Is Pleth Variability Index (PVI) a Surrogate for Pulse Pressure Variations (PPV) in Pediatric Spine Fusion (SF) Surgery?

Spine fusion is an involved procedure during which patients are at risk for significant intra-operative blood loss. The resulting hypovolemia increases the fluctuations in arterial pressure associated with positive pressure ventilation. These respiratory induced arterial pressure variations (RIAPV) appear as cyclical peaks and troughs on the arterial waveform. Different approaches have been used to quantify the RIAPV. One such approach has been to measure the pulse pressure variation (PPV), using invasive arterial monitoring. In previous studies, PPV has been shown to be a good indicator of fluid responsiveness intra-operatively, but this has not been specifically evaluated in patients undergoing spine fusion. This patient population is of particular interest because of their underlying scoliosis as well as their prone position during the operation. These two variables could potentially alter lung-thorax mechanics in a manner which may influence RIAPV, which is specifically determined by the interaction between intrathoracic pressure and venous filling of the heart. A second approach to quantifying RIAPV relies on non invasive technology initially developed by the Masimo Corporation for pulse oximetry. This parameter has been coined pleth variability index (PVI), as it specifically quantifies real time changes in the plethysmogram associated with respiration. PVI, which is based on arterial blood volume changes, is therefore analogous to PPV, which is derived from changes in arterial pressure. To date PVI has only been evaluated in adult patients undergoing cardiac surgery and the data suggest that it may be a useful indicator of fluid responsiveness. Given the non-invasive and continuous nature of PVI, it is appealing for potential use as a bedside monitor to guide fluid resuscitation. However, photoplethysmography is known to be sensitive to noise due to motion, light and electrical interference. Furthermore, PVI is based upon a degree of quantitative evaluation of the photoplethysmogram that is the first of its kind. For these reasons, it is important to understand the limits of agreement between PVI and PPV before we can accept PVI as a non-invasive surrogate measurement. This study intends to evaluate PVI by analyzing it in relation to PPV. More specifically, paired measurements of PPV and PVI will be compared to determine the limits of agreement between the two parameters in patients undergoing spinal fusion.