Question 19

College Answer

Following insertion of a pulmonary artery catheter

a)  list 3 tests which suggest appropriate Zone 3 positioning
PAWP < PADP
PAWP alters by < 50% of applied PEEP
PAWP increases by < 50% of changes in alveolar pressure
O2 satn in the wedged position greater than unwedged position
On the CXR, tip of catheter below level of LA.

b)  list 2 conditions where PAWP will read higher than LVEDP
Mitral stenosis
Atrial myxoma
Pulm venous obstruction – fibrosis, vasculitis
MR

non-zone 3 catheter placement

L to R shunt

COPD

IPPV+/-PEEP

c)  list 3 causes of inaccurate cold thermodilution cardiac output  measurements
1)  catheter malposition,
2)  injection mistakes (volume, injection speed, injectate temperature)
3)  inaccurate thermistor
4)  Tricuspid regurgitation
5)  Intra-cardiac shunts
6)  Wrong computation constant

d)  Is the pulmonary capillary hydrostatic pressure normally higher or lower than the pulmonary artery wedge pressure ?

Higher

Discussion

The PA catheter receives a thorough treatment in a series of chapters dedicated all to itself, somewhere deep in the Haemodynamic Monitoring section. . Appropriate zone positioning is also discussed.

The tip should be in West's 3rd Zone.

The following features confirm this position:

• On lateral CXR, the tip of the catheter is at or below the left atrium
• Respiratory variation of PAOP is < 50% of the static airway pressure (peak – plateau)
• Change the PEEP: PAOP changes by 50% of the change in PEEP
• The PAWP is less than the PA diastolic pressure
• The PAWP contour has recognizable a and v waves; in Zones 1 and 2 it is unnaturally smooth.
• Wedge PO2 minus Arterial PO2 = 19mmHg
• Arterial PCO2 minus Wedge PCO2 = 11mmHg
• Wedge pH minus Arterial pH = 0.008

Situations where the wedge pressure is higher than the LV end-diastolic pressure:

• Mitral stenosis (gradient across the mitral valve is high, LA pressure is increased)
• Atrial myxoma (same reason)
• Mitral regurgitation (large v waves interfere with wedge measurement, and LA pressure is high)
• Pulmonary fibrosis (obstruction to venous flow)
• Inappropriate placement (eg. into a high Wests zone)
• High PEEP
• High Auto-PEEP

Causes of inaccurate cold thermodilution cardiac output  measurements:

• Catheter is in the wrong position
• The thermistor tip is up against the wall
• The respiration is erratic
• There is an intracardiac shunt
• Tricuspid regurgitation
• Cardiac arrhythmia
• Rapid infusion happening via the IJ line
• Abnormal hematocrit
• Slow injectate delivery
• Injectate not cold enough, or not enough of it

Finally, pulmonary capillary hydrostatic pressure is usually higher than the wedge pressure. Normally, because of the venous resistance, Pcap will be higher than PAWP; in situations when this resistance is zero (i.e never) the two values might be equal.

 

An excellent online resource is available, which treats this subject with a massive amount of detail.

Also, the PA catheter section from The ICU Book by Paul L Marino (3rd edition, 2007) is a good source for most of this information.

Finally, Edwards Life Sciences has a booklet on invasive haemodynamic monitoring, which is a good solid overview.