Cardiac output monitors: lidco, picco, oesophageal doppler

How to talk through a CO monitor in a viva (structure)

Start with: what it measures, how it measures it, what you need to set it up, and what outputs you get
Then: calibration/validation, assumptions, artefacts, contraindications, and how it changes management (fluids/vasoactives/inotropes)
Finish with: safety, complications, and when to choose an alternative monitor

Practical clinical use (generic algorithm)

Use dynamic indices and/or SV response to a test (fluid challenge or passive leg raise) rather than chasing a single CO number
- Define a fluid challenge (e.g. 250 mL crystalloid over 5–10 min) and a responder threshold (commonly ≥10–15% rise in SV/CO)
- If not fluid responsive: consider vasopressor for low SVR or inotrope for low contractility, guided by MAP, SVR estimate, echo, lactate, urine output
Interpret in context: rhythm, ventilation mode, tidal volume, PEEP, vascular tone, temperature, and measurement site quality

LiDCO (LiDCOplus / LiDCOrapid): principles and outputs

Type: minimally invasive CO monitoring using arterial waveform analysis; may be calibrated (LiDCOplus) or uncalibrated/trending-focused (LiDCOrapid depending on configuration)
Core method: pulse power analysis — derives stroke volume from the arterial pressure waveform power after accounting for arterial compliance; CO = SV × HR
Calibration (LiDCOplus): lithium indicator dilution via peripheral/central venous injection and arterial detection using a lithium-sensitive electrode on the arterial line
- Inject small bolus lithium chloride; detect concentration-time curve on arterial line; compute CO by indicator dilution (Stewart–Hamilton principle)
- Calibration improves absolute accuracy; waveform analysis then provides beat-to-beat trending
Typical outputs: CO/CI, SV/SVI, SVV/PPV (if conditions met), trends, and derived indices (e.g. SVR estimate if CVP entered)

LiDCO: set-up, calibration, and interpretation

Set-up: reliable arterial line waveform (damping checked), venous access for lithium injection (if calibrated), correct patient demographics entered
- Square-wave test: underdamped waveform exaggerates systolic/PP and can distort SVV/PPV and SV estimates; overdamped underestimates PP
When to (re)calibrate: after major haemodynamic change (vasopressor/inotrope changes, major bleeding, significant temperature/vascular tone shifts), or if waveform quality changes
Dynamic indices validity: SVV/PPV require controlled mechanical ventilation, regular rhythm, adequate tidal volume (often ≥7–8 mL/kg ideal body weight), and no significant spontaneous breathing

LiDCO: limitations, contraindications, and complications

Waveform analysis limitations: reduced accuracy with marked changes in arterial compliance/vascular tone (sepsis, high-dose vasopressors), severe peripheral vasoconstriction, IABP, significant aortic regurgitation, and poor waveform quality
Arrhythmias: irregular rhythm reduces reliability of beat-to-beat SV and dynamic indices
Lithium dilution caveats: avoid/interpret cautiously with lithium therapy; potential interference with some neuromuscular blockers (historically noted with quaternary ammonium compounds) and with certain blood sampling/flush contamination
- Practical exam point: ensure no lithium-containing flush; inject via dedicated line; avoid sampling during indicator curve
Complications: those of arterial and venous cannulation; lithium bolus is very small dose but treat as drug administration (allergy rare, dosing errors possible)

PiCCO: principles and outputs

Type: minimally invasive transpulmonary thermodilution (TPTD) calibration + pulse contour analysis for continuous CO
Hardware: central venous catheter for cold bolus injection + specialised thermistor-tipped arterial catheter (commonly femoral; axillary/brachial options) for detection
TPTD provides: intermittent CO and volumetric variables; pulse contour provides continuous beat-to-beat CO trending between calibrations
Key derived variables (exam-relevant):
- GEDV/GEDVI (global end-diastolic volume): volumetric preload surrogate (4-chamber volume) — often more informative than CVP
- EVLW/EVLWI (extravascular lung water): estimate of pulmonary oedema burden
- PVPI (pulmonary vascular permeability index): helps differentiate hydrostatic vs permeability oedema (interpret in context)
- SVV/PPV (dynamic indices) and SVR estimate (if CVP/MAP available)

PiCCO: thermodilution concepts you may be asked to explain

Principle: inject known thermal indicator (cold saline) into central vein; measure downstream temperature-time curve in arterial system; apply Stewart–Hamilton to compute CO
Mean transit time (MTt) and downslope time (DSt) are used to derive volumes:
- Intrathoracic thermal volume (ITTV) ≈ CO × MTt
- Pulmonary thermal volume (PTV) ≈ CO × DSt
- GEDV ≈ ITTV − PTV (conceptual relationship used in device algorithms)
- EVLW derived from intrathoracic volumes (device-specific algorithm); interpret trends rather than isolated values
Calibration: typically 3 cold boluses (e.g. 15 mL) at end-expiration; average results; repeat after major haemodynamic change

PiCCO: limitations, contraindications, and complications

Requires arterial thermistor catheter and CVC: more invasive than LiDCO/oesophageal Doppler; complications include arterial thrombosis/ischaemia, bleeding, infection
Accuracy affected by: severe tricuspid regurgitation, intracardiac shunts, rapid temperature changes, ECMO/cardiopulmonary bypass, and poor injectate technique
Pulse contour limitations: reduced reliability with major changes in vascular tone/compliance, arrhythmias, IABP, and damping/line issues
EVLW/PVPI: interpret cautiously in focal lung disease, post-lung resection, large pleural effusions, and when lung perfusion distribution is abnormal

Oesophageal Doppler: principles and outputs

Type: minimally invasive Doppler ultrasound probe in oesophagus measuring blood flow velocity in descending thoracic aorta; estimates SV/CO
Method: SV ≈ (velocity–time integral) × (aortic cross-sectional area) × (correction factor for proportion of CO in descending aorta)
Outputs: SV/CO (estimated), corrected flow time (FTc), peak velocity (PV), and response to fluid challenges
- FTc: systolic flow time corrected for HR; low FTc suggests low preload/vasoconstriction; high FTc may suggest vasodilation (interpret with PV and clinical context)
- Peak velocity: surrogate for contractility (also affected by afterload and preload)

Oesophageal Doppler: set-up, interpretation, and limitations

Set-up: probe insertion similar to OGT; position to obtain stable aortic waveform (characteristic shape) and maximal signal; secure and re-check after movement
Strength: excellent for guiding intraoperative fluid optimisation (goal-directed therapy) with immediate SV response to small boluses
Limitations: assumes aortic diameter (or estimates it) and fixed proportion of CO to descending aorta; errors with changes in aortic size, distribution of flow, and aortic pathology
- Less reliable with significant aortic regurgitation, aortic aneurysm/dissection, coarctation, and major changes in splanchnic/upper body flow distribution
Signal issues: probe malposition, oesophageal spasm, surgical retraction, diathermy interference, and patient movement
Contraindications/risks: oesophageal pathology (varices, strictures, tumour, recent surgery), bleeding risk, perforation (rare), dental trauma

Comparing LiDCO vs PiCCO vs Oesophageal Doppler (exam table in words)

Invasiveness: Oesophageal Doppler (probe) < LiDCO (A-line ± venous injection) < PiCCO (CVC + thermistor arterial catheter)
Calibration: Doppler requires positioning/assumptions; LiDCOplus uses lithium dilution; PiCCO uses transpulmonary thermodilution
Best for: Doppler—perioperative fluid optimisation; LiDCO—less invasive continuous trending; PiCCO—ICU/sepsis/ARDS where EVLW and volumetric preload are useful
Dynamic indices: LiDCO/PiCCO provide SVV/PPV from arterial waveform (only valid under strict conditions); Doppler uses SV response and FTc/PV
Common failure modes: waveform quality/vascular tone (LiDCO/PiCCO), injectate technique (PiCCO), probe position/assumptions (Doppler)

Explain the principles of arterial waveform analysis for cardiac output monitoring.

A structured answer should cover what is measured, how SV is derived, and key assumptions/limitations.

Measures arterial pressure waveform; derives SV from waveform characteristics (area/power) and an estimate of arterial compliance; then CO = SV × HR
Assumes a relationship between pressure and flow that is stable; accuracy worsens when vascular tone/compliance changes rapidly (e.g. sepsis, vasopressors)
Requires high-quality, correctly damped arterial trace; artefacts (flushes, resonance, damping) distort SV and dynamic indices

Describe how LiDCOplus is calibrated and what could make the calibration inaccurate.

This is a frequent FRCA-style equipment question: method + practical pitfalls.

Calibration uses lithium indicator dilution: inject small lithium bolus via venous line; detect arterial concentration-time curve with lithium electrode; compute CO using indicator dilution
Inaccuracy from: incorrect injectate dose/technique, injection via line with residual drugs/flush contamination, sampling during curve, poor arterial waveform, extreme haemodynamic instability
Caution/avoid: patient on lithium therapy (baseline lithium), and consider potential interference with some drugs; follow local device guidance

What conditions must be met for SVV/PPV to predict fluid responsiveness reliably?

Often asked as a list; examiners want the limitations clearly stated.

Controlled mechanical ventilation with no spontaneous breaths
Regular rhythm (no AF/frequent ectopics)
Adequate tidal volume (classically ≥7–8 mL/kg IBW) and relatively stable chest wall/lung compliance; interpret cautiously with high PEEP/low VT
No major right heart failure/pulmonary hypertension and no significant valvular lesions that distort stroke volume variation

Describe PiCCO monitoring: what lines are required and what extra variables it provides beyond CO.

A classic 'describe the monitor' question.

Requires CVC for cold saline bolus injection and a thermistor-tipped arterial catheter (often femoral) for temperature detection
Provides intermittent transpulmonary thermodilution CO for calibration plus continuous pulse contour CO trending
Additional variables: GEDVI (volumetric preload), EVLWI (lung water), PVPI (permeability index), SVV/PPV (if conditions met)

Explain transpulmonary thermodilution and how GEDV and EVLW are derived conceptually.

You are not expected to reproduce proprietary equations, but should explain the concept and key time constants.

Inject cold indicator into central vein; detect arterial temperature-time curve; apply Stewart–Hamilton to calculate CO
Use curve-derived mean transit time and downslope time to estimate intrathoracic thermal volume and pulmonary thermal volume; their relationship is used to estimate GEDV
EVLW is derived from intrathoracic volumes (thermal distribution) to estimate water outside pulmonary vasculature; best used for trends and in context (ARDS, fluid balance)

List clinical situations where PiCCO values may be misleading or difficult to interpret.

Significant tricuspid regurgitation or intracardiac shunts (indicator dilution assumptions violated)
Rapid temperature changes, extracorporeal circuits (ECMO), cardiopulmonary bypass
Severe vasoplegia/high-dose vasopressors affecting pulse contour accuracy between calibrations
Focal lung pathology/altered perfusion distribution affecting EVLW interpretation (e.g. lobectomy, large pleural effusion)

Describe the principle of the oesophageal Doppler and how it estimates stroke volume.

Probe in oesophagus measures Doppler velocity of blood in descending thoracic aorta; integrates velocity over systole to obtain VTI
SV estimated from VTI × estimated aortic cross-sectional area × correction factor for proportion of total CO in descending aorta
Best for detecting changes (fluid responsiveness) rather than absolute CO in all patients

What are FTc and peak velocity on oesophageal Doppler, and how would you use them clinically?

FTc (corrected flow time): systolic flow time corrected for HR; used as a surrogate of preload/afterload balance; low FTc may suggest hypovolaemia or vasoconstriction
Peak velocity: surrogate for contractility (also influenced by loading conditions); low PV with adequate preload may suggest reduced contractility
Use alongside SV response to a fluid bolus: if SV rises ≥10–15% and then plateaus, you are near the flat part of the Frank–Starling curve

List contraindications and complications of oesophageal Doppler probe insertion.

Contraindications: oesophageal varices, strictures, tumours, recent oesophageal/gastric surgery, significant upper GI bleeding risk
Complications: mucosal trauma/bleeding, perforation (rare), aspiration risk if not managed appropriately, dental injury

A patient in septic shock has a LiDCO/PiCCO pulse contour CO that falls after starting noradrenaline, but lactate improves and capillary refill improves. How do you interpret this?

This tests understanding of vascular tone effects on waveform-derived CO and the need for calibration/context.

Noradrenaline increases vascular tone/compliance characteristics and may alter pulse contour-derived SV/CO; apparent CO change may be partly measurement artefact
Reassess waveform quality and consider recalibration (PiCCO TPTD or LiDCO lithium dilution if using calibrated system)
Prioritise clinical endpoints and perfusion markers (MAP, urine output, lactate trend, skin perfusion) and consider echo to assess LV/RV function

Compare LiDCO and PiCCO: give two advantages and two disadvantages of each.

LiDCO advantages: less invasive (standard arterial line ± venous injection), provides continuous trending and dynamic indices; quicker set-up than PiCCO
LiDCO disadvantages: waveform accuracy affected by vascular tone/compliance and arrhythmias; lithium calibration limitations (drug/technique considerations)
PiCCO advantages: provides volumetric preload (GEDVI) and lung water (EVLWI/PVPI) in addition to CO; robust intermittent calibration via TPTD
PiCCO disadvantages: more invasive (thermistor arterial catheter + CVC), risks of vascular complications; interpretation issues with shunts/TR and EVLW confounders

How would you troubleshoot an unexpectedly low CO reading on any of these monitors intraoperatively?

Examiners want a systematic approach: patient first, then equipment.

Patient: check pulse, rhythm, BP, ETCO2, SpO2, temperature, bleeding, anaesthetic depth; consider anaphylaxis, tamponade, pneumothorax, PE, myocardial ischaemia
Waveform/line: check arterial trace damping, transducer level/zero, flush system, kinks, air bubbles, stopcocks; confirm with manual BP and palpation
Monitor-specific: re-site/adjust Doppler probe; repeat thermodilution calibration (PiCCO) with correct bolus technique; repeat lithium calibration if indicated (LiDCOplus)
Confirm with an independent modality if uncertainty persists (TTE/TOE, lactate trend, urine output, ABG, clinical exam)