Introduction

Associated with post-operative mortality and morbidity, intra-operative hypotension (IOH) is a topic of increasing interest amongst both clinical researchers and clinicians responsible for the hemodynamic management of patients in the operating room.

Background

During non-cardiac surgery, variations in intraoperative blood pressure beyond normal physiological ranges are a common occurrence.

Hemodynamic instability

During the peri-operative period, surgical patients are often hemodynamically unstable, and this may result in a supply-demand mismatch¹.

A primary goal of anesthetic management is hemodynamic control, but little is known about the optimal targets for such management.

What is IOH?

IOH is a common and frequent occurrence in patients undergoing general anesthesia for non-cardiac surgery. A 2014 study of almost 17,000 anesthetic records revealed that 26% of the surgical patients involved had a peri-operative systolic blood pressure of <80 mmHg for >5minutes². 

Definition of IOH

At present, there is no widely accepted definition of IOH and this absence confounds the association between blood pressure deviations during surgery and adverse sequelae^3,4,5.

Measurement of IOH

Blood pressure signals are complex and demonstrate considerable variation over time. Multi-component, they include diastolic, systolic, mean and pulse pressures. The optimum method for characterizing these complex signals has yet to be defined. Several different approaches have been advocated and examined clinically. These include mean, time-weighted average, minimum, time under various thresholds, area under thresholds, time-weighted average under thresholds and minimum pressure maintained for various pre-defined periods.

Any identified association between blood pressure and outcome will have a primary dependency on the method chosen to characterize the pressure. More complex methodologies, e.g. those quantifying the duration and magnitude of pressure changes are likely to be more useful than simpler approaches such as mean and time-weighted average⁶.

IOH and unfavorable outcomes

It has long been recognized that IOH is associated with post-operative mortality³. The link between IOH and adverse outcomes such as acute kidney injury (AKI) and myocardial injury (MI) has been conclusively demonstrated by several landmark clinical studies^5,7,8Furthermore, accumulating evidence from other studies indicates that even brief periods of IOH may be harmful to patients⁹. Of concern is the fact that the threshold for harm may reside below blood pressure levels that are currently accepted as standard of care¹⁰.

Several clinical studies that have defined hypotension in terms of minutes or integrated pressures below various absolute limits have demonstrated associations between low mean arterial pressure (MAP) and organ injury¹¹.

IOH, defined in various ways, is weakly associated with AKI and strongly associated with MI and death¹¹.

In a study of 104,000 non-cardiac surgery patients, 30-day mortality was strongly related to time-weighted average intra-operative MAP¹².

The threshold for myocardial injury is a MAP ≤65mmHg. Recent clinical evidence suggests that the threshold for renal injury may be higher, possibly nearer 75mmHg. A few minutes of a MAP <55mmHg is associated with AKI and MI risk—both of which have been demonstrated to increase markedly with prolonged IOH⁸.

Adverse outcomes associated with IOH such as AKI and MI require additional treatment and length of stay. These incur substantial extra hospital expenditures¹⁴.

Benefits of preventing IOH

The prevention of IOH, by tailoring management of peri-operative blood pressure to individual patient physiology, may improve post-operative outcomes. In a randomized clinical trial of patients undergoing major abdominal surgery, those in the individualized blood pressure management arm demonstrated significantly lower rates of post-operative organ dysfunction than those managed with standard practice (38.1% vs. 51.7% respectively) ¹³.

A recent Monte Carlo modelling study based on current US epidemiological and cost outcomes literature has suggested that “improved intra-operative hypotension control in a hospital with an annual volume of 10,000 non-cardiac surgical patients is associated with mean cost reductions ranging from $1.2–$4.6 million per year” ¹⁴.

How can the problem be addressed?

Directed Systems Ltd (Cambridge, UK and Austin, USA) has developed and received FDA 510(k) clearance, for a new software medical device, Hypotension Decision Assist (HDA), which provides guidance to anesthesiologists in relation to the real-time management of patient cardiovascular physiology.

HDA is designed to help anesthesiologists respond to the problem of hypotension in patients undergoing anesthesia during surgical procedures.

The information provided by HDA is intended to complement, not replace, the information provided by the standard multi-parameter monitoring systems that are routinely used in the operating room. 

HDA continually processes and displays, in graphical charts and numeric format, hemodynamic data and derived variables in comparison with user defined targets. It detects and indicates to the user when blood pressure shown as MAP is above or below a defined target range.

HDA includes algorithms that provide insight into the determinants of cardiovascular function, including trends in cardiac output and systemic vascular resistance^15-20.

HDA allows the user to add labels to the graphic display chart to show the administration of vasopressors, as bolus or infused, and volume challenges.