Fluid Resuscitation for Refractory Hypotension

Abstract

Hypotension is a common occurrence, especially in anesthetized patients and in critical patients suffering from hypovolemia due to shock and sepsis. Hypotension can also occur in normovolemic animals, anesthetized or conscious, under conditions of vasodilation or decreased cardiac function. The main consequence of hypotension is decreased organ perfusion and tissue injury/dysfunction. In the human literature there is no consensus on what is the threshold value for hypotension, and ranges from < 80 to < 100 mmHg for systolic blood pressure and from < 50 to < 70 mmHg for mean arterial blood pressure have been referenced for intraoperative hypotension. In veterinary medicine, similar values are referenced, despite marked differences in normal arterial blood pressure between species and with respect to humans. Therapeutic intervention involves fluid therapy to normalize volemia and use of sympathomimetics to enhance cardiac function and regulate peripheral vascular resistance. Despite these therapeutic measures, there is a subset of patients that are seemingly refractory and exhibit persistent hypotension. This review covers the physiological aspects that govern arterial blood pressure control and blood flow to tissues/organs, the pathophysiological mechanisms involved in hypotension and refractory hypotension, and therapeutic considerations and expectations that include proper interpretation of cardiovascular parameters, fluid recommendations and therapy rates, use of sympathomimetics and vasopressors, and newer approaches derived from the human literature.

Effect of methylene blue used as medical dye on blood pressure in a dog undergoing partial pancreatectomy

“…13 When hypovolaemia is coupled with peripheral vasodilation, such as in case of anaesthetised or septic patients, fluid therapy alone has limited benefit if without concomitant use of vasopressor drugs. 14 During surgical procedures, haemorrhage can be the cause of hypotension, in which case the treatment with fluid therapy might be controversial. Both crystalloids and colloids can be effective in correcting hypotension secondary to blood loss, although in case of severe blood loss, whole blood, packed erythrocytes or plasma should be provided.…”

“…However, fluid therapy as a sole method to correct hypotension should be cautious as high rates of fluids can exacerbate complications leading to haemodilution of haemoglobin, packed cell volume, total protein, decrease in colloid osmotic pressure, arterial oxygen content and central‐venous oxygen content 13 . When hypovolaemia is coupled with peripheral vasodilation, such as in case of anaesthetised or septic patients, fluid therapy alone has limited benefit if without concomitant use of vasopressor drugs 14 …”

Clinical Application of the Fluid Challenge Approach in Goal-Directed Fluid Therapy: What Can We Learn From Human Studies?

“…In individuals admitted for emergency care with signs of circulatory failure, hypotension is often caused by poor circulating volume that may be associated or not with sepsis-induced loss of vasomotor tone. Otherwise, hypotension during anesthesia is often caused by drug-induced decreases in systemic vascular resistance (SVR) and/or CO and does not imply that there is a circulating volume deficit ( 46 ). Although, it has been recognized that responders to volume expansion are not necessarily hypovolemic ( 28 ), hypotension-inducing hemorrhage in isoflurane anesthetized dogs can lead to a state of preload dependence, as suggested by increases in PPV and PVI from pre-hemorrhage values ( 47 , 48 ).…”

“…Although, it has been recognized that responders to volume expansion are not necessarily hypovolemic ( 28 ), hypotension-inducing hemorrhage in isoflurane anesthetized dogs can lead to a state of preload dependence, as suggested by increases in PPV and PVI from pre-hemorrhage values ( 47 , 48 ). In case of intraoperative hypotension where the suspected cause is poor circulating volume, a fluid challenge approach may be attempted but cardiovascular support with sympathomimetics should not be delayed due to the dose-dependent depressant effects of inhalant anesthetics and/or neuraxial anesthesia on contractility and vasodilation ( 46 ). A drug with alpha-1 and beta-1 effects is ideal to improve both arterial blood pressure and CO, respectively ( 46 ).…”

“…In case of intraoperative hypotension where the suspected cause is poor circulating volume, a fluid challenge approach may be attempted but cardiovascular support with sympathomimetics should not be delayed due to the dose-dependent depressant effects of inhalant anesthetics and/or neuraxial anesthesia on contractility and vasodilation ( 46 ). A drug with alpha-1 and beta-1 effects is ideal to improve both arterial blood pressure and CO, respectively ( 46 ). Constant rate infusions of vasopressor drugs, such as norepinephrine and dexmedetomidine, should be carefully used as these drugs can mask hypotension and increases in PPV and SPV caused by poor circulating volume, probably by shifting blood from the unstressed to stressed volume via a venoconstrictive action ( 47 , 49 ).…”