Central Venous Pressure Indicators of Hypovolemia
A central venous pressure (CVP) reading less than 5 cm H₂O (or less than 4 mmHg) indicates hypovolemia. 1, 2
Understanding CVP and Volume Status
CVP is a hemodynamic parameter that can help assess intravascular volume status, though it has limitations as a standalone measurement:
- CVP less than 5 cm H₂O is considered indicative of hypovolemia 1
- CVP values between 6-10 cm H₂O typically represent normovolemia 1
- CVP greater than 10 cm H₂O may suggest hypervolemia 1
Clinical Application in Different Settings
Sepsis and Shock
- The Surviving Sepsis Campaign previously recommended targeting CVP of 8-12 mmHg during initial resuscitation 1
- However, more recent evidence suggests that CVP alone is not reliable for predicting fluid responsiveness in all patients 1, 3
- A very low CVP (≤ 5 mmHg) still has value in identifying significant hypovolemia, especially in younger patients 4
Neurosurgical Patients
- In neurosurgical patients with hyponatremia, CVP < 5 cm H₂O has been used to identify hypovolemic states requiring fluid resuscitation 1
- Patients with CVP < 5 cm H₂O respond well to normal saline administration (50 mL/kg/d) and salt supplementation 1
Mechanical Ventilation Considerations
- In ventilated patients, an internal jugular vein diameter ratio (30°/0° position) < 0.45 correlates with low CVP (≤ 5 mmHg) 2
- When patients are mechanically ventilated, a slightly higher CVP target (12-15 mmHg) may be needed to account for increased intrathoracic pressure 1
Limitations of CVP for Volume Assessment
- The pooled correlation coefficient between CVP and measured blood volume is only 0.16, indicating poor correlation 3
- Static CVP measurements alone have limited ability to predict fluid responsiveness 1, 3
- Dynamic changes in CVP may be more useful than absolute values in some patient populations 4
Alternative and Complementary Assessments
- Ultrasonographic measurement of inferior vena cava (IVC) diameter can complement CVP in assessing volume status 5
- Urinary chloride levels ≤ 20 mmol/L with sodium ≥ 20 mmol/L may indicate hypovolemia in ICU patients 6
- Dynamic measures such as pulse pressure variation and stroke volume variation during mechanical ventilation are more reliable than static CVP 1
Clinical Pearls and Pitfalls
- Avoid using CVP as the sole determinant for fluid management decisions 3
- Low CVP values (< 5 cm H₂O) have better specificity for hypovolemia than higher values have for hypervolemia 1, 2
- Consider patient-specific factors such as age, as CVP changes correlate better with fluid responsiveness in patients younger than 60 years 4
- In hypovolemic shock specifically, changes in CVP may have better predictive value for fluid responsiveness than in other shock types 4