Excavating the Unknown MAP

By Amanda Decimo, MSN, MPH, CRNA, from the IARS, AUA and SOCCA 2019 Annual Meetings*

During the session, What is the Magic Number? Blood Pressure Thresholds in the Operating Room, ICU, and Beyond, Ashish Khanna, MD, Wake Forest University School of Medicine, led the audience on a blood pressure journey from the perioperative period to the more ambiguous intensive care setting in hopes of getting some absolute answers to blood pressure management that will improve patient outcomes. Hypotension by itself means nothing; it’s more about the amount and duration.

A MAP of 65 seems to be the interoperative value that is comfortable for anesthesiologists. A landmark study by Dr. M. Walsh and colleagues, published in 2013, revealed that the interoperative hypotension threshold is a MAP of 65 for AKI and myocardial injury. It defined damage associated with time, where a 20-minute duration of hypotension exposure results in twice the risk of associated organ injury. We should not ignore these numbers, Dr. Khanna stressed.

With chronic hypertension, do we need to adjust this threshold MAP of 65? It was not an obvious answer, but research indicates that we can use the same intraoperative MAP >65 thresholds for our patients with chronic hypertension.

The robust INPRESS randomized controlled trial (RCT), by Dr. E. Futier and colleagues, compared individualized blood pressure management (using patient’s baseline MAP) to standard of care (SBP>80), using systemic inflammation and organ system injury as primary outcomes. The individualized care group did better. Blood pressure seems to matter in the intraoperative environment (MAP 65), and it’s even better to keep it close to baseline.

Peri-induction hypotension is critical. After induction, we are busy placing IVs, nasogastric tubes, and warming blankets. A recent study published in Anesthesia & Analgesia shows from all hypotensive intraoperative episode (MAP<65), 36% occur in the period between induction and incision. While we can’t control many surgical factors that affect outcomes, we can control our part — intraoperative hypotension. We can make a difference here in preventing organ system injury.

Tachycardia is a problem we shouldn’t take lightly. It is more problematic when considered in the context of extreme blood pressures and prolonged duration. It’s associated with myocardial injury. Other BP component thresholds that play a role in outcomes are SBP>90, DBP>45, and PP>45. SBP and MAP thresholds play a bigger role in their association with harm.

Hospital wards are believed to be low acuity settings, but that is not the case. Hypotension and hypoxia are common problems on the floor. Most MI occur in the first 1-3 postoperative days; most are silent. The general floor environment is where MI often occur, yet there is no analysis of how. This is where it happens and we are not intervening.

A recent study published in Anesthesiology examined continuous portable blood pressure monitoring on the floor after noncardiac surgery. Hypotensive episodes were missed up to 50% of the time with spot checks. Hypertensive episodes (MAP>110) were missed 75% of the time.

The POISE trial examined episodes of hypotension by phase of care. On POD, 1-4 patients had a risk of harm three times greater than the intraoperative risk of harm associated with hypotension. 

ICU hypotension is never easily managed. These patients are critically ill and dying of multiple comorbidities. It’s hard to get a magic number in this environment with vasopressors and propofol drips running, intubation happening, and don’t forget sepsis. Multiple studies suggest that these patients require a MAP anywhere from 70-85, slightly higher than our OR numbers.

The SEPSISPAM trial conducted a RCT looking at BP targets and ICU outcomes. This landmark trial compared MAP of 65mmHg to MAP of 85mmHg. Data showed increased AKI in the lower value group and increased atrial fibrillation with the higher MAP.

Sepsis guidelines suggest an ICU MAP threshold of 65mmHg; however, we are not meeting these goals in the ICU, according to Dr. Khanna. Data indicates that a vast majority of septic ICU patients have MAPs well below the 65mmHg threshold for significant durations. As hypotension happens, mortality increases.

A study titled, “Economic Burden of Hypotension in the ICU,” estimates that $4,450/patient is saved when an ICU patient’s MAP is maintained near 65mmHg.  More cost-effective analysis is needed, but early numbers indicate hypotension in the ICU loses money.

What are the true ICU thresholds? It is not always 65mmHg. Duration matters. Dr. Khanna’s research shows that any MAP< 80mmHg for over an hour or a MAP<70mmHg for any duration was associated with increased MINS, mortality, and AKI.

A study on delirium in the ICU showed that a MAP < 75 had an increased association with delirium. ICU delirium is complex, and the causes are multifactorial. However, hypotension is a major player.

When patients are intubated in ICU, BP often drops shortly thereafter. Dr. Khanna and colleagues are conducting a prospective study to determine preintubation BP thresholds. A SBP >130mmHg or MAP >95mmHg are cut-off points for predicting intubating conditions below which are associated with detrimental hypotension. Preintubation BP matters and requires a higher MAP.

A MAP ≥65 was a target of the past. The threshold was not tailor-made for every situation. We lack good data to provide definitive answers about ICU blood pressure thresholds. The best research we have up-to-date shows these magic threshold numbers:

• Critical Care (Intubation): SBP≥130, MAP≥95
• ICU: MAP≥ 80-85
• Interoperative: SBP≥90 DBP≥45 MAP≥65
• Floor: We don’t know.

Hypotension is serious, even for short periods. Please select a MAP based on rationale and defend it. Often clinicians modify their MAP expectations as their patient’s condition changes. It’s a slippery slope. Be more vigilant at sticking to your MAP.

One final thought Dr. Khanna offered: don’t forget perfusion! MAP means nothing without it. For example, in our septic ICU patient on vasopressors with sluggish inflammatory cells, there is poor peripheral perfusion and microcirculation. Dr. Khanna hopes that one day our monitoring capabilities will provide comprehensive readings of all of these components, and we will be able to find that sweet spot for blood pressure and perfusion.

*Coverage from SOCCA Review Course Lecture: What Is the Magic Number? Blood Pressure Thresholds in the Operating Room, ICU and Beyond during the IARS 2019 Annual Meeting