SAN FRANCISCO—Is central venous pressure (CVP) useful for fluid management in the intensive care unit (ICU)? It can be, asserts Michael R. Pinsky, MD.[1]

Dr. Pinsky, Director of Critical Care Research at the University of Pittsburgh, defended his position in a debate at the American Thoracic Society 97th International Conference. His opponent, Sheldon A. Magder, MD, Director of Medical Critical Care at McGill University in Montreal, took a more cautious approach, outlining the limitations of CVP measurements in the critically ill.

WHY CVP IS USEFUL

There are important advantages to assessing CVP, which reflects right atrial pressure, said Dr. Pinksy. “It is a poor man’s pulmonary artery occlusion pressure,” closely approximating that measurement under normal conditions and with volume loading. “Thus, if you have a low CVP, chances are you [have] a low pulmonary artery occlusion pressure,” he explained.

There are other advantages to CVP measurements, he stated. For example, a CVP above 10 mm Hg indicates that the patient is not hypovolemic (although, Dr. Pinsky said, fluid administration may be required for other reasons). Furthermore, an elevated CVP is a nonspecific marker for the presence of disease.

CVP may not accurately reflect pulmonary artery occlusion pressure in patients with right ventricular failure, Dr. Pinsky acknowledged. Further, he said, insertion of a central venous line is invasive and can lead to complications, such as hemorrhage, infection, pneumothorax, and air embolism. Thus, CVP measurements cannot be considered a perfect replacement for pulmonary artery catheterization.

However, CVP can predict the hemodynamic response to positive end-expiratory pressure (PEEP), a recent Austrian study suggests.[2] Among 22 ventilator-dependent patients with mild to severe acute lung injury, cardiac output invariably fell with PEEP administration if the baseline CVP was 10 mm Hg or less. “Only CVP predicted the subsequent fall in cardiac output at all levels of PEEP,” reported Dr. Pinsky.

Predictions of preload responsiveness are most accurate, however, when based on dynamic changes in CVP rather than on static or mean CVP values, Dr. Pinsky stressed. Indeed, research has shown that the rise in cardiac output after volume challenge is much better in critically ill patients who experience a greater than 1-mm Hg change in CVP during a single breath than in those with a less than 1-mm Hg fluctuation.

Other work supports this finding, including a French study that “showed that the pulse pressure variation … was highly predictive of an increase in cardiac output,” related Dr. Pinsky, “whereas a mean level of CVP or pulmonary artery occlusion pressure was no better than a coin flip.”[3]

THE ARGUMENT AGAINST CVP

Though he believes CVP monitoring in the ICU deserves respect, Dr. Magder thinks that the measurement has limited value as a component of fluid management in critically ill patients.[4] “The final output from the heart … is from the left [chamber of the] heart,” he noted. Thus, measurements that focus on the right atrium provide an incomplete picture of hemodynamics.

Left ventricular dysfunction is often not evident from the CVP, Dr. Magder added. Furthermore, the accuracy of CVP measurements is questionable when pleural pressures are high. In addition, CVP cannot be used to assess the mean circulatory filling pressure due to the venous resistance between the two measurements.

Since it gives no indication of unstressed volume or volume reserves, CVP is not helpful in evaluating total body water, either. For instance, it cannot pinpoint the capacitance change—the point at which further volume loss causes a sharp drop in cardiac output by overwhelming the sympathetic nervous system’s ability to maintain the mean circulatory filling pressure. And, CVP is a poor surrogate for end-diastolic volume.

In patients with excess volume, edema is a better marker than CVP, said Dr. Magder. However, he acknowledged that the CVP does provide evidence of the back pressure on the liver and kidneys. “The wedge pressure, of course, gives you the risk to the lungs,” Dr. Magder added.

Dynamic CVP changes do not always accurately predict the response to fluid challenge, he emphasized. Indeed, cardiac output sometimes fails to improve in patients with wide swings in CVP, whereas it may rise significantly in patients who experience small CVP variations.

Moreover, there were limitations to the French study Dr. Pinsky presented when arguing for the use of dynamic CVP changes, Dr. Magder stressed. Most important, many of the study subjects had only a 10% change in cardiac index. “In my own studies, I do not call this a change in cardiac output,” he commented.

—Timothy Begany

References
1. Pinsky MR. Pro: central venous pressure can be used for fluid management in the ICU. Paper presented at: American Thoracic Society 97th International Conference; May 20, 2001; San Francisco.
2. Jellinek H, Krafft P, Fitzgerald RD, et al. Right atrial pressure predicts hemodynamic response to apneic positive airway pressure. Crit Care Med. 2000;28:672-678.
3. Michard F, Boussat S, Chemla D, et al. Relation between respiratory changes in arterial pulse pressure and fluid responsiveness in septic patients with acute circulatory failure. Am J Respir Crit Care Med. 2000;162:134-138.
4. Magder SA. Con: central venous pressure can be used for fluid management in the ICU. Paper presented at: American Thoracic Society 97th International Conference; May 20, 2001; San Francisco.