What is the unit of measurement for atrial pressure and Central Venous Pressure (CVP)?

Units of Measurement for Atrial Pressure and Central Venous Pressure (CVP)

Both atrial pressure and Central Venous Pressure (CVP) are measured in millimeters of mercury (mmHg) or centimeters of water (cmH₂O), with mmHg being the most commonly used unit in clinical practice. 1, 2

Standard Units and Conversion

• Central venous pressure is expressed as a numerical value in either millimeters of mercury (mmHg) or centimeters of water (cmH₂O) 1
• The conversion between these units is: 1 mmHg ≈ 1.36 cmH₂O 3
• In clinical documentation and research studies, mmHg is the predominant unit used for reporting CVP values 4

Normal Values by Patient Status

• In non-mechanically ventilated patients, normal CVP ranges from 3-8 cmH₂O (approximately 2-6 mmHg) 3
• In mechanically ventilated patients, normal CVP ranges from 8-12 mmHg 3
• For patients with mechanical ventilation, pre-existing decreased ventricular compliance, or increased intra-abdominal pressure (>12 mmHg), a higher target of 12-15 mmHg is often recommended 3

Clinical Examples of CVP Measurement

• In hemodynamic studies of patients with sickle cell disease, CVP is reported in mmHg, with values of 10 ± 5 mmHg observed in patients with pulmonary hypertension compared to 6 ± 3 mmHg in those without pulmonary hypertension 4
• When measuring CVP for assessment of right heart function, values >12 mmHg may indicate right ventricular failure, volume overload, or significant tricuspid regurgitation 2
• In patients with shock, low CVP (<3 cmH₂O) often indicates hypovolemia and may require fluid resuscitation 3

Measurement Technique

• For accurate measurement, the patient should be positioned at 30-45 degrees elevation 2
• The reference point for measurement should be 5 cm below the sternal angle 2
• CVP is measured through a central venous catheter placed in the superior vena cava or right atrium 2
• When calculating pulmonary vascular resistance or assessing right heart function, right atrial pressure is measured in mmHg 4

Clinical Pitfalls in Interpretation

• Static CVP measurements have limited value in predicting fluid responsiveness, with a CVP of less than 8 mmHg predicting volume responsiveness with a positive predictive value of only about 50% 4, 3
• CVP measurements are affected by multiple factors including tricuspid regurgitation, positive end-expiratory pressure ventilation, right ventricular compliance changes, and patient positioning 2
• When using CVP for fluid management, dynamic changes in response to interventions are more valuable than static measurements 3

Special Considerations

• In research settings, right atrial pressure may be measured as part of calculating fractional flow reserve (FFR), though venous pressure is often not included in daily clinical practice calculations 4
• In patients with liver disease undergoing TIPS procedures, portal-caval pressure gradient (PCPG) is measured in mmHg, reflecting the consistency of pressure measurements throughout the cardiovascular system 4