Does the systolic pressure variation change in the prone position?

Objective: The Systolic pressure variation (SPV) is known to be a sensitive indicator of hypovolemia. However, the SPV may be elevated due to other reasons, such as changes in lung compliance or tidal volumes. Using the SPV to monitor the hemodynamic status of patients in the prone position may, therefore, be problematic due to possible effects of increased abdominal pressure on both venous return and lung compliance. The purpose of this study is to examine whether or not the SPV changes significantly when placing the patient in the prone position. Methods: The arterial pressure waveform was recorded and SPV measured in 25 patients undergoing spine surgery. Patients that were elderly (age > 65 years), obese (BMI > 30), or had history of lung disease (COPD, Asthma), were excluded. Measurements were taken in the supine and prone position and the results were compared using the Paired Student's t-test. A P < 0.05 was considered significant. Values expressed are for mean ± standard deviation. Results: The SPV was 6.9 ±1.9 and 7.0 ± 1.8 mmHg in the supine and prone position respectively. These two results were not statistically significant. Conclusions: This study is important because it shows for the first time that the SPV does not change significantly in the prone position, and may therefore continue to be used as an indicator of the volume status. It also would appear to indicate that our methods for protecting the chest and abdomen in the prone position are effective.