Question 21

a) List four patient factors that determine central venous pressure (CVP). (20% marks)

b) List four clinical conditions that may be detected from an abnormal central venous waveform in a euvolaemic patient and for each condition describe the associated waveform features.
(20% marks)

c) Explain how one performs and interprets a passive leg raise manoeuvre including its physiological basis, reliability, and limitations in clinical practice. (60% marks)

[Click here to toggle visibility of the answers]

College answer

Not available.

Discussion

Determinants of central venous pressure, where you can pick any four of the following:

Reference level of the transducer
Intravascular volume
- and the distribution of this volume between the venous and arterial compartment
Central venous compliance
Right ventricular compliance
- myocardial or pericardial disease; tamponade
Right ventricular systolic function
Cardiac rhythm (i.e. AF vs. sinus rhythm)
Tricuspid valve disease
Pulmonary vascular resistance
Intrathoracic pressure

Characteristic CVP waveforms are seen in the following settings (pick four, any four):

AF: absent a waves
Junctional rhythm, VT, complete heart block: cannon fused ac waves
Tricuspid regurgitation: fused cv waves
Triciuspid stenosis: prominent a wave
Pericardial constriction or poor RV compliance: bifid CVP wave
Cardiac tamponade: prolonged y descent

Passive leg raise autotransfusion:

Process:
1) Drop the patient's torso to supine position

2) Raise both legs to 45° using the mechanical bed

3) Keep them up for 60-90 seconds
4) Measure the change in stroke volume
Interpretation:
A 10% increase in the measured stroke volume is interpreted as a positive result.
Other acceptable surrogates would be an increase in cardiac output, an decrease in pulse pressure variation, or (less reliably) an increase in blood pressure
Physiological basis:
The autotransfusion of ~ 500ml of venous blood from the legs acts as a reversible fluid bolus. Ergo, if the patient's cardiac output increases as the result of this manoeuvre, it will also increase following a fluid bolus.
Reliability:
Sensitivity of 97% and a specificity of 94%, in predicting fluid responsiveness
Reliable irrespective of the mode of ventilation
Stroke volume and cardiac output measurements are the most reliable, whereas pulse pressure variation is less reliable (Cherpanath et al, 2016)
Limitations:
You need a patient with both legs intact
You rely on an intact pelvis, so this excludes a lot of messy trauma patients (in whom it would be very useful)
It can't be done if you have a balloon pump in situ, or post angiography (because you need to lie flat) - and thus a lot of low-cardiac-output cardiogenic shock patients are excluded, which is a pity
It can't be done if you are even slightly concerned about your intracranial pressure.

References

Magder, S. "More respect for the CVP." Intensive care medicine 24.7 (1998): 651-653.

Pittman, James AL, John Sum Ping, and Jonathan B. Mark. "Arterial and central venous pressure monitoring." International anesthesiology clinics 42.1 (2004): 13-30.

Marik, Paul E., and Rodrigo Cavallazzi. "Does the central venous pressure predict fluid responsiveness? An updated meta-analysis and a plea for some common sense*." Critical care medicine 41.7 (2013): 1774-1781.

Alzeer A et al. Central venous pressure from common iliac vein reflects right atrial pressure. Can J Anaesth 1998 Aug 45 798-801.

Magder, Sheldon. "Central venous pressure: A useful but not so simple measurement." Critical care medicine 34.8 (2006): 2224-2227.

Cherpanath, Thomas GV, et al. "Predicting fluid responsiveness by passive leg raising: a systematic review and meta-analysis of 23 clinical trials." Critical care medicine 44.5 (2016): 981-991.

[Submit a comment or correction]