Equipment review: new techniques for cardiac output measurement--oesophageal Doppler, Fick principle using carbon dioxide, and pulse contour analysis

Abstract

Measuring cardiac output is of paramount importance in the management of critically ill patients in the intensive care unit and of 'high risk' surgical patients in the operating room. Alternatives to thermodilution are now available and are gaining acceptance among practitioners who have been trained almost exclusively in the use of the pulmonary artery catheter. The present review focuses on the principles, advantages and limitations of oesophageal Doppler, Fick principle applied to carbon dioxide, and pulse contour analysis. No single method stands out or renders the others obsolete. By making cardiac output easily measurable, however, these techniques should all contribute to improvement in haemodynamic management.

Figure 1

Oesophageal Doppler. (a) Schematic representation of oesophageal Doppler probe in a patient, demonstrating the close relation between oesophagus and descending thoracic aorta. (b) Characteristic velocity waveform obtained in the descending aorta. The spectral representation shows that most red blood cells (orange-white color) are moving at the maximum velocity (close to the green envelope) during systole, and that diastolic flow is minimal.

Figure 2

Principle of stroke volume calculation from aortic velocity (V_Ao) measurements. The area under the maximum aortic velocity envelope (VTI) represents the stroke distance. Assuming that all red blood cells are moving at maximum velocity and that aortic cross-sectional area is constant during systole, stroke volume is obtained by multiplying stroke distance by aortic cross-sectional area.

Figure 3

Eighty-eight paired measurements of cardiac output (CO) variations between two time-points obtained simultaneously using thermodilution (TH) with a pulmonary artery catheter and oesophageal Doppler (ED). Ideal agreement is represented by a horizontal line. Contradictory information with the two techniques was observed in only three cases [10]. The open boxes and vertical bars indicate mean and standard deviation, respectively.

Figure 4

Illustration of the importance of various arterial mechanical properties in generating the aortic pressure waveform. With the measured instantaneous flow [Q(t)] as an input, a single resistance (R) model of the circulation (model 1) would generate a pressure waveform [P(t)] with morphology identical to that of the flow waveform, differing only in magnitude by a factor of R. When arterial compliance, represented by a capacitance element (C), is incorporated (model 2), the predicted pressure waveform begins to exhibit many of the morphological characteristics of its measured counterpart. If a third element representing characteristic impedance (Z) is introduced (model 3), the morphologies of the predicted and measured pressure waveforms become very similar [23].