A study in Anesthesia Analgesia 2012 describes how to visually estimate Systolic Pressure Variability (SPV) without relying on automated calculations of a monitoring system. How does a visual estimate compare to values calculated by the PICCO? Can we ‘manually’ estimate SPV with enough precision to use it to guide fluid therapy?
SPV is is one of the few working adjunct markers of fluid responsiveness in mechanically ventilated hypovolemic (e.g. sepsis) patients.
Mechanical ventilation induces cyclic changes in intrathoracic and transpulmonal pressures. Those cyclic changes will result in corresponding cyclic changes in preload, stroke volume and secondarily cyclic changes in arterial blood pressure.
Variations in stroke-volume and arterial pressure will be especially prominent in hypovolemic patients. By measuring this respiratory variation we can calculate Systolic Pressure Variation (SPV) as a percentage of mean systolic pressure. A formula would look like this.
SPV = 100 x (SBPmax – SBPmin)/SBPaverage
Calculating SPV (or derivatives like stroke volume variation or pulse pressure variation) is a built in function in several monitoring systems like PICCO or FloTrac. However most anaesthesia or ICU monitoring systems don’t have that function. Smaller hospitals or non-hospital sites might not have a PICCO monitor.
Do we really need a machine to calculate SPV and make assumptions on fluid responsiveness?
50 anaesthetists were presented with a basic theoretical background and the formula above. Then they were presented with a random sequence of 10 arterial pressure tracings. After each tracing they were asked to estimate SPV.
Based on prior studies it was assumed that
patients with SPV <7% were assumed to be not fluid responsive
patients with SPV >14,5% would be fluid responsive
patients with SPV values between 7-14 were indeterminate.
The estimated SPVs were compared to the true SPVs and placed in a Clinical Significance Analysis graph.
True SPV and estimated SPV were plotted in the graph below. Dots in the red areas means the anestethist over- or underestimated that SPV to an extent that resulted in incorrect decision to treat or withhold fluid treatment.
For example, one anaesthetist overestimated a measured SPV of 6% to 20% resulting in an incorrect decision to treat with fluid, risking fluid overload.
Only in 4,4% of instances did the subjects calculate an incorrect SPV that resulted in incorrect treatment.
Visual estimation of systolic pressure variation could be useful method for guiding fluid therapy in situations before PICCO/FloTrac monitoring is established or not available.
Brief report: the ability of anesthesia providers to visually estimate systolic pressure variability using the “eyeball” technique.
Thiele RH, Colquhoun DA, Blum FE, Durieux ME.
Anesth Analg. 2012 Jul;115(1):176-81. Epub 2012 Mar 30.