Pulmonary artery diastolic pressure

This chapter has no relevance to any specific Section of the 2017 CICM Primary Syllabus, and like the rest of the PA catheter chapters, it is probably irrelevant to most ICU trainees. At least the pulmonary artery diastolic pressure is slightly more relevant than the wedge pressure.  The wedge pressure measurements have become rather unpopular over the years, but people keep inserting PA catheters for cardiac output monitoring, which means continuous PA pressure monitoring is still seen. Ergo, one is more likely to encounter PA diastolic measurements than PAWP measurements out in the wild. The following discussion briefly summarises the most important concepts in the event you might ever be tempted to draw some sort of significance from these values. 
 
In summary:
 
  • PA diastolic pressure and the wedged pressure are supposed to have a stable relationship.
  • Thus, PADP is a reasonable surrogate for PAOP
  • PADP is usually within about 5mmHg of PAOP
  • PADP will be more than 5mmHg different if the patient is tachycardic or there is a condition which increases pulmonary vascular resistance
  • The relationship between PAOP and PAOP will usually remain stable over hours.

That's probably all you need to know. The image below used borrowed tracings from the original paper by Swan and Ganz (1970) to illustrate this concept:

PA catheter - PA diastolic pressure

The PADP is usually higher than the PAWP. 
  • The diastolic pressure in the pulmonary arteries is higher because of the resistance to flow in the pulmonary arterial network;
  • Thus if  the flow is abolished (by occluding the artery) the pressure drops.
  • Ergo, if the PADP is lower than the PAWP, the PAWP measurement is probably wrong.
  • It may mean your catheter tip position needs to be changed.

The difference between PADP and PAOP is a surrogate measure of pulmonary vascular resistance

  • The diastolic pressure in the pulmonary arteries is higher because there is flow, and resistance to this flow.
  • Once you abolish flow, you also abolish resistance, and the pressure drops.
  • Thus, the difference between PADP and PAOP is a surrogate measure of pulmonary vascular resistance.
  • For example, if the PADP is markedly elevated while the PAWP is relatively normal,  there is probably pulmonary hypertension.

How great would the discrepancy have to be before you call it pulmonary hypertension?  6mmHg is the usually quoted number, which seems to come from Wilson et al (1988).

References

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.

Lappas, Demetriuos, et al. "Indirect measurement of left-atrial pressure in surgical patients–pulmonary-capillary wedge and pulmonary-artery diastolic pressures compared with left-atrial pressure." Anesthesiology: The Journal of the American Society of Anesthesiologists 38.4 (1973): 394-397.

Swan HJ, Ganz W, Forrester J, Marcus H, Diamond G, Chonette D (August 1970). "Catheterization of the heart in man with use of a flow-directed balloon-tipped catheter"N. Engl. J. Med. 283 (9): 447–51.

Wilson, Robert F., et al. "Pulmonary artery diastolic and wedge pressure relationships in critically ill and injured patients." Archives of Surgery 123.8 (1988): 933-936.