[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anaesthesia and ICU](https://durgical.com/en/anaesthesia/) › [Critical care (ICU)](https://durgical.com/en/anaesthesia/areas/critical-care/) › Haemodynamic monitoring: arterial line, CVP, cardiac output and lactate

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

- Concept

Haemodynamic monitoring measures pressure, blood flow and tissue oxygen so the ICU team can tell whether the circulation is enough.

Updated Nov 13, 2026

## Key points

- Cardiac output = heart rate × stroke volume; stroke volume depends on preload, afterload and contractility.
- Mean arterial pressure is roughly diastolic plus one third of the pulse pressure; it drives perfusion of most organs.
- Central venous pressure reflects right atrial pressure; it is a poor predictor of fluid responsiveness but useful as a trend and when very high.
- A low central venous oxygen saturation suggests low delivery or high extraction; a high one in sepsis may mean poor extraction.
- Lactate above the normal range signals hypoperfusion or impaired clearance; its trend matters more than one value.

On this page

- The basics Key points
- What it is
- Key points
- Components
- How it works
- In practice
- Next step
- Elsewhere in DurgiPedia

- Safety What each monitor tells you

- Study Test yourself

- Connections Connections

## 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

| Monitor | Tells you | Limits |
| --- | --- | --- |
| Arterial line | Beat-to-beat pressure, waveform, blood gases | Damping errors, bleeding, ischaemia, infection |
| Central venous pressure | Right atrial pressure trend | Poor predictor of fluid response |
| Echocardiography | Ventricle size and function, tamponade, stroke volume | Operator-dependent, intermittent |
| Thermodilution | Measured cardiac output and filling | Invasive, needs expertise |
| Lactate and venous saturation | Global balance of oxygen delivery and use | Other causes raise lactate; trends matter |

## 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.

## Test yourself

Think of the answer, then tap the question to see it.

## Why can blood pressure be normal while cardiac output is low?

Pressure equals flow times resistance; strong vasoconstriction can hold pressure up while flow, and so oxygen delivery, is low.

## Where should a pressure transducer be levelled?

At the phlebostatic axis, the fourth intercostal space in the mid-axillary line, which approximates the level of the right atrium.

## What does a low central venous oxygen saturation suggest?

That oxygen delivery is low for the demand (low output, anaemia, hypoxaemia) or demand is high, so tissues extract more oxygen.

## Why is central venous pressure a poor guide to giving fluids?

It is influenced by heart function, intrathoracic pressure and vascular tone, so a given value does not predict whether stroke volume will rise.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anaesthesia/haemodynamic-monitoring-arterial-line-cvp-cardiac-output-and-lactate/)

## Elsewhere in DurgiPedia

- [Arterial line insertion](https://durgical.com/en/clinical-skills/arterial-line-insertion/)
- [Central venous catheter: sites, Seldinger technique and checks](https://durgical.com/en/clinical-skills/central-venous-catheter-sites-seldinger-technique-and-checks/)
- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [Central venous pressure and the jugular pulse](https://durgical.com/en/physiology/central-venous-pressure-and-the-jugular-pulse/)
- [Oxygen content, delivery and extraction](https://durgical.com/en/physiology/oxygen-content-delivery-and-extraction/)
- [Lactate](https://durgical.com/en/biochemistry/molecules/lactate/)
- [Central venous catheter care](https://durgical.com/en/nursing/central-venous-catheter-care/)

## Connections

## Related 6

- [Fluid responsiveness: when more fluid helps](https://durgical.com/en/anaesthesia/fluid-responsiveness-when-more-fluid-helps/)
- [Shock: a bedside approach by type](https://durgical.com/en/anaesthesia/shock-a-bedside-approach-by-type/)
- [Central lines in the ICU: choice, checks and removal](https://durgical.com/en/anaesthesia/central-lines-in-the-icu-choice-checks-and-removal/)
- [Standard intraoperative monitoring](https://durgical.com/en/anaesthesia/standard-intraoperative-monitoring/)
- [Vasopressors and inotropes by receptor and mechanism](https://durgical.com/en/anaesthesia/vasopressors-and-inotropes-by-receptor-and-mechanism/)
- [Capnography: the waveform and what it tells you](https://durgical.com/en/anaesthesia/capnography-the-waveform-and-what-it-tells-you/)

## Related conditions 4

- [Cardiogenic shock](https://durgical.com/en/conditions/cardiogenic-shock/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/)
- [Cardiac tamponade](https://durgical.com/en/conditions/cardiac-tamponade/)

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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](https://www.ncbi.nlm.nih.gov/books/NBK470455/) (StatPearls (NCBI Bookshelf), 2023); [Central Line Management](https://www.ncbi.nlm.nih.gov/books/NBK539811/) (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anaesthesia/haemodynamic-monitoring-arterial-line-cvp-cardiac-output-and-lactate/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anaesthesia/haemodynamic-monitoring-arterial-line-cvp-cardiac-output-and-lactate/