Central Venous Pressure

What It Is and What It Reflects

Central venous pressure (CVP) is the pressure in the great veins/right atrium, measured through a central line with its tip at the SVC–right-atrium junction and zeroed at the phlebostatic axis. It represents the right atrial pressure and therefore the preload — the volume returning to the right heart — which is why it is used as a guide to volume status. Normal is about 3–8 mmHg (≈ 5–12 cmH₂O).

Interpretation

• Low CVP → hypovolaemia (the tank is under-filled).
• High CVP → fluid overload, right heart failure, or an obstructive cause (cardiac tamponade, tension pneumothorax); also raised by mechanical ventilation with high PEEP.

It is most useful dynamically: a fluid challenge (a small bolus) applies the Frank-Starling principle — an under-filled heart takes the extra preload with little rise in CVP, whereas a full or failing heart shows a sustained rise, signalling that more fluid will only congest.

The CVP Waveform

The trace follows the cardiac cycle — the a wave (atrial contraction), c wave (tricuspid bulge during ventricular contraction) and v wave (atrial filling). Abnormalities are diagnostic: giant a waves (tricuspid stenosis/pulmonary hypertension), cannon a waves (complete heart block), absent a waves (atrial fibrillation), and giant v waves (tricuspid regurgitation).

Uses & Risks of the Line

A central line is placed by the Seldinger technique under ultrasound (internal jugular, subclavian or femoral vein) not only to measure CVP but to give vasopressors, TPN and rapid fluids and for dialysis access. Insertion carries real risks — pneumothorax, arterial puncture, air embolism, arrhythmias, and later catheter-related bloodstream infection — so it is done aseptically and removed early.

The Fluid Challenge & Frank-starling

CVP is measured with a transducer zeroed at the phlebostatic axis, but its real clinical use is the fluid challenge: a small bolus (e.g. 250 mL) is given and the CVP watched. This applies the Frank-Starling principle — on the steep part of the curve (an under-filled heart) the extra preload raises stroke volume and the CVP barely moves; on the flat part (a full or failing heart) the same bolus gives little extra output but a sustained rise in CVP, warning that more fluid will only congest.

Alternatives & Adjuncts

Because CVP is imperfect, it is increasingly supplemented by better tools: an arterial line gives beat-to-beat pressure and pulse-pressure variation; bedside echocardiography shows cardiac filling and function directly (and IVC collapsibility); and dedicated monitors (PiCCO, oesophageal Doppler, or a pulmonary-artery (Swan-Ganz) catheter) measure cardiac output. These have largely displaced reliance on CVP alone in critical care.

Indications for Central Access & Sources of Error

A central line is placed for monitoring (CVP and central venous oxygen saturation) and for therapy — giving drugs that damage peripheral veins (vasopressors, concentrated potassium, chemotherapy, TPN), rapid large-volume infusion, temporary dialysis or a pacing wire, and reliable access when peripheral veins fail. A CVP reading is only as good as its technique: common errors include an incorrectly set zero reference (it must sit at the phlebostatic axis), measuring during coughing or straining, a blocked or malpositioned catheter, and high intrathoracic pressure in a ventilated patient — another reason the trend is trusted over any single value.

The trend in CVP is far more useful than any single reading.

• CVP = right-atrial/great-vein pressure via a central line; reflects preload. Normal ≈ 3–8 mmHg (zeroed at phlebostatic axis).
• Low → hypovolaemia; high → overload, right heart failure, obstruction (tamponade/tension pneumothorax); PEEP raises it.
• Waveform: a (atrial contraction), c (tricuspid bulge), v (atrial filling); use the fluid challenge (Frank-Starling).
• Line also used for vasopressors/TPN/dialysis; risks — pneumothorax, arterial puncture, air embolism, line sepsis.
• A single CVP poorly predicts fluid responsiveness — prefer dynamic measures + overall perfusion.