Cardiac output monitoring

Abstract

Cardiac output monitoring has become commonplace amongst the standard monitoring in anaesthesia for major surgery and in intensive care. Multiple techniques are used, employing different models to calculate flow, which cannot be directly measured. Each method has differing advantage and disadvantage profiles and so the choice of device should consider a balance of invasiveness, usability, reliability and validity. Trend monitoring is generally emphasized over absolute values.

Learning objectives

After reading this article, you should be able to:

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understand the importance of cardiac output monitoring in anaesthesia and intensive care
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understand the physiology and physics behind the commonly used methods of cardiac output monitoring – Fick principle, pulse contour analysis, bioimpedance, aortic Doppler and echocardiography
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understand the strengths, common uses and limitations of the above methods of measuring cardiac output

Introduction

Cardiac output (CO) is an important parameter, representing the total cardiovascular blood flow through the heart. It is defined as the volume of blood ejected from the right or left ventricle, per unit time commonly being described as litres per minute.

CO = Stroke volume (SV) × Heart rate (HR)

Oxygen delivery is vital to tissues, especially in critical illness or post-surgery where oxygen demand is increased due to the inflammatory response. To ensure adequate organ function and aerobic respiration in these situations perfusion and oxygen delivery must be maintained. Delivery of oxygen is a result of both the cardiac output and arterial oxygen content:

DO ₂[ml/minute] = Cardiac output [litres/minute] × Arterial oxygen content [ml/dl] × 10

Therefore, by utilizing techniques to calculate the CO the information can be combined with the measurable arterial oxygen content to give a good estimation of oxygen delivery to tissues, impacting how the patient may be treated.

For almost a century there have been a number of ways to calculate cardiac output, with each in turn having issues around potential sources of error, cost or limitations of use. Most forms of CO measure are limited to controlled areas such as critical care or the operating theatre due to their invasive nature. Each technique can be potentially classified into three main types:

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Invasive: utilizes a central line, arterial line or pulmonary catheter.
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Minimally invasive: ultrasound techniques exploiting the oesophagus’ proximity to the aorta.
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Non-invasive: a number of techniques are used including bioimpedance.

All these methods aim to accurately infer the stroke volume of the patient to give a value for the CO.

This article aims to explain the principles and physiology behind each of these methods and include the limitations of each.

Invasive methods of analysis

Fick principle

This principle holds that the uptake or excretion of a substance by an organ is equal to the difference between the amount of substance entering the organ and the amount leaving ( Figure 1 ).

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It is important to realize that because the entire blood volume passes through the lungs, changes in oxygen concentration can give an accurate picture of the CO in a patient. However, the use of oxygen is not practical because it requires a pulmonary artery (PA) catheter and also makes assumptions about the system within which it is estimating concentration difference (e.g. that the pulmonary and systemic volumes are equal). Additionally, to properly ensure accurate measurement of oxygen a sealed hood is required for measuring gases, which again adds complexity to utilizing oxygen as the measured substance in the Fick principle.

Subsequently, dye or thermodilution techniques are utilized instead and assume that the rate of decay in the concentration of the substance/temperature change is proportional to the flow rate through the organ.

Dye dilution techniques

Method of use: in this instance a known volume/amount of a marker substance is chosen and injected into the pulmonary artery or central vein (therefore it requires a central venous catheter (CVC) or PA catheter). A peripheral arterial line is then used to continuously sample the concentration of the dye. This then can produce a graph which measures the concentration over time ( Figure 2 ).

Traditionally, indocyanine green was the dye used to create this graph as it had a low toxicity and short half-life. This limited the amount of recirculation after the initial passage through the systemic vasculature. Lithium is now more commonly used as it is not an ion present normally in plasma so only very small doses are required to gain a high signal to noise ratio, again this ensures patients are not exposed to toxic or pharmacologically significant doses.

CO can then be calculated from the graph produced by using the area under the curve (integral of the curve) and a modification of the Fick equation known as the Stewart–Hamilton equation ( Figure 1 ).

Limitations

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Logarithmic transformation of the concentration is necessary to give a more accurate estimation of the area under the curve and mitigate for recirculation of dye.

Thermodilution

To avoid the problem with potential recirculation of dye, a thermodilution technique was developed. This then became the ‘gold standard’ against which other CO measures are compared to. It utilizes the same principle as dye dilution except it uses 10–15 ml cold saline which is then injected though a PA catheter. The change in blood temperature is then measured from the proximal port where it’s injected to the thermistor in the distal portion of the catheter. A modification of the Stewart–Hamilton equation is used to calculate the CO ( Figures 1 and 2 ).

Method of use: it is inserted by inflating a balloon at the tip of a PA catheter and ‘floating’ the catheter through the right atrium, the right ventricle and letting it rest in the pulmonary artery (shown in Figure 2 ). The cold saline is injected and CO calculated.

Benefits

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It requires no blood sampling and there is no recirculation issue as the blood has warmed back up to normal by the time it reaches the pulmonary circulation.
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This system continues to remain accurate in the presence of balloon pumps and arrhythmias and any inaccuracies can be reduced by taking multiple measurements.

Complications

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Those linked to pulmonary catheter insertion including arrhythmias, infection, valvular damage and pulmonary artery rupture.

A study (PAC-MAN study group) in 2005 aimed to assess the mortality associated with PA catheter insertion in critically ill patients. Despite this randomized controlled trial finding no evidence of benefit or harm, there has not only been a move to less accurate methods of measuring CO, but indeed a move towards their cessation of use in clinical practice. The reasons for this change are multifactorial and extend beyond the remit of this article; however, recognizing this form of CO monitoring as the ‘gold standard’ by which others are measured is important.

Pressure waveform analysis

The clinical use of pulse pressure, assessing its character and volume, has been utilized long before more quantitative measures in current practice. This has been achieved with the use of arterial cannulation and the assessment of a pulse pressure waveform and now form the most widely used techniques for monitoring cardiac output.

Windkessel effect: the left ventricle (LV) contracts generating a systolic pressure and typically ejects approximately 70 ml (the stroke volume) of blood into the aorta.

Blood within the aorta then causes the walls to expand and stretch to accommodate this new volume increasing the pressure from 80 mmHg to 120 mmHg. The ejected blood has kinetic energy that propels it onwards into the systemic vasculature while the stretched elastic walls of the aorta have potential energy. This is then converted to kinetic energy when the walls recoil and helps ensure that blood continues to be moved though the systemic vasculature in diastole and helps maintain a diastolic BP. The phenomenon described as the Windkessel effect is how this sinusoidal pressure generated in the heart is then converted into a positive and constant pressure at the tissues. Otto Frank, the German physiologist who first described this phenomenon, likened it at the time to the ‘Windkessel’ air chamber used in fire engines that converted the sinusoidal pump pressure generated into a constant water stream ( Figure 3 ).