Directly measured and derived PA catheter variables

Direct measurements came up inQuestion 28.1 from the first paper of 2011, among various duplicate versions.  Thus far, the college examiners have not asked any sadistic questions regarding the derived variables.

Direct measurements

• The following "direct" measurements can be made:

• Core temperature
• RA pressure
• PA pressure
• PA wedge pressure
• Mixed venous saturation

Normal values for the direct measurements

This has come up in Question 30.2 from the second paper of 2012. The candidates were offered a set of data where there was a pressure gradient between the RV and the PA, suggesting that an RVOT obstruction or pulmonic stenosis were present.

Variable	Normal Value
RA pressure	[ 2-6mmHg ]
RV pressure	[ 15/2 - 30/8 mmHg ]
PA pressure	[ 15/8 - 30/15 mmHg ]
PAOP	[ 6 - 12 mmHg ]
CI	[ 2.5 - 4 L/min/m2 ]

Key features:

• RV diastolic pressure must be lower than the CVP, so that the RV can fill in diastole
• The PAWP waveform is very similar to the CVP waveform, but slightly higher
• The PA diastolic pressure must be higher than the RV diastolic pressure

Importance of directly measured right heart catheter data

• The PA catheter is the gold standard for diagnosis of pulmonary hypertension

First degree derived variables

• Thermodilution cardiac output (CO)
• Stroke volume (SV)

Second degree derived variables

These are the derived variables which are dervived from other derived variables, and are thus two orders of error removed from actual measurements. Any error in the aforementioned direct measurements will therefore be significantly amplified in these calculations.

• Cardiac index (CI)
• Stroke volume index (SVI)
• Systemic vascular resistance index (SVRI)
• Pulmonary vascular resistance index (PVRI)
• Right ventricular ejection fraction (RVEF)
• Right Ventricular End-Diastolic Volume (RVEDV)
• Left Ventricular Stroke Work Index (LVSWI)
• Right Ventricular Stroke Work Index (RVSWI)
• Oxygen delivery (DO₂)
• Oxygen Uptake or Oxygen Consumption (VO₂)
• Oxygen extraction ratio (OER)

Calculation of the SVR

The SVR is  80 × (MAP- CVP) / CO)

The product of this equation is in dynes.s.cm^-5.

Normal is 700-1600.

The candidates have not been expected to calculate this since early 2006.