Haemodynamic monitoring: arterial line, CVP, cardiac output and lactate

What it is

Haemodynamic monitoring tracks how well the circulation delivers oxygen. Pressure is not flow: cardiac output equals heart rate times stroke volume, and oxygen delivery depends on cardiac output, haemoglobin and oxygen saturation. In the ICU several layers are combined. The arterial line gives beat-to-beat blood pressure and easy blood sampling, and its waveform shows damping, the effect of breathing on stroke volume and clues to the heart's function. The central venous catheter measures central venous pressure, which reflects the right atrial pressure and is useful as a trend or when very high, but predicts poorly whether fluids will help; it also allows central venous oxygen saturation to be sampled. Cardiac output can be measured by echocardiography, by thermodilution through a pulmonary artery catheter or a transpulmonary system, or estimated from pulse contour analysis of the arterial waveform. Markers of tissue perfusion complete the picture: lactate, central venous oxygen saturation, the venous-to-arterial carbon dioxide gap, capillary refill time, urine output and mental state. No single number is enough: trends, the clinical picture and the response to a treatment are what guide decisions. Every invasive device carries risks of infection, thrombosis, bleeding and wrong readings, so each one must earn its place and be removed when no longer needed.

Key points

• Pressure is not flow: a normal blood pressure can hide a low cardiac output when vessels are constricted.
• Transducers must be zeroed to air and levelled at the phlebostatic axis (fourth intercostal space, mid-axillary line); a wrong level shifts every reading.
• An over-damped arterial trace underestimates systolic pressure; an under-damped trace overestimates it; the mean is least affected.
• Echocardiography at the bedside shows ventricular size and function, pericardial fluid and an estimate of stroke volume.
• Dynamic indices such as pulse pressure variation are more useful than static pressures for predicting the response to fluids.

Components

• Arterial line: radial or femoral cannula, pressurised saline flush, transducer and monitor; gives a waveform and samples.
• Central venous catheter: internal jugular, subclavian or femoral line; measures central venous pressure and gives central venous saturation.
• Cardiac output monitors: echocardiography, pulmonary artery catheter thermodilution, transpulmonary thermodilution, pulse contour analysis.
• Perfusion markers: lactate, central venous oxygen saturation, venous-to-arterial CO2 gap, capillary refill, urine output, mental state.

How it works

• The transducer turns the pressure wave in a fluid-filled tube into an electrical signal; air bubbles, clots and kinks distort it.
• Thermodilution injects cold fluid and measures the temperature change downstream; the area under the curve is inversely related to output.
• Pulse contour analysis estimates stroke volume from the shape of the arterial wave; it needs a good trace and calibration in many systems.
• Central venous oxygen saturation falls when delivery falls or demand rises, because the tissues extract more oxygen.
• A wide venous-to-arterial CO2 gap means blood flow is too slow to wash out tissue CO2, even if venous saturation looks normal.

What each monitor tells you

In practice

• Do a square-wave flush test on the arterial line before trusting a sudden change in pressure.
• Re-zero and re-level the transducer when the patient or bed position changes.
• A very high or rising central venous pressure with shock suggests right heart failure, tamponade or fluid overload: stop fluids and look with echo.
• Rising lactate with a normal blood pressure: look for low output, gut or limb ischaemia, or another cause such as liver failure or adrenaline.
• Remove invasive lines as soon as they are no longer needed to reduce infection and thrombosis.

Next step

• Check zeroing, levelling and damping at every shift and before acting on a sudden change.
• Combine at least one pressure, one flow or perfusion marker and the clinical picture before changing treatment.
• Call a senior when output monitoring is needed to choose between fluids, inotrope and vasopressor.
• Review each invasive line daily and remove it when it no longer changes management.
• Follow line sites for infection, distal limb perfusion after arterial lines and thrombosis after central lines.

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Elsewhere in DurgiPedia

• Arterial line insertion
• Central venous catheter: sites, Seldinger technique and checks
• Cardiac output and what sets it
• Central venous pressure and the jugular pulse
• Oxygen content, delivery and extraction
• Lactate
• Central venous catheter care

Connections

← All of anaesthesia

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: Physiology, Cardiac Output (StatPearls (NCBI Bookshelf), 2023); Central Line Management (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.