Using the PA Catheter in the Diagnosis of Hemodynamic Instability
Forget the biopsychosocial approach.
Some say you can make the diagnosis with only 3 parameters.
- Central venous pressure (CVP)
- Cardiac Index (CI)
- Systemic Vascular Resistance (SVRI)
For example, a patient with a low SVRI, low CI and high CVP had decreased vascular resistance (thus likely sepsis) as well as decreased cardiac output (likely owing to heart failure) with a high preload, which suggests they have a combination of distributive and cardiogenic shock.
To identify shock, one also needs to demonstrate decreased tissue perfusion.
- Otherwise, its just a low-output state with adequately perfused tissues.
- VO2 is the marker of tissue oxygen consumption.
- Thus, in states of shock, VO2 will be low.
- It allows one to differentiate a low-output state from shock.
Most would agree
- The only thing the catheter can be reliably depended upon is the thermodilution cardiac output measurement.
- You can track this over time to establish whether what you are doing with the inotropes is helping or hurting.
- It allows one to differentiate a low-output state from shock of other types.
Specific applications of the PA catheter for investigation of right heart failure
There are several situations where it is particularly helpful to have a transduced catheter in the pulmonary artery. One such situation is the diagnosis of the causes of right heart failure, as well as the monitoring of a response to therapy.
The PA catheter may reveal raised LA pressures and/or a waveform characteristic of mitral regurgitation, which would lay the blame for the RV decompensation upon the failure of the left heart.
Or, the pulmonary vascular resistance (PVR) might turn out to be absurdly high, which would also explain RV failure (this time due to overwhelming afterload).
Or one might discover a RV-PA gradient in excess of 25mmHg, which would suggest RVOT obstruction.
Or, the PAWP and PVR may turn out to be completely normal, and one might find oneself disappointed by the right ventricular contractility.
In summary, a right heart catheter offers a direct measurement of right heart pressures, and a thermodilution-based assessment of right heart function.
Additionally, PA catheterisation is still the gold standard for the differential diagnosis of pulmonary hypertension. Comparison of TTE and PAC have revealed a discordance of the two measurements of PA pressures, though the authors hasten to point out that this is not due to the inaccuracy of one modality, but rather due to the constant variation of PASP.
A 2009 article from Circulation discusses the history of changes in the utility of the PA catheter, and suggests a list of modern indications for its use.
Other bedside uses
The PA catheter offers the "direct" measurement of several parameters.
Additonally, useful information can be derived from analysis of the waveform.
The following variables are directy measured, and could be useful in the management of hemodynamic instability, in the right setting:
- Core temperature - it is the gold standard of temperature monitoring
- RA pressure, PA pressure and PAWP - useful in the assessment of right ventricular failure, particularly if cardiac MRI is unavailable
- Use of continuous PA pressure monitoring to titrate pulmonary vasodilators
- Waveform analysis
- Is to look for the large v-wave on the occlusion pressure trace
- This makes you think that the mitral valve is incompetent
- Its useful in patients recovering from cardiac surgery
- Its especially useful if you’ve just replaced the mitral valve.
- Mixed venous saturation -a means of assessing the oxygen extraction ratio.