Cardiac output monitoring

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Chapter 16 Cardiac output monitoring

Robert Sekun Kong, Luigi Vetrugno

Chapter contents

Pulmonary artery catheter

Oesophageal Doppler method for measurement of cardiac output

Arterial pressure waveform analysis

Many anaesthetic drugs adversely affect the cardiovascular system to some degree. This response is often exacerbated by pre-existing medical or surgical conditions. Failure to take account of these factors may lead to irreversible organ dysfunction in the perioperative period. Monitoring the efficiency of the cardiovascular system and taking therapeutic steps to improve its function has become an essential part of high-quality anaesthesia. In the past, non-invasive measurements of variables such as blood pressure and pulse, gave limited information as to the state of the cardiovascular system. However, it is now recognized that inferences of cardiac output (CO) made from these measurements can be wildly inaccurate and insufficient for therapeutic intervention in the perioperative period or during critical illness.

Newer methods of assessing and optimizing cardiovascular function have been developed that allow vastly improved therapeutic intervention by anaesthetists in the perioperative period. Today, at least a dozen devices are available to measure or estimate CO quite accurately. Some of the more popular ones, based on their market presence or adoption in clinical practice, will be described in this chapter. Table 16.1 is a more comprehensive list of the available equipment and their associated technologies.

Table 16.1 A representative sample of the variety of devices available for measuring CO and their technological bases.

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Pulmonary artery catheter

The advent of the Swan-Ganz pulmonary artery catheter (PAC) in 1970 enabled clinicians to measure cardiac output (CO) at the bedside.1

For most of the last 40 years, the PAC has been the preferred tool for haemodynamic monitoring in critically ill patients. As well as CO, the PAC can measure right heart and pulmonary artery pressures, estimate cardiac preload, and evaluate global oxygen delivery and consumption. Although such a comprehensive assessment of the circulation would be assumed to enhance patient care, there is little evidence to support this notion. Several studies have failed to show that using a PAC improved the outcome of critically ill patients in general2,3 or those with congestive cardiac failure4 or acute respiratory distress syndrome.5 Now and consequent to the emergence of the other less invasive technologies for measuring CO discussed later in this chapter, the last ten years has seen a continuing and dramatic decline in PAC usage.

The original PAC was a double lumen balloon-tipped catheter. A wide range of PACs is now available with one manufacturer producing around 30 different models. The PAC for adult use is typically a multi-lumen catheter, 110 cm in length and 7.0 or 7.5 F (French gauge) in external diameter. It is inserted via an introducer sheath (8.0–9.0 F depending on catheter model) into the internal jugular, subclavian or femoral vein. The proximal ports on the catheter are intended for administration of fluids or drugs, with their lumens appearing 19–30 cm from the distal tip of the catheter. To position the PAC, the balloon is inflated with air (1.5 ml) and the catheter is advanced with continuous pressure monitoring from the distal lumen. The typical waveforms and pressures encountered as the catheter traverses the right atrium and right ventricle on its way to the pulmonary artery (PA) should be familiar to anyone using this device (Fig. 16.1).

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Figure 16.1 A. Catheter advancing through right heart chambers and B. corresponding pressures in right atrium (RA), right ventricle (RV), pulmonary artery (PA) and wedged (PAWP).

To measure the pulmonary artery wedge pressure (PAWP), which under normal circumstances will approximate left ventricular end diastolic pressures, the catheter must be advanced until it is wedged in one of the medium-sized pulmonary arteries. The balloon is deflated as soon as the PAWP measurement is completed to minimize the risk of causing pulmonary ischemia. A locking mechanism is present at the balloon port to prevent inadvertent inflation of the balloon. Passage of the catheter into the PA depends on an adequate blood flow. If the patient has a very low CO, the catheter may not float into the intended final position, but tend to coil in the right ventricle. In such situations there is a risk that attempts to advance the catheter could literally tie it into a knot.

Cardiac output (CO) with PAC

The PAC measures CO by thermodilution – an adaptation of the indicator-dilution principle according to Stewart and Hamilton. Intermittent CO measurements are made using bolus injections of cold saline or 5% glucose solution as the thermal indicator. Following each injection, a thermodilution curve is obtained by plotting the temperature change of the blood, measured by a rapid-response thermistor at about 4 cm from the distal tip of the catheter. The Stewart-Hamilton equation shows that CO is indirectly proportional to the area under the curve (∫TB(t)dt) and where V = volume of injectate, TB = blood temperature, TI = injectate temperature, and K1 and K2 are computational constants. The relationship is given by the formula:

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Modified pulmonary artery catheters

A PAC equipped with a heating element can measure CO continuously by using a modified thermodilution principle in which the thermal indicator is heat. The catheters made by Edwards Lifesciences and OptiQ (ICU Medical) differ in the way the heating element is constructed and the duration of the heat pulses used to analyze the thermodilution curves. The displayed CO value is the average of intermittent, but frequent measurements performed automatically at intervals of several minutes. Continuous CO in this sense is not the same as beat-to-beat CO. The averaging time may be prolonged in unstable haemodynamic states or when there is thermal interference following rapid infusion of fluids. Nevertheless, continuous thermodilution CO monitoring is more convenient and may be less prone to operator-dependent errors when compared to intermittent thermodilution.

Data from the continuous CO catheter can be synchronized with the electrocardiogram to obtain right ventricular stroke volume, right ventricular end-diastolic volume, and calculate right ventricular ejection fraction. Oximetric catheters contain a fibre-optic cable and permit continuous measurement of mixed venous oxygen saturation (SvO2) from the distal pulmonary artery. Some catheters have embedded pacing wires for atrial, ventricular or dual chamber pacing. Like other central vascular catheters, the PAC can be coated with an antimicrobial agent to reduce the risk of catheter-related infection. A latex-free PAC is also available.

Limitations

1. Central venous cannulation is an invasive procedure with risks of arterial puncture, haemorrhage and pneumothorax.

2. The PAC can become knotted during insertion or later and may need to be removed surgically.

3. The indwelling introducer sheath and PAC are potential causes of sepsis and thrombus formation. The risk of infection appears to increase significantly if the catheters are left in for more than 72 h.

4. The PAC may cause arrhythmias particularly in the presence of hypothermia or electrolyte disturbances.

5. Improper balloon inflation and monitoring (e.g. failure to recognize an inadvertently wedged catheter) can result in arterial rupture, pulmonary infarction or haemorrhage.

6. The presence of the PAC may worsen pre-existing tricuspid regurgitation (TR). Over and underestimation of the true CO have been reported when there is TR. Continuous CO devices may be less affected by TR than when CO is measured intermittently.

7. CO measurement may be inaccurate in the presence of intracardiac shunts.

Oesophageal doppler method for measurement of cardiac output

Doppler effect

Oesophageal Doppler cardiac output monitors measure blood flow velocity in the descending aorta. An ultrasound probe (see below) is inserted into the distal third of the oesophagus. The ultrasound signal is backscattered by red cells travelling in the descending aorta by virtue of their differing acoustic impedence to the surrounding plasma (see Chapter 31). The returning ultrasound, is at a lower frequency (if flow direction is away from the source) and the difference between the two frequencies – the Doppler shift – is proportional to the velocity of the aortic blood flow as shown in the equation:

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where:

V is velocity of red blood cells (blood flow)

C is speed of ultrasound travelling through biological tissues

F is Doppler frequency shift

Fe is emitted frequency from ultrasound device

Cosφ is cosine of angle (φ) between the sound beam axis and the direction of blood flow (angle of insonation).

In the following sections the explanation of how this velocity measurement is translated into a calculation of cardiac output is primarily based on the CardioQ-ODM device (Deltex Medical, UK) (see below), which is currently the most widely available oesophageal Doppler cardiac output monitor in the UK. For comparison another device is briefly mentioned at the end.

CardioQ-ODM (Deltex Medical)

In the 1990s Abbott Medical marketed an early version of the oesophageal Doppler monitor, the ODMII, from this manufacturer. A major revision of the monitor resulted in the appearance of the CardioQ in 2000. The current model, the CardioQ-ODM (Fig. 16.2), was introduced in 2009. Compared to the CardioQ, the monitor screen has a higher resolution and a USB has replaced the RS232 ports to facilitate downloading of data and screenshots. The original algorithm to convert flow velocity to stroke volume is still used in the latest model, although some software enhancements have improved the monitor’s facility to store and display a variety of haemodynamic data.

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Figure 16.2 The Cardio Q-ODM by Deltex Medical, UK.

Photograph courtesy of Deltex Medical.

Aortic flow signal

Flow signals acquired by the probe are processed by fast Fourier transformation (FFT) and represented on the monitor as a spectral density display of the distribution of red blood cell flow velocities (vertical axis) against time (horizontal axis). Descending aortic blood flow gives a positive deflection and is approximately triangular in shape (Fig. 16.3).

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Figure 16.3 Ideal aortic waveform. The brightness of the signal is on the periphery of the waveform.

Note that flow in the descending aorta occurs predominantly during systole; there is minimal forward flow in diastole. Correct identification of the descending aortic waveform is a prerequisite for oesophageal Doppler monitoring. Ideally, the aortic waveform should appear triangular with the ‘brightness’ confined to the peripheral edge and an absence of signal in the central part of the triangle. This type of waveform is characteristic of a plug flow profile (present in the descending aorta) with a narrow spread of red cell velocities. Turbulent flow or unsatisfactory orientation of the transducer towards the descending aorta would show evidence of spectral dispersion (Fig. 16.4).

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Figure 16.4 Unsatisfactory aortic waveform. This waveform shows spectral dispersion (see text).

Signals from other vessels (e.g. pulmonary artery, celiac artery, azygous vein) may be encountered, although these are significantly different in appearance from the descending aortic waveform.

The Doppler frequency is proportional to flow velocity and being in the audible range may be transmitted through speakers. The two rotary controls on the monitor (Fig. 16.2) adjust the volume of the audio output and amplify the signal (gain) seen on the waveform display until the optimal signal is obtained. The same rotary controls may also be used to navigate through various menu options.

The nomogram

The device traces the maximum velocity of the spectrum described above and by calculating the area under the curve a value is derived for stroke distance; this being the distance a nominal column of blood moves in the aorta during systole.

The patient’s age, weight and height are entered in the monitor and stored in the memory chip of each new probe. The biometric variables are used in a proprietory nomogram to calculate stroke volume from the measured stroke distance. The conversion is based on a database of patients who underwent CO monitoring simultaneously with PAC thermodilution and the oesophageal Doppler. From this, the relationship between the velocity time integral (area under the curve) of the aortic flow waveform and stroke volume was formulated. Theoretically, the two are related by the cross-sectional area of the aorta, which is mostly determined by age and size of the individual. In fact the nomogram is in effect a patient-specific calibration constant which also allows for the fact that a significant proportion of cardiac output (brain, coronary and upper limb flow) does not reach the descending aorta. The calibration formula appears to be robust in that it has not had to be modified since its inception over 20 years ago. The accuracy of oesophageal Doppler CO is similar to other less invasive CO devices and considered to be clinically acceptable.

Doppler probe

The CardioQ-ODM uses continuous wave Doppler ultrasound. This is transmitted through an oesophageal probe that consists of a 50 cm length of a tightly coiled, steel spring, mounted with send-and-receive piezo-electric transducers (Fig. 16.5).

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Figure 16.5

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