This chapter has no relevance to any specific Section of the 2017 CICM Primary Syllabus, and is probably irrelevant to most ICU trainees, as they will probably never be asked exam questions about this topic, nor use this derelict technology in their daily practice, what with the TOE probes these young people are swinging all over the place. However, it seems reasonable to briefly touch on this topic, purely for the purpose of historical reference.
Remember, that with an inflated balloon the flow (Q) in the pulmonary artery is zero. In the absence of flow, the resistance of the pulmonary circulation becomes irrelevant, and the only pressure measured here should be the pressure of the left atrium. Or is it?|
In generic terms, one would have to acknowledge that the assumption that PAWP = LAP is sound. If a static column of blood connects the left atrium and the wedged catheter tip, this should allow the measurement of left atrial pressure (Pascal's principle, people). However, there is blood flow into the left atrium, which means any sources of resistance along the path of that flow could give rise to a spuriously elevated PAWP. Theoretically, these sources of resistance could take several different forms:
However, in practice, their influence would have to be minimal in order for the organism to function. This practical objection seems to be supported by the literature. Connoly et al (1954) catheterised a whole series of patients, including those with mitral stenosis and pulmonary hilar malignancy, measuring both atrial and wedge pressures to make comparisons. On average, the PAWP was quite close to the LA pressure: 8 mm Hg vs 7mm Hg in normal hearts, and different only by about 5mm Hg in patients with relatively severe mitral disease. This makes some sense, as an excessive resistance to left atrial or pulmonary venous flow, giving rise to a huge pressure gradient, would be a recipe for severe pulmonary oedema and substantially decreased cardiac output. In short, though PAWP and LA pressure could be very different, in practice they are usually very similar, because the alternative would be incompatible with life.
PAWP can accurately reflect LA pressure only when there is an uninterupted column of blood between the LA and the wedged tip. This describes Wests Zone 3, where pulmonary arterial pressure is greater than alveolar air pressure.
The bottom line is, you need to have some blood around the catheter to measure any sort of blood pressure; and you can only guarantee this in Zone 3, where respiration does not squish blood out of the capillaries. And the alternative leads to...
So far, we have described scenarios where the PAWP exceeds left atrial pressure, due to some sort of venous or atrial resistance problem. Another (more exotic) possibility is for the PAWP to be lower than left atrial pressure. At first glance, that might make no sense, as the unidirectional flow of blood out of the pulmonary circulation depends on the left atrial pressure being lower than the pulmonary venous pressure. However, there are several situations where this relationship might be reversed. The excellent article by the young Ray Raper and William Sibbald (1986) discusses them in some detail.
The PA catheter section from The ICU Book by Paul L Marino (3rd edition, 2007) is the source for most of this information.
Kumar, Anand, et al. "Pulmonary artery occlusion pressure and central venous pressure fail to predict ventricular filling volume, cardiac performance, or the response to volume infusion in normal subjects." Critical care medicine 32.3 (2004): 691-699.