a) List the determinants of central venous pressure (CVP).
b) Discuss the role of CVP monitoring in the critically ill.
Determinants of CVP:
Intravascular volume status
Mean systemic filling pressure
Right and left ventricular status and compliance
Pulmonary vascular resistance
Venous capacitance / tone
For example: CVP is the pressure recorded from the right atrium or superior vena cava and is representative of the filling pressure of the right side of the heart. CVP monitoring in the critically ill is established practice but the traditional belief that CVP reflects ventricular preload and predicts fluid responsiveness has been challenged.
Most critically ill patients have central venous vascular access with multi-lumen catheters, making CVP monitoring easy to do.
Information derived from the waveform and/or measured value assists with / assists the diagnosis of:
Confirmation of correct line placement.
Tricuspid regurgitation or stenosis.
Complete heart block.
Right ventricular infarction.
Differential diagnosis of shock state.
Determining mechanical atrial capture with AV pacing.
Determining the presence of P waves in cases of SVT.
Traditionally, CVP measurement has been used to assess fluid responsiveness – including assessment of change in CVP after fluid boluses – and the use of target values as resuscitation end-points as recommended in the Surviving Sepsis Guidelines. However increasing evidence including a recent meta-analysis (Marik in Chest) has shown there is no correlation between CVP and fluid status and targeting a certain CVP value can lead to overload in one patient and to another remaining hypovolaemic. Current thinking suggests that interpretation of CVP should be in association with information relating to other haemodynamic variables.
Complications associated with CVC insertion means that CV monitoring is not risk-free. Correct placement, calibration and measurement (at end-expiration) are needed to obtain an accurate recording. Simultaneous fluid administration through the CVC leads to inaccuracies.
Alternative monitoring modalities include devices such as PiCCO and Vigileo analysing stroke volume variation, pulse contour analysis, global end-diastolic blood volume, etc. and bedside echo.
For example: CVP monitoring may contribute information relating to the haemodynamic state of a patient but the value must be interpreted in the context of what else is known about that patient's cardiac function. Use of CVP as a measure of fluid responsiveness is flawed. The increasing use of bedside echo in the ICU is decreasing the utility of CVP monitoring.
This question is a repeat, closely resembling Question 16 from the first paper of 2001, "What are the determinants of central venous pressure? How may its measurement guide patient management?"
Detailed discussions of this can be found elsewhere:
- Determinants of central venous pressure
- Utility of CVP measurement in the ICU
- Information derived from analysis of the CVP waveform
- Factors which influence the accuracy of CVP measurement
In brief, the answer may resemble this:
Factors which determine CVP:
- Transducer position
- Timing of measurement with the cardiac cycle
- Timing of measurement with the respiratory cycle
Central venous blood volume
- Venous return
- Cardiac output (which determines venous return)
- Volume of blood in the central capacitance vessels
Central venous vascular compliance
- Vascular tone of the central venous walls
- Right atrial and right ventricular compliance
- Pericardial compliance
- Myocardial compliance
- Incompressible fluid in the pericardium, eg. tamponade
- Pulmonary arterial compliance
- Right ventricular outflow tract obstruction
- Pulmonary hypertension
Tricuspid valve competence
- Tricuspid stenosis will increase the CVP
- Tricuspid regurgitation will also increase the CVP
- The absence of atrial contraction decreases the CVP (eg. AF)
- Asynchronous atrial contraction (eg. during ventricular pacing) increases the CVP
Compartment pressures in the thorax and abdomen.
- An increase in intrathoracic pressure will increase the CVP:
- Intermittent psitive pressure ventilation
- Tension pneumothorax
- Intrabdominal pressure may increase OR decrease the CVP.