(207) Baseline Shock Index and Autonomic Function in Older Adults

Introduction: Elevations in shock index (SI), defined as heart rate (HR)/systolic blood pressure (BP) is an important indicator of acute critical illness in the setting of shock due to hypovolemia, trauma, and sepsis. Possibly due to age-related autonomic changes, SI has been shown to be less discriminative of impending circulatory collapse in the older adult population, although the underlying mechanisms for this remain unclear.

Methods: 51 older adults without critical illness (29 women, 23 men, age >= 65; mean age 77.8±0.9 years) were recruited from outpatient clinics in an academic center. SI, the modified shock index (MSI, HR/mean BP), and the age corrected SI (Age * SI) were measured at rest and during 3 minutes orthostatic stress. Baroreflex function (using the sequence method), and heart rate variability (HRV) were also measured at baseline.

Results: SI, the modified shock index (MSI, HR/mean BP), and the age corrected SI (Age * SI) were measured at rest and during orthostatic stress. Baroreflex function (using the sequence method), and heart rate variability (HRV) were also measured at baseline. Age * SI had a significant positive association with baroreflex sensitivity (BRS, p=0.016) but this was not present for either SI (p=0.093) or MSI (p=0.139). All shock indices also showed a significant positive association with BRS for upward blood pressure swings (SI, p=0.007; MSI, p=0.007; age * SI, p=0.001). Age * SI also showed a significant correlation with high frequency HRV (p=0.044). By repeated measures analysis of variance, all shock indices increased over time with increasing duration of orthostatic stress (SI, p< 0.001; MSI, p< 0.001; age * SI, p< 0.001).

Conclusions: Autonomic dysfunction is associated with lower shock indices at baseline in older adults that are not critically ill. Although more work needs to be done, age and chronic illness-related changes in autonomic function might be one of the explanations for the reduced diagnostic power of shock indices in the older adult population.