Puzzling out PAC Pressures

Labs and Lytes 019

Author: David Tuxen
Reviewers: Chris Nickson, Paul Nixon

This problem set is designed to help develop an understanding of the important diagnostic patterns of pulmonary artery catheter (PAC) pressure measurements. Much can be discovered from just 3 parameters: right atrial pressure (RAP), pulmonary diastolic pressure (PDP) and pulmonary artery occlusion pressure (PAOP; also known as the pulmonary artery wedge pressure (PAWP)).

The reference range for PA pressure values is:

  • RAP = 0-6 mmHg
  • RV = 25/0 mmHg
  • PAP = 15-30/5-15 mmHg
  • PAOP = 2-10 mmHg

Q1. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 9 mmHg
PDP 17 mmHg
PAOP 16 mmHg

PDP and PAOP are elevated in tandem, with only a mild increase in RAP.

This pattern is consistent with an elevated left atrial pressure (LAP) usually due to left ventricular failure (elevated LV EDP) or mitral valve disease (rarely an atrial myxoma).

A useful rule of thumb is that the PAOP should be no more than 2 mmHg higher than RAP. PAOP reflects LAP – and is increased in the presence of either an elevated LV EDP from LV failure or mitral valve disease.

Q2. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 8 mmHg
PDP 18 mmHg
PAOP 9 mmHg

There is an isolated elevation in PDP.

This pattern is consistent with pulmonary vascular disease or pulmonary hypertension. Underlying diagnoses may include conditions such as ARDS, COPD, PE or pneumonia.

A useful rule of thumb is that the PDP should be no more than 2 mmHg higher than PAOP (different sources suggest 1-5 mmHg), unless there is pulmonary vascular disease/ pulmonary hypertension.

Q3. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 15 mmHg
PDP 17 mmHg
PAOP 16 mmHg

All 3 values are elevated in unison.

This pattern is consistent with fluid overload.

The rule of thumb to remember is that ventricles don’t usually fail equally, biventricular failure is unlikely to cause this pattern. Early pericardial tamponade is an alternative explanation.

Q4. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 14 mmHg
PDP 21 mmHg
PAOP 8 mmHg

PDP and RAP are elevated.

This pattern is consistent with cor pulmonale, i.e. right ventricular failure due to pulmonary hypertension.

The rule of thumb is that RAP is always less than PAOP unless there is right ventricular failure.

Q5. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 24 mmHg
PDP 26 mmHg
PAOP 25 mmHg

All 3 values are markedly elevated in unison.

This pattern is consistent with pericardial tamponade.

As for Q3, ventricles tend not to fail equally. The additional rule of thumb is that fluid alone rarely causes pressures >20 mmHg.

Q6. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 7 mmHg
PDP 9 mmHg
PAOP 14 mmHg

This pattern doesn’t make physiological sense!

The PAOP should be less that PDP. Thus the PAOP measurement may be inaccurate, it might represent mean PA pressure if there is a damped PA pressure waveform.

Q7. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 5 mmHg
PDP 7 mmHg
PAOP 6 mmHg

This is normal!

All the values are within standard reference ranges and the normal pattern is present: PAOP <2 mmHg higher than RAP and PDP <2 mmHg higher than PAOP.

Q8. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 32 mmHg
PDP 9 mmHg
PAOP 8 mmHg

This normal, but with an artefactually elevated RAP due to fluid infusion through the RA line.

As a rule of thumb, RAP is only higher than PDP due to a measurement error or artefact.

Q9. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 9 mmHg
PDP 21 mmHg
PAOP 14 mmHg

The PAOP is elevated, with a more marked increase in PDP. The RAP is also mildly elevated.

This is consistent with a combination of left ventricular failure and pulmonary hypertension.

This is most commonly in elderly patients with acute or chronic lung disease, or chronic left ventricular failure causing pulmonary hypertension.

Q10. Interpret the PAC pressure measurements below and give the likely diagnosis:

RAP 9 mmHg
PDP 21 mmHg
PAOP unable to wedge

PDP is elevated, and RAP is mildly elevated.

This is abnormal but the underlying diagnosis is uncertain, it could be PE, a milder form of Q4 or if the actual PAOP is elevated the same as Q9.

When unable to wedge the PA catheter, if air returns to the syringe then the catheter is probably not advanced far enough into the pulmonary arterial tree. If air does not return to the syringe, then the balloon is probably ruptured.

References and Links

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