Critically ill patients who demonstrate an elevated modified shock index (MSI) within the first 24 hours of ICU admission have a significant risk for death.
MSI, which is the ratio of elevated heart rate to mean arterial pressure, may be a better predictor of mortality than shock index (SI) because MSI incorporates both systolic and diastolic blood pressures, while SI incorporates only systolic blood pressure, according to Nathan J. Smischney, MD, an anesthesiologist at Mayo Clinic, in Rochester, Minn., who was an author of this study.
Diastolic blood pressure is important for coronary perfusion. In septic patients, myocardial dysfunction is not uncommon. “Thus, MSI may better capture the spectrum of myocardial disturbance noted in septic and nonseptic patients,” said Dr. Smischney, who presented his team’s findings at the 2017 Society of Critical Care Medicine’s annual meeting (abstract 919).
“Given that MSI is easily calculated at the bedside, clinicians may institute interventions earlier, which could improve survival,” the researchers said.
Between Jan. 1, 2013, and Dec. 31, 2014, researchers from Mayo Clinic looked at the association between high MSI and in-hospital mortality among patients admitted to the medical and/or surgical ICU. They developed a case-control study in which cases were matched on three factors: age, end-stage renal disease and diagnosis on admission to the ICU.
“ICU admission diagnosis was divided into several categories,” Dr. Smischney explained. The categories were chosen based on physiologic rationale. For example, MSI is based on two parameters: heart rate and mean arterial pressure. Shock of any type would lead to derangements in these parameters and the diagnosis of sepsis.
They analyzed various parameters among 83 cases (patients who died) and 159 live controls. They found that the following parameters were associated with mortality: lorazepam administration (P≤0.01), shock requiring vasopressors (P≤0.01), maximum MSI (P≤0.001) and elevated APACHE (Acute Physiology, Age, Chronic Health Evaluation) III prognostic score at one hour (P≤0.001). In a multivariate analysis, maximum MSI (P=0.03) and the APACHE III score at one hour (P=0.003) remained significant for mortality. The optimal cutoff point for high MSI and mortality was 1.8.
Although the researchers did not demonstrate poor outcomes with a low MSI value, “high MSI values were significantly associated with increased mortality,” Dr. Smischney said.
“MSI could also serve as an easily calculated bedside variable that would provide additional information for family discussion with regard to goals of care.”
Dr. Smischney said neither MSI nor SI is “used all that much. The evidence is still coming out for both SI and MSI. Until both are fully validated, adoption will be slow. However, they are easily calculated at the bedside, unlike lactate, and would likely serve as valuable parameters to be aware of, with treatments directed at correcting both—similar to lactate.”
The response to an elevated MSI and/or SI depends on the clinical situation, Dr. Smischney said. “Similar to lactate, it may be that the response consists of fluid resuscitation with or without vasoactive agents. The key point is that it serves as a marker of physiologic derangements that, like lactate, if corrected with treatments, may improve survival. However, unlike lactate, it is captured instantaneously in real time with standard equipment,” Dr. Smischney said.
“As we seek tools to better predict patient outcomes without adding risks of invasive monitors or increased cost, this hemodynamic index that is easily calculated at the bedside could certainly add to our armamentarium,” said Marcos Lopez, MD, MS, assistant professor in the Department of Anesthesiology at Vanderbilt University Medical Center, in Nashville, Tenn., who was not part of the study but was asked to comment.
Dr. Lopez added, “A majority of the study group had shock or sepsis; thus this predictor may be most relevant to acutely ill or hemodynamically unstable patients, and findings should be validated prospectively and in other populations.”
Given the feasibility of calculation, Dr. Smischney said, “we feel that it should be calculated during the entirety of a patient’s ICU stay. One can think of it as a sixth vital sign.”