We appreciate the insightful comments of Dr. Nelson and the opportunity to respond to him as it relates to our recent Clinical Focus Review on the “Perioperative Management for Complex Spine Fusion Surgery.”  In his letter, Dr. Nelson relates concern regarding our discussion of the predictive potential of pulse pressure variation in this particular cohort of patients.  The word limits for this type of review did not permit us to provide a comprehensive discussion or evaluation of each component of perioperative anesthesia management for complex spine surgery, and so we appreciate this opportunity to expand on the topic of volume assessment for fluid therapy in the prone position in these patients.

Bernard Baruch, financier and advisor to several U.S. Presidents through two world wars once quipped, “A speculator is a man who observes the future, and acts before it occurs.” The role of our individual intraoperative monitoring systems is to provide a snapshot of the current state of our patient in real-time. It is the responsibility of the anesthesiologist to know how best to assimilate and integrate all of the resultant data, taking into account patient demographics, anesthetic-specific factors, and surgical circumstances, with knowledge of the predictive potential and limitations of those monitoring systems.

We agree with Dr. Nelson that the predictive power of pulse pressure variation is reduced in patients with decreased lung compliance.  All anesthesiologists will recognize the inverse association between body mass index and static pulmonary compliance and the fact that prone positioning introduces additional strain to this relationship by increasing intraabdominal, and obligatorily, intrathoracic pressure. These patient-related factors do, in fact, limit the utility of pulse pressure variation, and are well recognized confounders in the literature, as well as conditions disposing to right ventricular dysfunction, cardiac arrythmias, and low tidal volume ventilation, or a spontaneous breathing patient.  However, as with any monitor, these data cannot and should not be interpreted in a vacuum. Messina et al. published a meta-analysis and meta-regression of pulse pressure variation in the intraoperative setting accounting for many patient-related factors as well as prone positioning and differences in surgical and ventilatory strategies.  Their results for studies conducted in the prone position provided a pooled area under the curve of 0.78 (95% CI 0.69 to 0.88) for a threshold of 11.2% pulse pressure variation. The predictive power of pulse pressure variation increased to area under the curve of 0.85 (95% CI 0.76 to 0.94) for the subgroup of prone patients with tidal volume ventilation greater than 8 ml/kg.4 

Given that pulse pressure variation can be easily and continuously monitored from an in situ inta-arterial catheter in many modern manufacturer-integrated electronic anesthesia workstations, this has become a low cost, “non-invasive,” and valuable tool to guide fluid therapy in the prone spine patient. By no means is it perfectly accurate in this setting. But using a therapeutic threshold greater than 15% pulse pressure variation to predict fluid responsiveness in the prone spine patient, we would argue, is far better than the nil alternative, and this is borne out of the literature.