The relationship of PAWP to left atrial pressure

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 However, it seems reasonable to briefly touch on this topic, purely for the purpose of historical reference.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?|

Scenarios where PAWP is higher than left atrial pressure

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:

• Poor LA compliance (eg. scarring following radiotherapy or Cox-Maze procedure)
• Mechanical atrial obstruction (eg atrial myxoma)
• Pulmonary venous obstruction, by
  • Lung tumour (extrinsic compression or invasion)
  • Consolidation
  • Pulmonary fibrosis
  • Radiation therapy
  • Pulmonary veno-occlusive disease

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.

Relationship to catheter tip position

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...

Scenarios where PAWP is lower than left atrial pressure

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. 

In short:

• Extremely high intrathoracic pressure: where the patient has extremely high alveolar pressure, the PAWP may drop to below left atrial pressure because the wedge catheter tip ends up being isolated behind some occluded capillaries. In other words, instead of measuring an uninterrupted column of blood leading to the left atrium, one ends up measuring a very short column of blood leading to some collapsed capillaries wrapped around an overdistended alveolus. It would be logical for the PAWP to approach PEEP in this scenario, and that indeed appears to be the case. For example, Lozman et al (1974) found that a PEEP of 15 or so was required for this discrepancy to develop, and at a PEEP of 15, some patients had PAWP of around 15-17mmHg, with an LAP of around 27 mmHg. 
• Non-Zone 3 position: A high PEEP is not required for the PAWP to be pointless - you just need to put the catheter into a position where it measures the pressure of alveoli and collapsed alveolar capillaries. In West's Zone 1, by definition, there is no uninterrupted column of blood to the left atrium, and all you are measuring is the transmitted pressure of the alveolar gas. In fact, one of the ways of confirming a non-Zone-3 position is to increase the PEEP and demonstrate that the PAWP pressure increased by about 50% of the PEEP increment.
• Extreme hypovolaemia: you don't need a malpositioned catheter tip to find Zone 1; in haemorrhagic shock Zone 1 finds you. The territory of underperfused lung extends lower in shock, producing an impressive amount of dead space. 
• Severe mitral regurgitation: because of how the pressures are reported by automatic monitors, this will make LA pressure look higher. The mean pressure is calculated as an integrated area under the pressure/time curve, and the high peaks of pressure due to MR will give a high mean LA pressure. To be fair, flow reversal in the pulmonary veins does actually occur with this. The same may be seen in aortic regurgitation, except it increases the expected LVEDP instead of the left atrial pressure. 
• Poor LV compliance: the atrial kick at the end of diastole may be pushing against a particularly stiff ventricle, which would produce a large increase in pressure without much increase in volume. This would produce a huge left atrial a-wave, and therefore also fool the monitoring software into reporting a high mean LA pressure, i.e. higher than the mean PAWP.