Recent findings point to new ways to monitor fluid therapy perioperatively and to gear fluid therapy protocols to each individual animal.
PHOTO BY GREGORY KINDRED
In this final article of the series, I discuss advances in monitoring fluid therapy, and I describe a more individualized and goal-directed approach to tailoring intravenous fluid therapy based on an animal's needs before, during, and after surgery.
MONITORING: OLD AND NEW METHODS
Traditional methods for monitoring perioperative intravenous fluid administration are not reliable.1 Other than increases in postoperative body weight, there have been few accurate indicators of excessive fluid administration.2 Traditional static methods for monitoring fluid therapy, such as central venous pressure (CVP), ventricular end diastolic pressure, and pulmonary capillary wedge pressure, have been shown to be unreliable and generally late indicators of changes in blood volume, fluid responsiveness, and fluid overload (see sidebar "Perioperative fluid therapy dilemmas" in Rethinking your approach to perioperative fluid therapy).1,3-6
Trends in cardiac filling pressures, including CVP, can indicate when too much fluid is administered too quickly but are very dependent upon concurrent changes in heart rate, ventricular function, and venous capacitance, which are frequently altered by disease and anesthesia.1,3
A new method
1. PVI is a noninvasive method to determine both the need for and the response to fluid therapy. It is based on the pulse oximeter plethysmographic (pleth) pulse pressure waveform. The Masimo pulse oximeter uses the pleth variability index (PVI) to monitor fluid responsiveness during anesthesia.
Dynamic variables (indices evaluating the response to cyclical changes in venous return, or preload) are more predictive of fluid responsiveness. Among these, arterial pulse pressure variation induced by assisted (manual) or mechanical ventilation has been demonstrated to be a specific and sensitive guide to fluid therapy.6,7 Respiratory variations in the amplitude of the noninvasively recorded pulse oximeter-derived plethysmographic (change in volume) pulse pressure waveform have been shown to predict fluid responsiveness.6,8
The plethysmographic variability index (PVI) measures the dynamic changes in perfusion index (PI) over respiratory cycles and is calculated as follows:
PVI = (PImax – PImin)/PImax × 100
The PVI is accurate and far less expensive than esophageal Doppler measurement of variations in stroke volume.4 The greater the PVI, the more likely the patient will respond to fluid administration.
The use of PVI-based goal-directed fluid management in surgical patients reduces the volume of intraoperative fluid infused and decreases intraoperative and postoperative lactate concentrations.8 PVI was less accurate in predicting fluid responsiveness during spontaneous breathing than during mechanical ventilation, but PVI was still better than CVP, at least in people.9
An automatic PVI measurement
The Masimo pulse oximeter (Masimo Corp.) uses an algorithm to continuously quantify changes in pulse volume (i.e. PVI) (Figure 1). PVI is better than CVP for determining the animal's response to fluid therapy.6
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