Stroke volume variations and implications in Anesthesia
Stroke volume variation (SVV) is a naturally occurring phenomenon in which the arterial pulse pressure falls during inspiration and rises during expiration due to changes in intra-thoracic pressure secondary to negative pressure ventilation (spontaneously breathing).
SVV and its comparable measurement, pulse pressure variation (PPV), are not indicators of actual preload but of relative preload responsiveness. SVV has been shown to have a very high sensitivity and specificity when compared to traditional indicators of volume status (HR, MAP, CVP, PAD, PAOP), and their ability to determine fluid responsiveness.
Hemodynamic fluctuations, including cardiac output (CO) decrease and hypotension, are common adverse events during induction of general anesthesia; they occur because of the vasodilation and myocardial suppression caused by anesthetic drugs and are exacerbated by preoperative dehydration and fasting.
As normovolaemia is not a well-defined clinical concept, an established method of assessing fluid status is to evaluate stroke volume response to i.v. infusion of a fluid bolus.
Due to varying shapes that the Frank–Starling curve could take depending on the ventricular systolic function, a fluid challenge could lead to either a significant or a negligible increase in stroke volume and cardiac output.
Frank- Starling’s Relationship:
The variations of stroke volume (SVV), and of surrogates, that are induced by mechanical ventilation were the first methods to be developed for the dynamic assessment of preload responsiveness. The rationale is that, during positive pressure ventilation, insufflation decreases preload of the right ventricle. When transmitted to the left side, this induces a decrease in preload of the left ventricle. If left ventricular stroke volume changes in response to cyclic positive pressure ventilation, this indicates that both ventricles are preload dependent.
By measuring SVV before induction of general anesthesia, anesthesiologists can perform prophylactic volume expansion and vasopressor administration in patients at high risk of decreased CO and hypotension.
Conditions where pulse pressure and stroke volume variations are less reliable:
Low Vt/low lung compliance
Increased intra‐abdominal pressure
Very high respiratory rate (HR/RR < 3.6)
Right heart failure
In the operating room setting, PPV and SVV monitoring (invasively or non-invasively obtained) retain their predictive value since the conditions of their applicability are generally fulfilled. The limitations of PPV and SVV must always be kept in mind by the intensivists or anaesthesiologists, since ignoring them could lead to serious misinterpretations.
The Summary of fluid strategy may be depicted as given below: