Haemodynamic Monitoring in Critical Care
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Direct answer
Haemodynamic monitoring in critical care measures pressure, flow and tissue perfusion to guide fluid and vasoactive therapy. Static pressures such as central venous pressure predict fluid responsiveness poorly, so dynamic tests such as the passive leg raise and pulse pressure variation are preferred. The monitoring chosen ranges from a simple arterial line to pulse-contour analysis and echocardiography.
What you must remember
- Central venous pressure: reflects right atrial pressure, with normal values around 2 to 6 mmHg, and is a poor predictor of fluid responsiveness.
- Arterial line: gives beat-to-beat blood pressure and allows waveform analysis and measurement of pulse pressure variation.
- Pulse pressure variation: a respiratory variation of about 13 per cent or more predicts fluid responsiveness in ventilated patients with a regular rhythm and adequate tidal volume.
- Passive leg raise: raising the legs to about 45 degrees autotransfuses blood, and a rise in stroke volume or cardiac output of about 10 per cent predicts fluid responsiveness.
- Pulmonary artery catheter: wedge pressure approximates left atrial pressure and thermodilution gives cardiac output, though routine use has declined.
- Pulse-contour methods: devices such as PiCCO and LiDCO estimate cardiac output from the arterial waveform, calibrated by thermodilution or lithium dilution.
- Central venous oxygen saturation: a low value suggests inadequate oxygen delivery relative to consumption.
Deciding whether a septic patient needs more fluid
A 55-year-old man with septic shock has a mean arterial pressure of 60 mmHg and a central venous pressure of 4 mmHg. Static pressures cannot answer whether more fluid will help, so the team runs a dynamic test first.
- Define the goal: Set targets for mean arterial pressure, urine output, capillary refill and lactate clearance before choosing monitors.
- Assess responsiveness: Perform a passive leg raise and measure stroke volume or cardiac output before and after, where a rise of about 10 per cent suggests fluid responsiveness.
- Check pulse pressure variation: In a ventilated patient with regular rhythm and adequate tidal volume, variation of 13 per cent or more supports a fluid trial.
- Give a fluid challenge: Infuse a crystalloid bolus, commonly 500 mL over 15 to 30 minutes, and reassess the same parameters.
- Stop when unresponsive: If stroke volume does not rise, further fluid risks pulmonary oedema, so a vasopressor is started or escalated instead.
- Confirm with perfusion: Track lactate, urine output and central venous oxygen saturation, since a low value shows that oxygen delivery is still inadequate.
Where the exam tries to catch you
Monitoring questions reward the dynamic test over the static number, and the options often offer the static number first.
- Trap: giving fluid because the central venous pressure is low. Correct: central venous pressure is a poor predictor of fluid responsiveness, so a dynamic test is used instead.
- Trap: using pulse pressure variation in a spontaneously breathing or arrhythmic patient. Correct: pulse pressure variation needs controlled ventilation and a regular rhythm, so a passive leg raise is used in those settings.
- Trap: placing a pulmonary artery catheter as routine. Correct: its routine use has declined, and it is reserved for selected complex cases with a specific question.
Frequently asked questions
Why is central venous pressure a poor guide to fluid responsiveness?
Central venous pressure reflects right atrial filling pressure, which does not reliably track ventricular preload. A low or normal reading therefore does not predict whether a patient will respond to a fluid bolus, and dynamic tests are preferred.
What is the passive leg raise test?
The passive leg raise test lifts the legs to about 45 degrees to autotransfuse blood from the limbs. A rise in stroke volume or cardiac output of about 10 per cent suggests fluid responsiveness, and the test does not itself require any fluid.
What conditions limit the use of pulse pressure variation?
Pulse pressure variation needs controlled mechanical ventilation with an adequate tidal volume and a regular heart rhythm. It is unreliable in spontaneously breathing patients and in arrhythmias, where a passive leg raise is preferred.
What do pulse-contour methods measure in critical care?
Pulse-contour methods estimate cardiac output from the shape of the arterial pressure waveform, calibrated by thermodilution or lithium dilution. They give continuous output data without requiring a pulmonary artery catheter.
What does a low central venous oxygen saturation suggest?
A low central venous oxygen saturation suggests that oxygen delivery is inadequate for the patient's oxygen consumption. It is read together with lactate and urine output to judge whether perfusion has been restored.