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CVP monitoring for MD anaesthesia exam answer

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Central Venous Pressure (CVP) Monitoring - MD Anaesthesia Exam Answer

Definition

CVP is the pressure exerted by blood against the walls of the intrathoracic venae cavae. Because pressure in the venae cavae is generally within 1 mmHg of right atrial pressure, CVP reflects the pressure under which blood is returning to the heart. It is measured by placing a catheter tip in any of the great systemic veins of the thorax or the right atrium.
  • Normal CVP: 2-8 mmHg (approximately 3-10 cmH2O)
  • Conversion: 1 mmHg = 1.36 cmH2O

Physiology

CVP has two important hemodynamic effects:
  1. Preload determinant - promotes filling of the heart during diastole (reflects RV end-diastolic volume)
  2. Venous return back-pressure - opposes return of blood from the peripheral venous circulation to the heart
CVP values are determined by the interaction of:
  • Intravascular volume
  • Right atrial and ventricular function
  • Venomotor tone
  • Intrathoracic pressures
CVP reflects right heart filling pressure only. It does not reliably reflect left heart pressures unless both ventricles function normally.
  • Miller's Anesthesia, 10e, p. 4806

The CVP Waveform

The normal CVP waveform has three positive deflections (waves) and two negative deflections (descents):
Normal CVP waveform showing a, c, v waves and x, y descents alongside ECG and arterial trace

Waveform Components (Table)

ComponentPhase of Cardiac CycleMechanical Event
a waveEnd-diastoleAtrial contraction ("a = atrial kick")
c waveEarly systoleIsovolumic ventricular contraction; tricuspid valve closure moves toward RA
x descentMid-systoleAtrial relaxation + descent of tricuspid annulus toward apex (systolic collapse)
v waveLate systoleVenous filling of atrium while tricuspid valve is closed
y descentEarly diastoleTricuspid valve opens; blood flows from atrium to ventricle (diastolic collapse)
h waveMid- to late diastoleDiastolic plateau (only seen with slow heart rate or elevated CVP)
Timing relative to ECG:
  • a wave follows the P wave
  • c wave follows the QRS (R wave)
  • v wave peaks just after the T wave
  • Miller's Anesthesia, 10e, p. 4806-4808

Abnormal CVP Waveforms

ConditionWaveform ChangeMechanism
Atrial fibrillationLoss of a wave; prominent c waveNo organised atrial contraction; greater atrial volume at onset of systole
AV dissociation / junctional rhythm / ventricular pacingCannon a wavesAtrium contracts against closed tricuspid valve
Tricuspid regurgitationTall systolic c-v wave; loss of x descentRetrograde systolic filling of RA; "ventricularised" trace
Tricuspid stenosisTall a wave; attenuated y descentIncreased atrial work; slow diastolic emptying
RV ischaemia / pericardial constrictionTall a and v waves; steep x and y descents; M or W configurationImpaired RV compliance
Cardiac tamponadeDominant x descent; attenuated y descentPericardial fluid limits diastolic filling
Key exam pearl: In tricuspid regurgitation, the monitor displays an overestimated mean CVP. True RVEDP is best measured at the ECG R wave, before the regurgitant wave.
  • Miller's Anesthesia, 10e, p. 4811-4813

Indications for CVP Monitoring

  1. Acute circulatory failure / haemodynamic instability
  2. Major surgery with anticipated large fluid shifts (esophagectomy, pneumonectomy, hepatic resection, cardiac surgery)
  3. Anticipated massive blood transfusion
  4. Cautious fluid replacement in patients with compromised cardiovascular status
  5. Suspected cardiac tamponade
  6. Fluid resuscitation in severe sepsis (goal-directed therapy)
  7. Additional uses of the central venous catheter (not purely for pressure monitoring):
    • Administration of vasoactive drugs, total parenteral nutrition, chemotherapy, concentrated electrolytes
    • Transvenous cardiac pacing
    • Temporary haemodialysis
    • Pulmonary artery catheter introduction
    • Aspiration of venous air embolism
    • Inadequate peripheral IV access
  • Roberts & Hedges' Clinical Procedures in Emergency Medicine, p. 518; Miller's Anesthesia, 10e, p. 4792-4793

Contraindications

AbsoluteRelative
Other resuscitative interventions that take priorityCoagulopathy (prefer site compressible with direct pressure, e.g., internal jugular over subclavian)
SVC syndromeSevere emphysema (subclavian approach risky)
Large vegetations on tricuspid valveInfection at proposed insertion site
Right atrial tumour or thrombusThrombocytopenia

Sites of Insertion

SiteAdvantagesDisadvantages
Right internal jugularBest route to right heart; highest success rate; safe in coagulopathy (compressible); preferred for PA catheter and pacingRisk of carotid puncture; patient discomfort; difficulty in obese/short necks
SubclavianComfortable for patient; lower infection riskHigher pneumothorax risk (~2%); non-compressible if arterial puncture; avoid in emphysema/bilateral disease
FemoralEasiest in emergency/resuscitation; no pneumothorax riskHigher infection rate; DVT risk; less reliable CVP if intra-abdominal pressure elevated; slower drug delivery to central circulation
External jugularVisible, safeMay kink; tortuous angle; less reliable positioning
Catheter tip position: Superior vena cava, at the cavo-atrial junction. Confirmed by CXR. Tip within 2 cm of the cardiac silhouette on AP film.
Ultrasound guidance is strongly recommended (mandated by most guidelines) for internal jugular cannulation and should be considered for subclavian or femoral access.
  • Miller's Anesthesia, 10e, p. 4792-4795

Measurement Technique

Manometric method (water column):
  • Zero the manometer at the level of the right atrium (mid-axillary line, 4th intercostal space, with patient supine)
  • Fill the column with IV fluid; open to the patient
  • Water level falls and oscillates with respiration
  • Read at end-expiration
Electronic transducer method:
  • Transducer zeroed at the phlebostatic axis (same reference point as above)
  • Provides continuous waveform display and digital mean value
  • Read at end-expiration in spontaneously breathing patients; read at end-expiration (before ventilator breath) in mechanically ventilated patients
Sources of error in CVP measurement:
  • Failure to zero or calibrate the transducer
  • Incorrect reference point (transducer not at right atrial level)
  • Catheter tip malposition
  • Air bubbles in the circuit
  • Catheter obstruction
  • Reading during the wrong phase of ventilation
  • Increased intrathoracic pressure (IPPV, PEEP, coughing, straining, pneumothorax)
  • Vasopressors (may falsely elevate readings)
  • Roberts & Hedges' Clinical Procedures in Emergency Medicine, p. 518-519

Normal Values and the Fluid Challenge

Static CVP value alone is a poor predictor of volume status or fluid responsiveness - many studies have confirmed this. In shocked patients, there is no single "normal" CVP target; some patients need a CVP of 5 cmH2O while others may require 15 cmH2O or more. Ventricular compliance changes rapidly in shock, making CVP a poor reflection of RVEDP.
Dynamic assessment - the fluid challenge:
  • Infuse 250-500 mL rapidly over 5-10 minutes
  • Assess CVP response:
ResponseInterpretationAction
No change in CVPVolume depleted - empty ventricle, steep portion of Starling curveFurther fluid resuscitation
Rise of 2-5 cmH2O that drifts back to baseline over 10-20 minutesNormal/adequate responseContinue monitoring
Large, sustained rise in CVPHigh preload - cardiac insufficiency or volume overloadStop fluids; consider inotropes
  • Bailey & Love's Short Practice of Surgery, 28e, p. 40

Limitations of CVP

This is a high-yield exam area. The main limitations are:
  1. Does not reflect left heart filling - PCWP (pulmonary artery occlusion pressure) is needed to estimate left atrial pressure
  2. Poor predictor of fluid responsiveness - static CVP cannot reliably predict whether a patient will respond to a fluid challenge (landmark meta-analyses have confirmed this)
  3. Affected by ventricular compliance - conditions like RV ischaemia, tamponade, pericardial constriction, and PEEP alter the CVP-volume relationship
  4. Intrathoracic pressure effects - IPPV, PEEP, and pneumothorax elevate CVP without a corresponding increase in preload
  5. Venomotor tone changes - vasoconstriction may maintain a "normal" CVP despite hypovolaemia
  6. Tricuspid valve disease - distorts waveform and numeric value
  7. Right vs. left heart discordance - in conditions with primary left heart failure or pulmonary hypertension, CVP may not reflect left-sided haemodynamics at all
  8. Practical sources of error (zeroing, tip position, ventilation phase - as above)
Current status: CVP has largely been supplanted in the ICU by dynamic preload indicators (pulse pressure variation [PPV], stroke volume variation [SVV]), point-of-care echocardiography, and functional haemodynamic tests. However, the central venous catheter remains indispensable for drug delivery, pacing, and haemodialysis.
  • Barash's Clinical Anesthesia, 9e, p. 3161; Miller's Anesthesia, 10e, p. 4789-4790

Complications of CVP Catheterisation

Mechanical (immediate)

  • Arterial puncture (most common acute complication; 1.9-15%)
  • Pneumothorax (~2% with subclavian; less with internal jugular)
  • Haemothorax, haemomediastinum, hydrothorax, chylothorax
  • Cardiac tamponade - most life-threatening; results from tip perforation of intrapericardial SVC, RA, or RV (especially with malpositioned tip abutting at steep angle)
  • Air embolism
  • Catheter malposition (coiling, migration into wrong vessel)
  • Nerve injury (brachial plexus, phrenic nerve)

Thromboembolic

  • Deep vein thrombosis (especially femoral)
  • Catheter-related thrombosis; pulmonary embolism

Infectious

  • Catheter-related bloodstream infection (CRBSI / CLABSI)
  • Up to 15% of patients experience some adverse event related to central venous catheterisation

Prevention

  • Ultrasound guidance
  • Maximal sterile barrier precautions (cap, mask, sterile gown and gloves, large sterile drape)
  • Checklist/protocol use
  • Tip confirmation by CXR before use
  • Waveform manometry or pressure measurement to confirm venous placement before securing
  • Miller's Anesthesia, 10e, p. 4795-4796

CVP vs. Other Haemodynamic Monitors

MonitorMeasuresAdvantage over CVP
PCWP (PA catheter)Left atrial pressure surrogate; CO by thermodilutionLeft heart filling; cardiac output; SVR
PPV/SVVDynamic preload responsivenessSuperior predictor of fluid response (in mechanically ventilated, sinus rhythm patients)
Echocardiography (TTE/TOE)Direct ventricular volume; wall motion; valve functionReal-time, visual, no assumptions
Passive Leg Raise (PLR)Functional fluid challengeReversible; valid even in arrhythmia, spontaneous breathing

Key Exam Points to Remember

  1. CVP = RAP = RVEDP (only when tricuspid valve is normal and no pericardial constraint)
  2. The a wave is absent in atrial fibrillation
  3. Cannon a waves = AV dissociation (atrium contracts against closed tricuspid)
  4. Tricuspid regurgitation = "ventricularised" trace (c-v wave, loss of x descent)
  5. Tamponade = prominent x, attenuated y descent
  6. Constrictive pericarditis = deep x AND deep y descent (M/W pattern)
  7. Always read CVP at end-expiration
  8. Fluid challenge (250-500 mL) is more informative than a static CVP reading alone
  9. CVP is NOT a reliable single-time-point predictor of volume status or fluid responsiveness
  10. Ultrasound guidance is the current standard of care for central venous cannulation
Sources: Miller's Anesthesia, 10e (Elsevier); Barash's Clinical Anesthesia, 9e; Roberts & Hedges' Clinical Procedures in Emergency Medicine; Bailey & Love's Surgery, 28e
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