5. Measurement of Cardiac Output

Cardiac output (CO): Delivers oxygen to tissues. Its prime determinants are heart rate (HR) and stroke volume (SV), in turn influenced by venous filling (preload), systemic vascular resistance (afterload) myocardial contractility. Low CO states predict adverse outcomes both in the critically ill and in patients undergoing major surgery.

although the use of the PAC has all but disappeared from most critical care units, it is still regarded as the most accurate method of determining CO and is the one against which other techniques are judged. The principles of measurement are described under ‘Measurement of Organ Blood Flow’ in the immediately previous section.

UK 2005 PAC vs Not in critically ill Hospital mortality – no significant difference No significant difference in: ICU length of stay Hospital length of stay Number of days of organ days in ICU 10% complication rate The fact that 80% of the patients in the no-PAC group had CO measured by a different means was potentially confounding, and sceptics were quick to point out that the outcomes in the other 20% of patients who had no CO measurement at all were very similar to those in patients who had PACs

Oesophageal Doppler monitor (ODM): this non-invasive ultrasonic device measures the velocity of blood flow in the descending thoracic aorta. The shift in frequency is proportional to the velocity of ejected blood. The device generates a velocity/time waveform to which is applied a calibration factor derived from the patient’s height, weight and age. The SV is then derived from the flow velocity, ejection time and aortic area. (A second transducer measures the cross-sectional area of the aorta.)

The useful indices that the ODM produces are the CO, the SV and the corrected systolic flow time (FTc). The FTc is normally between 330 and 360 ms and is an indicator of volaemic status; a low (short) FTc is associated with inadequate filling. Softer and smaller probes introduced nasally allow this technique to be used in the awake patient. Flow through the descending aorta is only around 70% of the total CO (the remainder is distributed through the subclavian arteries to the head, neck and upper limbs), but the device corrects for this proportion.

in 2011, NICE published guidance for oesophageal Doppler monitoring during surgery and explicitly named the Deltex Medical CardioQODM as the recommended device. NICE cited reductions in postoperative complications, reduced length of stay and a saving of around £1,100 per patient. The recommendation has been roundly criticized, and not without reason, as they were based on relatively few (eight) small trials, not all of which reported length of stay. One astringent commentator also noted that the main differentiation between controls and the study group was the intraoperative infusion of around 500 ml of colloid and queried whether such a modest intervention could really save more than a thousand pounds per patient. The point is well made.

Transoesophageal echocardiography (TOE): This technique also uses Doppler ultrasound. The TOE probe allows 180’ views of the heart, and not only measures CO but also gives a range of information about ventricular function, wall motion abnormalities and valvular anatomy. It is however very operator-dependent.

it has long been recognized that arterial pressure changes during respiration may be an indicator of volaemic status, and that a systolic pressure variation between the maximum and minimum recorded during one cycle of IPPV of more than 10 mmHg suggests at least a 10% reduction in circulating volume. Pulse contour analysis examines the arterial waveform, quantifies the SV and calculates the stroke volume variation (SVV). It is used both in LiDCO and PiCCO. As the devices are utilizing the arterial waveform, an optimal trace is essential for accurate determination.

This is a bolus indicator dilution method of measuring CO. After intravenous injection of a small dose of lithium (0.15 mmol), the plasma concentration is measured by an ion-selective electrode attached to the arterial line. The resulting concentration/time curve allows the calculation of CO (given by dose x 60/mean concentration [AUC] x time [s]). LiDCO can be used in combination with pulse contour analysis to provide continuous readings of CO, SV and SVV. (The technique cannot be used in patients who are receiving therapeutic lithium, nor in the first trimester of pregnancy.) LiDCO correlates well with invasive thermodilution catheter derived measurements.

This technique uses a thermodilution technique in conjunction with pulse contour waveform analysis. Cold saline is injected through into a central vein, and temperature is measured by a thermistor in an arterial cannula sited in a large artery (such as the brachial or femoral). This thermodilution CO measurement calibrates the system, after which the arterial waveform is analyzed to produce beat-to-beat determinations of SV, SVV and a continuous measure of CO. (It is also claimed that PiCCO can provide information about other volumetric parameters: global end-diastolic volume [GEDV], intrathoracic blood volume [ITBV] and extravascular lung water [EVLW]. You will not be asked how it does so.) PiCCO also correlates well with PA catheter–derived measurements.

This technique uses a thermodilution technique in conjunction with pulse contour waveform analysis. Cold saline is injected through into a central vein, and temperature is measured by a thermistor in an arterial cannula sited in a large artery (such as the brachial or femoral). This thermodilution CO measurement calibrates the system, after which the arterial waveform is analyzed to produce beat-to-beat determinations of SV, SVV and a continuous measure of CO. (It is also claimed that PiCCO can provide information about other volumetric parameters: global end-diastolic volume [GEDV], intrathoracic blood volume [ITBV] and extravascular lung water [EVLW]. You will not be asked how it does so.) PiCCO also correlates well with PA catheter–derived measurements.

Thoracic electrical bioimpedance: as the name suggests, this technique involves measuring the resistance to current flow through the thorax. This resistance changes both with the respiratory cycle and with pulsatile blood flow. High-frequency low amplitude alternating current is emitted and sensed via electrodes on the neck and lower chest wall. Changes in impedance are detected as blood distends and then leaves the aorta. These changes allow continuous determinations of CO, SV, contractile state and SVR. The technique tends to overestimate CO.