Why does central venous pressure measured in the right atrium or proximal superior vena cava represent preload?

Why CVP Represents Preload Despite Being Measured in the Right Heart

Central venous pressure (CVP) measured in the right atrium or proximal superior vena cava represents preload because it directly reflects the pressure gradient driving venous return to the heart, which is the primary determinant of right ventricular filling and, through serial circulation, ultimately determines left ventricular preload. 1

The Physiologic Basis

CVP as Right Atrial Pressure

• CVP directly measures right atrial end-diastolic pressure, which is the back-pressure against which systemic venous blood must flow to return to the heart 2, 3
• The pressure gradient from the systemic venous reservoir to the right atrium is normally only 4-8 mmHg, making CVP a critical determinant of this gradient 1
• Because this gradient is so small, even minor changes in CVP (right atrial back pressure) exert relatively large effects on venous return and cardiac output 1

The Venous Return-Cardiac Output Relationship

• Right atrial pressure is a potent determinant of venous return regardless of its cause (external pressure, impeded RV ejection, or increased intrathoracic pressure) 1
• CVP indicates the relationship between circulating blood volume and the capacity of the heart at any given time 4
• When properly measured, CVP reflects cardiac preload by representing the filling pressure of the right ventricle at end-diastole 4

Why Right-Sided Measurement Reflects Overall Preload

Serial Circulation Principle

• Blood flows sequentially through the right heart, pulmonary circulation, and left heart - what fills the right ventricle ultimately determines left ventricular filling over subsequent cardiac cycles 1
• The transient inspiratory increase in RV filling is passed to the LV over the next few beats, demonstrating the serial connection 1
• Under steady-state conditions, right and left ventricular outputs must be equal, making right atrial filling pressure a surrogate for overall cardiac preload 1

Practical Measurement Considerations

• CVP measured in the right atrium or superior vena cava accurately represents the pressure throughout the central venous system when there is no obstruction to blood flow 5
• The measurement must be taken at end-expiration with proper zeroing at the level of the right atrium (5 cm below the sternal angle) to ensure accuracy 4, 6
• Transmural right atrial pressure (right atrial pressure minus pleural pressure) provides the most accurate assessment of true preload, though pleural pressure is rarely measured clinically 1

Important Caveats and Limitations

CVP as a Static Measure

• CVP is a poor predictor of fluid responsiveness when used as an isolated static value 1
• A CVP >10 mmHg indicates low probability of cardiac output increase with volume infusion, making it a reasonable upper limit for fluid challenges 6
• Even patients with CVP <5 mmHg may not respond to fluid administration (25% are non-responders), highlighting that CVP alone cannot predict preload responsiveness 6

Factors That Complicate Interpretation

• Mechanical ventilation significantly affects CVP interpretation - positive pressure ventilation increases intrathoracic pressure, raising CVP independent of true intravascular volume 1
• Right ventricular dysfunction or increased RV afterload (pulmonary hypertension, ARDS) can elevate CVP without reflecting adequate left ventricular preload 1
• Ventricular interdependence means RV dilation can impair LV filling through septal shift, making CVP misleading in acute cor pulmonale 1
• Pericardial constraint and chest wall compliance alter the relationship between measured CVP and true transmural filling pressure 1

Clinical Application Algorithm

When using CVP to assess preload:

1. Ensure proper measurement technique - zero at the phlebostatic axis, measure at end-expiration, confirm central catheter position 4

2. Interpret in clinical context - CVP trends are more informative than isolated values 4

3. Consider confounding factors:

   • Positive pressure ventilation present? (increases CVP artificially) 1
   • RV dysfunction or pulmonary hypertension? (elevates CVP disproportionately) 1
   • Spontaneous breathing with large negative swings? (decreases measured CVP) 7

4. Use dynamic assessment - observe CVP response to interventions (fluid bolus, passive leg raise) rather than relying on static values 1

5. Integrate with other parameters - combine CVP with cardiac output assessment, echocardiography, and clinical examination 1, 6

The key principle: CVP represents preload because it is the back-pressure determining venous return, which is the ultimate source of cardiac filling in a serial circulation system, but it must always be interpreted as transmural pressure in the context of intrathoracic pressure changes and right ventricular function. 1, 7