Blood pressure management is a core component of anaesthetic practice. Yet despite advances in perioperative care, intraoperative hypotension (IOH) remains one of the most common haemodynamic disturbances encountered during surgery.
Growing evidence suggests that even relatively short periods of hypotension may be associated with adverse outcomes, including acute kidney injury, myocardial injury and increased postoperative morbidity. For this reason, preventing and managing hypotension has become an important focus of perioperative optimisation strategies.¹⁻⁴
While clinicians can easily identify a drop in blood pressure, understanding why that drop has occurred is often far more challenging.
The Limitation of Focusing on Blood Pressure Alone
When hypotension occurs, the natural response is to focus on restoring blood pressure to an acceptable level. However, blood pressure is not a direct measure of cardiovascular function. Rather, it is the result of multiple physiological processes occurring simultaneously.
A low arterial pressure reading tells us that a problem exists. It does not necessarily reveal its cause.
This distinction is important because two patients may present with identical blood pressure values while experiencing completely different haemodynamic conditions. One patient may require vasopressor support, another fluid therapy and a third intervention aimed at improving cardiac performance. Treating all three patients in the same way risks addressing the symptom rather than the underlying problem.¹,²
Three Physiological Drivers of Hypotension
In most cases, intraoperative hypotension can be traced back to one or more of three fundamental haemodynamic mechanisms.
1. Reduced Vascular Tone
Following induction of anaesthesia, vasodilation is a frequent cause of hypotension.
Many anaesthetic agents reduce sympathetic nervous system activity, causing blood vessels to relax and systemic vascular resistance to fall. As vascular tone decreases, arterial pressure may fall despite adequate circulating volume and cardiac output.¹,²
This phenomenon is particularly common in elderly patients and those with pre-existing cardiovascular disease.¹
2. Reduced Cardiac Output
Blood pressure may also decrease when the heart is unable to maintain sufficient blood flow to the circulation.
A reduction in cardiac output may result from myocardial dysfunction, arrhythmias, changes in preload or the cardiovascular effects of anaesthetic agents. In these situations, restoring vascular tone alone may not fully resolve the underlying haemodynamic disturbance.¹,³
3. Reduced Circulating Volume
Hypovolaemia remains a well-recognised cause of intraoperative hypotension.
Whether due to fasting, bleeding or fluid shifts associated with surgery, inadequate circulating volume can reduce venous return and stroke volume, ultimately leading to lower arterial pressure.²,³
Importantly, these three mechanisms rarely occur in isolation. Many hypotensive episodes involve a combination of vasodilation, altered cardiac performance and fluid deficits.¹,²
Why Correct Diagnosis Matters
The traditional question during a hypotensive event is often:
“How can we increase the blood pressure?”
Increasingly, clinicians are asking a different question:
“What is causing the blood pressure to fall?”
The answer can significantly influence treatment decisions.
For example:
- Vasodilation may respond to vasopressor therapy.
- Fluid depletion may require volume replacement.
- Cardiac dysfunction may require a different therapeutic strategy altogether.
Understanding the haemodynamic mechanism behind hypotension allows interventions to be tailored to the patient’s specific physiological needs rather than relying on a one-size-fits-all approach.²,³
Moving Towards a More Complete Haemodynamic Assessment
As perioperative care becomes increasingly focused on individualised treatment, there is growing recognition that blood pressure represents only one component of haemodynamic status.
Additional parameters, such as cardiac output, stroke volume and fluid responsiveness, can provide valuable context when evaluating haemodynamic instability. Together, these measurements may help clinicians better understand the factors contributing to hypotension and support more informed clinical decision-making.¹,²
This shift reflects a broader evolution in perioperative medicine, from monitoring physiological variables in isolation to building a more comprehensive understanding of cardiovascular function.
Conclusion
Intraoperative hypotension is not a diagnosis in itself but a sign that cardiovascular physiology has been altered.
Although blood pressure remains an essential parameter, focusing exclusively on the number displayed on the monitor may overlook the mechanisms driving haemodynamic instability. By considering vascular tone, cardiac output and circulating volume together, clinicians can gain a more complete understanding of the patient’s condition and make more targeted treatment decisions.
Ultimately, looking beyond blood pressure may help support a more individualised approach to perioperative haemodynamic management and contribute to safer patient care.¹⁻³
References
- Kouz K, Hoppe P, Briesenick L, Saugel B. Intraoperative hypotension: pathophysiology, clinical relevance, and therapeutic approaches. Indian J Anaesth. 2020;64(2):90-96.
- Guarracino F, Bertini P. Perioperative hypotension: causes and remedies. J Anesth Analg Crit Care. 2022;2:45.
- Hoppe P, Kouz K, Saugel B. Perioperative hypotension: clinical impact, diagnosis, and therapeutic approaches. J Emerg Crit Care Med. 2020;4:31.
- Yerdon A, Sherrer DM, Chappell D. Intraoperative Hypotension: A Public Safety Announcement for Anesthesia Professionals. Anesthesia Patient Safety Foundation Newsletter. 2024 Jun.
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