What does Central Venous Pressure (CVP) measure in a patient?

What Central Venous Pressure Measures

Central venous pressure (CVP) is an estimate of right atrial pressure that reflects the interaction between cardiac function and venous return, providing information about right-sided heart preload, volume status, and right ventricular function. 1, 2

Primary Physiologic Parameters

CVP directly measures:

• Right atrial pressure - CVP serves as a direct approximation of right atrial pressure when measured through a catheter positioned in the superior vena cava or right atrium 1, 3
• Right ventricular preload - The pressure measurement reflects the filling pressure of the right ventricle, though this relationship is complex and influenced by ventricular compliance 1, 2
• Venous return dynamics - CVP is determined by the interaction between cardiac function (how well the heart pumps blood forward) and venous return (how blood flows back to the heart) 2

Normal Values and Reference Standards

• Normal CVP ranges from 8-12 mmHg when measured relative to a reference point 5 cm below the sternal angle in non-critically ill patients 1
• In non-mechanically ventilated patients, normal CVP is 3-8 cm H₂O (approximately 2-6 mmHg), with values <3 cm H₂O indicating hypovolemia and >10 cm H₂O suggesting fluid overload or cardiac dysfunction 4
• In mechanically ventilated patients, the normal range shifts higher to 8-12 mmHg due to increased intrathoracic pressure 4

Clinical Information Provided by CVP

Volume Status Assessment

• Low CVP (<3 cm H₂O or <8 mmHg) often indicates hypovolemia and may warrant fluid resuscitation, though CVP alone predicts fluid responsiveness with only 50% positive predictive value 4, 5
• Elevated CVP (>10-12 mmHg) may indicate right ventricular failure, volume overload, or significant tricuspid regurgitation 1

Right Heart Function

• CVP >12 mmHg suggests right ventricular dysfunction, volume overload, or significant tricuspid regurgitation according to the European Heart Journal 1
• In right ventricular failure, elevated CVP reflects impaired forward flow and increased filling pressures 1

Critical Limitations in Interpretation

CVP alone cannot be used to guide fluid resuscitation because static CVP measurements within the relatively normal range (8-12 mmHg) have limited ability to predict fluid responsiveness 6

Key limitations include:

• Multiple confounding factors affect CVP including tricuspid regurgitation, positive end-expiratory pressure (PEEP) ventilation, right ventricular compliance changes, and patient positioning 1
• Poor correlation with left heart pressures - CVP rarely correlates with left atrial pressure in patients with acute heart failure 7
• Static measurements are unreliable - Even at CVP <5 mmHg, 25% of patients fail to respond to volume infusion with increased cardiac output 5
• CVP >10 mmHg indicates low probability of cardiac output increase with fluid administration 5

Proper Measurement Technique

CVP must be measured correctly to provide meaningful data:

• Catheter tip position should be at the right atrial-superior vena cava junction 7
• Patient positioning at 30-45 degrees elevation with measurements referenced to the phlebostatic axis (approximates right atrial level) 1
• Equilibration time of at least 2 minutes in supine position and 1 minute in upright position before measurement 1

Integration with Other Parameters

CVP should never be interpreted in isolation but rather combined with:

• Dynamic measures such as passive leg raises, pulse pressure variation (sensitivity 0.72, specificity 0.91 in sepsis), or stroke volume changes with mechanical ventilation 6
• Echocardiographic assessment of inferior vena cava diameter and collapsibility, ventricular function, and chamber sizes 1
• Clinical parameters including cardiac output, mixed venous oxygen saturation, urine output, and perfusion markers 1, 4

Common Pitfalls to Avoid

• Using CVP as the sole guide for fluid therapy - The Surviving Sepsis Campaign explicitly states CVP alone can no longer justify fluid resuscitation decisions 6
• Ignoring mechanical ventilation effects - PEEP and mean airway pressure artificially elevate CVP readings 1, 8
• Misinterpreting elevated CVP as always requiring diuresis - In restrictive cardiomyopathy, higher CVP (12-15 mmHg) may be necessary for adequate ventricular filling 4
• Assuming CVP reflects left-sided filling pressures - This correlation is poor, especially in heart failure patients 7