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Abstract
Objective
This papers objective is to discuss the pathophysiology of intraoperative hypotension, or low blood pressure, with a focus on the causes of hypotension occurring at the cellular/receptor level.
Background
Intraoperative hypotension (IOH) is a major factor associated with postoperative complications and high rates of mortality following non-cardiac surgery. The incidence varies according to the definition of IOH used by physicians and the causes of the condition. Even though there is no single definition of what can be considered intraoperative hypotension, physicians commonly use the indication of mean arterial pressure less than 65 mmHg. Despite the variations in defining hypotension and its symptoms, a strong connection has been between IOH and a number of adverse postoperative outcomes. These include myocardial injury after non-cardiac surgery, acute kidney injury, and other organ injuries.
Results
The current research has identified several major causes of intraoperative hypotension. They include intravascular hypovolemia, vasodilation, high intrathoracic pressure, low cardiac output, and compromised baroreflex regulation. These conditions act as causative mechanisms resulting from the use of anesthetic medication and lead to the reduction of blood pressure. Risk factors are associated with included advanced age, the history of hypotensive episodes, emergency surgery, and the vulnerability to tissue ischemia. Although several conditions and processes have been identified as causes of intraoperative hypotension, more research needs to be done on the treatments and perioperative management of IOH. Patients that receive non-cardiac surgery under general anesthesia still face the risks of a decrease in blood pressure to dangerous levels.
Pathophysiology of Intraoperative Hypotension
Introduction
Intraoperative hypotension is among the most common side effects of general anesthesia used during surgery. Most frequently, IOH or intraoperative hypotension is determined as a mean arterial pressure (MAP) lower than the 25% average value of the patient. Even short periods of the decrease in MAP have appeared to be connected to unfavorable outcomes. IOH is considered to be a major factor associated with adverse postoperative complications, such as ischemic myocardial injury after extracardiac surgery, kidney injury, organ injuries, and other dangerous conditions. Annually, over 8 million patients suffer from postoperative myocardial injury, which is also one of the leading causes of patients dying within the first month after surgery. Thus, IOH is a major factor linked to a high postoperative mortality rate, as it increases the risk of adverse postoperative outcomes leading to death.
Incidence
Studies report the different incidence since the definitions of IOH used by researchers vary. Thus, IOH defined as a mean arterial pressure below 65 mmHg occurs in approximately 65 per cent of operations, and IOH defined as a 20 per cent decrease in MAP from baseline occurred in 94 per cent . Fourteen of the studies reviewed by Wesselink et al. have investigated mortality, and showed an outcome incidence between 0.03% (follow-up: <1 day) and 5.6% (during hospital admission) . Twelve studies reviewed showed a connection between IOH and acute kidney injury, with AKI incidence between 2.8% (7 days) and 72% (7 days) . Nine of the studies investigated the cases of myocardial injury preceded by intraoperative hypotension, and the incidence reported varied between 0.09% (in-hospital) and 30% (1 day) . Even though hypotension is believed to be a strong signal and a significant factor involving adverse postoperative outcomes, and duration of its episodes has not been clearly defined yet.
Etiology
To be able to treat intraoperative hypotension and avoid its dangerous postoperative outcomes, health professionals have to identify its causes. The causative mechanisms that lead to IOH vary in different phases, which are defined according to the different stages of surgery under general anesthesia. Post-induction hypotension (PIH) and early intraoperative hypotension (IOH) have been identified as the main phases induced by intravenous anesthetics and other anesthetic medication.
Patients undergoing surgery under general anesthesia can have hypotension caused by various pathophysiologic mechanisms, which means that the etiology of IOH involves a range of factors. Among the most common ones are vasodilation, intravascular hypovolemia, low cardiac output, high intra-thoracic pressure, and impairment of the sympathetic nervous system or compromised baroreflex regulation. During vasodilation, blood vessels widen as a result of the relaxation of their muscular walls. This mechanism leads to a decrease in systemic vascular resistance (SVR) and an increase in blood flow , which causes a decrease in blood pressure. Vasodilation and venodilation, in particular, is considered to be the main cause of hypovolemia induced by anesthetic drugs. It can also be caused by the reduction in circulating blood volume following massive blood loss (absolute hypovolemia) or inadequate increases in the capacitance of the blood vessels as in vasodilatory shock (relative hypovolemia). As a result, when compensatory mechanisms are impaired or function inadequately, relative hypovolemia can reduce arterial blood pressure.
Decreased cardiac output can be caused by a sinus rate that is too slow and leads to conditions like bradycardia or low stroke volume. Abnormally high heart rates, in turn, can lead to a decrease in stroke volume by reducing ventricular filling time . As a result of ventricular fibrillation, cardiac output falls to zero, which, in turn, causes hypotension. Hypotension can also be a consequence of baroreflex failure in patients undergoing and/or following radical neck dissection, neck injury, throat irradiation, and carotid artery surgery. Along with bradycardia, hypotension is common in patients when they are resting or relaxing. This is due to the loss of buffering ability, which occurs during baroreflex failure. It results from the interruption of the afferent limb of the baroreflex at the level of the carotid sinus, baroreceptor afferents, or medulla , and, apart from orthostatic hypotension, can lead to acute or fluctuating hypertension4. Risk factors associated with intraoperative hypotension have appeared to include advanced age, the history of hypotensive episodes, emergency surgery, and the vulnerability to tissue ischemia.
Signs and Symptoms
Patients sedated with anesthetic medication during surgery do not convey most of the common signs of low blood pressure, such as dizziness, nausea, or blurred vision. Therefore, during surgery, physicians have to depend on common definitions of what is considered to be the lowest blood pressure that can be managed without severe health outcomes. That is why the definitions of what constitutes intraoperative hypotension still vary. However, it is commonly accepted that values below a mean arterial pressure threshold of 65 mmHg are dangerous and entail cases of postoperative complications and death5. It can result in permanent organ injury, as it means that blood is not reaching the patients major organs.
Conclusion
It can be concluded that while the causes of intraoperative hypotension have been identified, the ways to prevent them and treat hypotension have not been clearly defined. IOH remains to be a major signal associated with adverse outcomes and high rates of postoperative mortality. Therefore, further research might focus on investigating the ways to manage.
References
Wijnberge, M., Schenk, J., Bulle, E., et al. Association of intraoperative hypotension with postoperative morbidity and mortality: systematic review and meta-analysis. BJS Open. 2021; 5(1), p.2.
Wesselink, E., Kappen, T., Torn, H., Slooter, A. and van Klei, W. Intraoperative hypotension and the risk of postoperative adverse outcomes: a systematic review. British Journal of Anaesthesia. 2018; 121(4), pp.706-721.
Saugel, B., Kouz, K., Hoppe, P. and Briesenick, L. Intraoperative hypotension: Pathophysiology, clinical relevance, and therapeutic approaches. Indian Journal of Anaesthesia. 2020; 64(2), p.90.
Ramanlal, R. and Gupta, V. 2021. Physiology, Vasodilation. Ncbi.nlm.nih.gov. Web.
McGee, S. 2021. Hypovolemia. ScienceDirect. Web.
Fitzgerald, J., Fedoruk, K., Jadin, S., Carvalho, B. and Halpern, S. Prevention of hypotension after spinal anaesthesia for caesarean section: a systematic review and network metaanalysis of randomised controlled trials. Anaesthesia. 2019; 75(1), pp.109-121.
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