Assessment of Volume Status and Fluid Responsiveness in Small Animals

Abstract

Intravenous fluids are an essential component of shock management in human and veterinary emergency and critical care to increase cardiac output and improve tissue perfusion. Unfortunately, there are very few evidence-based guidelines to help direct fluid therapy in the clinical setting. Giving insufficient fluids and/or administering fluids too slowly to hypotensive patients with hypovolemia can contribute to continued hypoperfusion and increased morbidity and mortality. Similarly, giving excessive fluids to a volume unresponsive patient can contribute to volume overload and can equally increase morbidity and mortality. Therefore, assessing a patient's volume status and fluid responsiveness, and monitoring patient's response to fluid administration is critical in maintaining the balance between meeting a patient's fluid needs vs. contributing to complications of volume overload. This article will focus on the physiology behind fluid responsiveness and the methodologies used to estimate volume status and fluid responsiveness in the clinical setting.

Ultrasonographic measurement of caudal vena cava to aorta ratio during fluid resuscitation of dogs with spontaneous circulatory shock

“…Blood lactate levels are frequently used to evaluate tissue hypoperfusion (Rosenstein et al 2018b). However, blood lactate levels are non-specific and hyperlactataemia may be associated with any cause of decreased oxygen delivery, regardless of volume status, drug treatments or underlying disease (Rosenstein et al 2018a, Boysen & Gommeren 2021.…”

Performance of four cardiac output monitoring techniques vs. intermittent pulmonary artery thermodilution during a modified passive leg raise maneuver in isoflurane-anesthetized dogs

“…A considerable pressure gradient coupled with lower right atrial pressure corresponds to higher preload. Mean systemic filling pressure propels forward flow toward the right atrium, influenced by global blood volume and vascular compliance ( 9 , 65 ). A specific resting fluid volume termed the “unstressed volume,” occupies the vascular bed without exerting force on vessel walls or contributing to venous flow.…”

“…A specific resting fluid volume termed the “unstressed volume,” occupies the vascular bed without exerting force on vessel walls or contributing to venous flow. By contrast, the “stressed volume” stretches vessel walls, exerting an outward force that facilitates fluid movement from vessels ( 9 , 65 ). The Frank-Starling cardiac function curve stipulates that when ventricular preload increases, the interaction between actin and myosin filaments intensifies, bolstering intrinsic cardiac contractility and CO.…”

“…Dynamic indices such as stroke volume variation (SVV) and pulse pressure variation depend on heart-lung interactions induced by mechanical ventilation and measure the subsequent effect of positive-pressure ventilation on fluctuations in stroke volume across a respiratory cycle. While these indices hold clinical relevance in both human medicine ( 2 – 4 ) and veterinary medicine ( 8 , 9 ), their accuracy diminishes during spontaneous breathing, low tidal volume ventilation, cardiac rhythm disturbances, right-sided heart failure, and open chest conditions ( 2 , 10 , 11 ). In such scenarios, researchers have introduced the “passive leg raising” test that can trigger a preload challenge similar to a traditional fluid bolus or heart-lung interactions, effectively predicting FR based on multiple trials and meta-analyses ( 12 – 15 ).…”

Fluid Therapy in Pulmonary Disease: How Careful Do We Need to Be?

“…We are currently reliant on physical examination, calculated indices such as shock index and biomarkers, such as lactate to identify significant hypoperfusion ( 16 , 17 ). There is increasing focus on the use of point-of-care ultrasound to provide more objective measures which can be used alongside other indicators to improve and refine our approach to this ( 18 , 19 ).…”