Invasive monitoring

Overview & definition

The invasive monitoring of the anaesthetised patient provides the continuous, beat-to-beat measurement of the haemodynamic and the neurological variables that the non-invasive cuff, the pulse oximeter and the clinical observation cannot. It includes the arterial line (the direct blood pressure and the waveform), the central venous catheter (the right-heart filling pressure and the drug access), the cardiac output monitor (the flow and the volume responsiveness for the goal-directed therapy), the transoesophageal echocardiography (the structure and the function), and the depth-of-anaesthesia monitor (the BIS and the entropy). The principle is that the more unstable or the higher-risk the patient, the more monitoring is warranted — the low-risk patient for a minor surgery needs no more than the standard non-invasive monitoring, while the cardiac or the major-vascular patient needs the full invasive array. [1]

The arterial line: technique and indications

The arterial line is a cannula placed in a peripheral artery (most commonly the radial), connected to a column of saline and a pressure transducer, providing the continuous, beat-to-beat direct blood pressure and the arterial waveform. The insertion is by the Seldinger or the direct cannulation technique under the aseptic precautions, and the transducer is zeroed to the atmospheric pressure and levelled to the patient's mid-axillary line.[3]

The indications for the arterial line include: the major surgery (the cardiac, the vascular, the thoracic, the major abdominal) where the rapid blood pressure changes are expected; the controlled hypotension (the neurosurgery, the ENT) where the tight BP control is required; the haemodynamic instability (the shocked, the septic, the vasoactive-infusion patient); the frequent blood gas sampling (the ARDS, the long case); and the patient where the non-invasive cuff is unreliable (the morbidly obese, the arrhythmia).[3]

A meta-analysis found that the non-invasive continuous BP devices, though improving, remain less accurate than the arterial line in the rapidly changing haemodynamics — the arterial line remains the gold standard for the high-acuity patient.[2]

The arterial waveform: the physiology

The arterial waveform has a characteristic shape that conveys information beyond the single peak pressure value. The systolic upstroke (the anacrotic limb) reflects the left ventricular ejection; the dicrotic notch marks the aortic valve closure; the diastolic decline reflects the peripheral runoff. The area under the waveform (the pulse pressure) is proportional to the stroke volume — the basis of the pulse-contour cardiac output analysis.[3]

The waveform morphology can reveal: the pulsus paradoxus (the exaggerated fall in the systolic with inspiration, in the tamponade or the severe asthma); the pulsus alternans (the alternating large and small amplitudes, in the severe LV failure); the slow rise (in the aortic stenosis); the bounding pulse (in the aortic regurgitation, the PDA). The respiratory swing on the arterial trace is a sign of the hypovolaemia (the exaggeration of the normal minor respiratory variation).[3]

The central venous pressure: technique and interpretation

The central venous catheter is placed in a large central vein (the internal jugular, the subclavian, or the femoral), with its tip in the superior vena cava. It provides the central venous pressure (CVP) — an estimate of the right atrial pressure and, by extension, the right ventricular preload.[1]

The CVP has been the subject of much debate as a volume-responsiveness marker. The review by Lloyd-Donald concludes that a single CVP value is a poor predictor of the fluid responsiveness, but the trend (the rise in response to a fluid bolus or the passive leg raise) is more informative. The CVP is better used as an indicator of the right-heart function (the high CVP in the heart failure, the pulmonary hypertension, the tricuspid regurgitation) and as the access for the vasoactive drugs, the pacing and the post-resuscitation blood sampling.[1]

The CVP waveform: the components

The CVP trace has three positive waves (a, c, v) and two negative descents (x, y):

• The a wave — the right atrial contraction (absent in the atrial fibrillation; the 'cannon' a wave in the complete heart block).
• The c wave — the tricuspid valve bulging back during the ventricular contraction.
• The x descent — the atrial relaxation.
• The v wave — the atrial filling against the closed tricuspid valve (the large v wave in the tricuspid regurgitation).
• The y descent — the ventricular filling after the tricuspid opens. [1]

The waveform analysis can identify the tricuspid regurgitation (the large v wave), the tamponade (the blunted y descent), and the constrictive pericarditis (the rapid x and y descents, the 'M' or 'W' shape).[1]

The cardiac output monitoring: the methods

The cardiac output monitoring provides the flow (the cardiac output, the stroke volume) and the derived variables (the systemic vascular resistance, the oxygen delivery). The methods include: [1]

• The thermodilution (the pulmonary artery catheter) — the gold standard; a bolus of cold saline is injected and the temperature change downstream is used to compute the cardiac output. Invasive and reserved for the most complex cases.
• The pulse contour analysis (the LiDCO, the PiCCO, the FloTrac/Vigileo) — the cardiac output is derived from the arterial waveform by a mathematical analysis of the pulse contour, calibrated by a lithium dilution (LiDCO), a thermodilution (PiCCO), or a demographic autocalibration (FloTrac). These are the modern standard for the perioperative goal-directed therapy.[4]
• The oesophageal Doppler — a probe in the oesophagus measures the blood flow velocity in the descending aorta, from which the cardiac output is derived. Minimally invasive and widely used.
• The transoesophageal echocardiography (TOE) — the direct visualisation of the cardiac chambers, the valves and the contractility. The gold standard for the structure and the function; used in the cardiac and the major-vascular surgery.[4]

Goal-directed therapy: the principles

The goal-directed therapy (GDT) is the perioperative optimisation of the cardiac output, the oxygen delivery and the fluid balance using the cardiac output monitoring to guide the fluid, the vasopressor and the inotrope administration. The principle is that the patient with a low oxygen delivery (a low cardiac output or a low haemoglobin) is at a higher risk of the perioperative morbidity (the wound infection, the organ failure), and that the targeted optimisation (the fluid challenge to achieve the maximum stroke volume, the transfusion to maintain the oxygen carriage, the inotrope for the low-output state) reduces the complications.[4]

The modern GDT, as the review of Wichmann describes, has evolved from the fixed algorithm (the oxygen delivery target of 600 mL/min/m2) to the personalised haemodynamic management — the targeting of the patient's own baseline cardiac index and the individualised fluid and drug response, guided by the dynamic markers of the fluid responsiveness (the stroke volume variation, the pulse pressure variation, the passive leg raise).[4]

The pulse contour analysis: LiDCO, PiCCO, FloTrac

The pulse contour devices derive the cardiac output from the arterial waveform:

• LiDCO — calibrated by a lithium dilution; the lithium is injected intravenously and a sensor on the arterial line measures the concentration-time curve.
• PiCCO — calibrated by a transpulmonary thermodilution (a cold saline bolus through a central line, measured by a thermistor on a femoral arterial line); also provides the extravascular lung water and the global end-diastolic volume.
• FloTrac/Vigileo — autocalibrated (no external calibration needed); the cardiac output is derived from the arterial waveform using a demographic-based algorithm. Less accurate in the extreme haemodynamics but the simplest to set up.[4]

The transoesophageal echocardiography (TOE)

The TOE is the most structurally detailed cardiac monitor available to the anaesthetist. A probe in the oesophagus provides the high-resolution images of the cardiac chambers, the valves and the great vessels. In the cardiac and the major-vascular surgery, the TOE is used to assess the LV and the RV function, the valve competence (after the repair or the replacement), the preload, the contractility and the detection of the intracardiac air, the thrombus or the embolism. It is also used in the non-cardiac surgery for the unstable patient (the cause of the hypotension — the LV failure, the hypovolaemia, the tamponade, the pulmonary embolism). [1]

The complications of the invasive monitoring

• The arterial line: the haematoma, the thrombosis, the infection, the distal ischaemia (rare with the radial; more with the femoral), the nerve injury (the median nerve), the disconnection haemorrhage (the arterial line can exsanguinate rapidly if disconnected — the alarms and the tight connections are essential).
• The central venous catheter: the pneumothorax (the subclavian and the internal jugular), the arterial puncture (the carotid artery for the IJ), the haematoma, the infection (the line-related bloodstream infection — the major source of the nosocomial sepsis), the arrhythmia (the guidewire irritating the RV), the air embolism.
• The pulmonary artery catheter: the pulmonary artery rupture (the most feared, usually fatal), the arrhythmia, the knotting, the valvular damage.
• The TOE: the oesophageal injury (the rare but serious — the perforation, the tearing), the dental trauma, the gagging. [1]

When to use the invasive monitoring (indications)

The principle is proportionate monitoring — the monitoring matched to the risk. The standard non-invasive monitoring (the ECG, the pulse oximeter, the NIBP, the capnograph) is sufficient for the low-risk patient having minor surgery. The invasive monitoring is added for:[3][4]

• The arterial line — the major surgery, the vasoactive infusion, the controlled hypotension, the instability, the frequent blood gases.
• The central venous line — the major surgery (the drug access, the CVP), the poor venous access, the pacing, the post-resuscitation.
• The cardiac output monitor — the high-risk surgical patient (the goal-directed therapy), the cardiac and the vascular surgery, the ICU patient with the shock.
• The TOE — the cardiac and the vascular surgery, the unstable patient with the unknown cause of the hypotension. [1]

The depth of anaesthesia monitoring (BIS)

The bispectral index (BIS) is a dimensionless processed-EEG value from 0 to 100 that summarises the hypnotic depth: 100 is awake, 40 to 60 is the surgical range, below 40 is over-sedated. It is used to titrate the volatile or the TIVA depth (see the IV induction agents topic for the full discussion), and to reduce the risk of the awareness under general anaesthesia. [1]

The processed EEG (entropy, Narcotrend)

The entropy (the state and the response entropy) and the Narcotrend are alternative processed-EEG monitors that, like the BIS, summarise the depth from the frontal EEG. They are interchangeable in principle; the choice is one of the local availability and the familiarity. [1]

[1] [1]

Red flags

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References

1. [1]Lloyd-Donald P, Fujino M, Waldman B, et al. Measurement and interpretation of central venous pressure: a narrative review Anaesthesia, 2025.PMID 40457939
2. [2]Lischker S, Pestel GJ, Schmidtmann I. Accuracy and precision of continuous, non-invasive blood pressure devices: a systematic review and meta-analyses Anaesthesia, 2026.PMID 42290528
3. [3]Kane AD, et al. Continuous ARterial monitoring in Elderly and Frail patients for hip fractUre surgery to prevent Low blood pressure - the CAREFUL Study Protocol Anaesth Rep, 2026.PMID 41971679
4. [4]Wichmann P, et al. [From Goal-Directed Therapy to Personalized Hemodynamic Management] Anasthesiol Intensivmed Notfallmed Schmerzther, 2026.PMID 42009030