Cardiac Output Measurement
Cardiac Output Measurement
The pulmonary artery catheter was the mainstay of clinical cardiac output measurement for many years, but because of its relatively invasive nature and the lack of improvement of clinical outcome with its use, it is now seldom used in a modern clinical environment. Any perceived accuracy of the technique is now considered unnecessary in the face of the risks of its use, and with the introduction of newer non-invasive techniques. Nevertheless, it is worth describing, partly because of its historical interest, and partly because of the technologies involved. A catheter passed into the right atrium from an easily accessible central vein can be passed through the right ventricle and out into the pulmonary arterial tree while the vascular waveforms are visualised. Figure 13.1 shows the waveforms as they appear to the user. A small balloon at the tip of the catheter allows it to be flow directed and wedged in a pulmonary arterial vessel. At this point the pulsatile waveform is lost and the tip of the catheter is looking ahead, down the pulmonary arterial tree towards the left atrium, a system with a relatively low pressure drop from one end to the other, the flow in that vessel having been brought temporarily to a standstill. Thus the pulmonary artery occlusion pressure (PAOP) or pulmonary capillary wedge pressure (PCWP) can be considered a reasonably accurate representation of left atrial pressure or left ventricular filling pressure. This assumes that there is no pulmonary vascular disease, such as pulmonary hypertension, or mitral valve disease, in which case PAOP would not be an accurate representation of left atrial pressure. If the catheter is placed in the apical region of the pulmonary vascular tree, the excess of the alveolar pressure in inspiration over pulmonary capillary pressure becomes significant, and the latter is a less accurate reflection of left atrial pressure. The balloon should not be over-inflated for fear of rupturing the pulmonary artery, and this is one of its perceived risks that has led to less usage. Once the measurement has been made, the balloon should be deflated so that the pulmonary arterial waveform is once again visible, if necessary withdrawing the catheter a bit to achieve this; failure to do so would result in regional lack of perfusion and may result in ischaemia.
Keywords: Fick principle, LiDCO, Penlon Nuffield, PiCCO, Windkessel model, pulmonary artery catheters, pulmonary artery occlusion pressure (PAOP), pulmonary capillary wedge pressure (PCWP), pulse contour analysis, thermodilution method, transoesophageal Doppler, transthoracic electrical impedance
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