The introduction of the pulmonary artery catheter into clinical practice in the early 70's ^{1, 2} permitted the clinician, for the first time, to make a quantitative analysis of cardiovascular performance. The catheter allowed measurement of cardiac output by a thermal dilution technique ³ and estimation of left ventricular end-diastolic pressure (preload) by an occlusion technique. In conjunction with right sided pressure measurements, the physician was then able to calculate afterload and so build a picture of the 'haemodynamic state'.

The design of the early catheters ² included a terminal latex balloon, a pre-terminal thermistor, and distal and proximal pressure monitoring lumens (Figure 1).

The terminal balloon served two functions:

- It could be inflated with a gas (usually air) whilst in the central venous circulation and could then be used to 'flow-direct' the catheter through the right atrium and ventricle into the pulmonary artery. Thus the need for catheter direction under image intensification was eliminated. Once in the artery, it could then be slowly advanced until it became wedged in a medium-sized arterial branch.

- At this point, it was believed that a static column of blood now directly connected the distal lumen of the catheter, via this occluded segment of the pulmonary vasculature, with the left atrium (Figure 2). As left atrial pressure could be used as a measure of left ventricular end-diastolic pressure (LVEDP), this 'Pulmonary Capillary Wedge Pressure' ** or Pulmonary Artery Occlusion Pressure (PAOP), was thought to be a reflection of the LVEDP. Deflation of the balloon would now lead to 'unwedging' of the catheter and permit pulmonary arterial pressure measurements to be made. Subsequent measurements of PAOP could be made by re-inflation of the balloon.

The pre-terminal thermistor was used to sense the changes in blood temperature which followed the administration of a fixed volume of cold injectate given rapidly through the proximal catheter lumen. This bolus of cold injectate produced a variation in pulmonary artery blood temperature which could be sensed by the thermistor and which was dependent on right ventricular output at the time of injection. With appropriate recording and computing techniques, it was now possible to make cardiac output measurement using a 'thermodilution' technique (Figure 3) which was essentially a modification of the traditional indicator dilution method.

The distal pressure lumen served several functions:

- At the time of insertion, this lumen was used to monitor pressure waveforms during passage of the catheter. It was found that characteristic patterns for the right atrium, right ventricle, pulmonary artery and occluded pulmonary artery could be reliably identified and thus be used to determine catheter location.

- Once correctly positioned in the pulmonary artery, it could be used to measure pulmonary arterial pressures if the balloon were deflated, or the PAOP if inflated.

- The final role of the distal lumen was to permit sampling of the true mixed venous blood. There may be a considerable disparity in the oxygen content of the superior and inferior vena caval blood - IVC blood having a higher saturation than SVC blood in the resting patient. Passage of the blood through the right heart results in complete mixing of these two streams, so that by the time it arrives in the pulmonary artery, it is a representative sample of the 'average' venous blood. Under these circumstances, specimens drawn from the distal lumen could legitimately be used in the calculation of various metabolic indices.

The proximal pressure lumen served two functions:

- When the catheter was correctly positioned, this lumen would be in the superior vena cava and would provide an accurate measure of the central venous pressure.

- When cardiac output determinations were made, it was a suitable site for the administration of the bolus of injectate.

** Footnote:

The term Pulmonary Artery Occlusion Pressure, rather than Pulmonary Capillary Wedge Pressure, is probably more correct and is therefore used throughout this text.

1. Swan HJ, Ganz W, Forrester J, Marcus H, Diamond G, Chonette D. Catheterization of the heart in man with use of a flow-directed, balloon-tipped catheter. N Engl J Med 283:477, 1970.

2. Forrester JS, Ganz W, Diamond GA et al Thermodilution cardiac output determination with a single, flow-directed catheter. Am Heart J 83:306, 1972

3. Branthwaite MA, Bradley RD. Measurement of cardiac output by thermodilution in man. J Appl. Physiol 24:434, 1968

Last edited on: 13/11/2000