique has progressed clinically (often called BioZ, as promoted by the leading manufacturer in the US) and allowing 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
- <math> Flow = \frac{Pressure} {Resistance}</math>
When applied to the circulatory system, we get:
- <math> Cardiac\ Output = \frac{ABP - RAP}{TPR}</math>
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:
- <math>Cardiac\ Output \approx \frac{Aortic\ Blood\ Pressure}{Total\ Peripheral\ Resistance}</math>
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:
- <math>Heart\ Rate \times Stroke\ Volume \approx \frac{ABP}{TPR}</math>