Use of systolic pressure variation to predict the cardiovascular response to mini-fluid challenge in anaesthetised dogs
Introduction
Hypovolaemia is a common cause of arterial hypotension in human beings during anaesthesia and intensive care (Morris et al., 2005). This condition requires prompt fluid administration to increase the blood pressure. When controlled mechanical ventilation (CMV) is used in hypovolaemic human beings (Cavallaro et al., 2008) and dogs (Perel et al., 1987), stroke volume values exhibit wide cyclic variations in accordance with the ventilation cycle. During hypovolaemia, when the blood arterial pressure wave is monitored, such variations in stroke volume values are detectable by cyclical variations in arterial systolic pressure values.
The physiological reason for such haemodynamic changes can be explained by the decrease in venous return to the right atrium of the heart due to two concurrent mechanisms: (1) compression of the vena cava secondary to an increase in pleural pressure, and (2) an increase in right atrial pressure (Jardin and Vieillard-Baron, 2003). These effects cause a reduction in right ventricular stroke volume due to the Frank–Starling mechanism (Jardin and Vieillard-Baron, 2003). Positive pressure ventilation initially increases the left ventricle preload by forcing the blood contained within the lungs to the left side of the heart, which results in the afterload being reduced as a consequence of the reduction in thoracic blood volume. These effects give rise to an increase in stroke volume and, therefore, an increase in systolic pressure (maximum systolic pressure) (Pinsky et al., 1985). After a few heart beats, the inspiratory decrease in the right ventricular stroke volume causes a decrease in left heart refilling and, consequently, a reduction in stroke volume and systolic pressure (minimum systolic pressure) (Jardin et al., 1983).
The difference between maximum and minimum systolic pressure over a respiratory cycle has been termed systolic pressure variation (SPV). The SPV value is strictly correlated and directly proportional to the severity of hypovolaemia in dogs anaesthetised and undergoing CMV (Perel et al., 1987). Despite its validation as a reliable index of hypovolaemia in dogs, SPV has never been applied in clinical practice due to the lack of a usable value to discriminate between dogs that should receive a bolus of fluids and those in which fluid administration may have few or even detrimental effects.
Hypovolaemia induces a cascade of physiological responses, such as tachycardia and arterial vasoconstriction, to maintain blood pressure homeostasis. In a hypovolaemic subject, an adequate intravenous bolus of fluid is able to reverse these compensatory mechanisms, thus decreasing heart rate and increasing blood pressure (Chien, 1967). We hypothesised that the cardiovascular response to a bolus of fluids can be predefined using an SPV value with good specificity and sensitivity. The relevant cardiovascular response was defined as >10% increase in mean arterial blood pressure and/or >10% decrease in heart rate after a mini-fluid challenge (MFC) of 3 mL/kg crystalloids in dogs with normal cardiovascular function undergoing CMV with low peak inspiratory pressure (PIP). An MFC can be described as the intravenous (IV) administration of a small volume of fluid over a short period of time, which allows the cardiac preload to be increased by a high flow rate rather than by the total volume of fluid infused (Muller et al., 2011). The fluid challenge is a test, the purpose of which is to increase the cardiac preload in order to study the cardiovascular response.
Section snippets
Materials and methods
The study was approved by the Ethical Committee of the University of Padua (protocol number 24713; date of approval May 2012) and all owners gave informed consent. Dogs were excluded from the study if there was a history and/or clinical signs of cardiovascular disease or cardiac arrhythmias, or other systemic disease, if they were <1 year of age, or if their temperament precluded the use of the selected standardised anaesthetic technique.
Following exclusions, 28 dogs, presented to the Clinica
Results
The study included 26 dogs (10 males, 16 females), with a median age of 48 months (range 19–132 months) and a median weight of 15 kg (range 6–38 kg). The median propofol induction dose was 5.5 mg/kg (range 4.2–7.6 mg/kg). Ten dogs were classified as responders and 16 as non-responders according to the predefined criteria. Responders and non-responders did not differ significantly in pre-bolus HR or MAP values (P > 0.05; Table 1). Responders had significantly higher SPV-% values than
Discussion
SPV is detectable in all animals undergoing positive pressure ventilation, but its presence is not always an indicator that the administration of a bolus of fluids will significantly improve the haemodynamic status. Once clinical evidence emerged that aggressive fluid administration could severely affect clinical outcome (Lowell et al., 1990), it became necessary to understand the risk/: benefit ratio of administering fluids, prompting researchers to introduce the concept of predictive indices
Conclusions
This study has provided practical information and cut-off values for the use of SPV as a dynamic index of cardiac preload as a guide for fluid therapy in dogs under anaesthesia (isoflurane in oxygen and air) and CMV (PIP 8 cmH2O) in daily clinical practice. A recorded SPV value > 4.5% in anaesthetised and ventilated dogs with normal cardiovascular function most likely predicts a cardiovascular response (>10% increase in mean arterial blood pressure and/or >10% decrease in heart rate) after an
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