Page 26 - Journal of Special Operations Medicine - Spring 2016

Figure 7 Heart rate (top) and compensatory reserve (bottom) Figure 8 The compensatory reserve measured continuously
measured during a 5-minute orthostatic test in a patient with over 2 hours in a healthy adult man during baseline rest
postural orthostatic tachycardia syndrome. (green line), heat stress (orange line, right panel), progressive
reduction in central blood volume (red line), and restoration
of central blood volume (blue line) (personal communication,
Dr Craig Crandall and coworkers).

Each bar represents 5 seconds. Bar colors: green, Compensatory Re- LBNP, lower-body negative pressure.
serve Index (CRI) >0.6; yellow, CRI ≤0.6 and >0.3; red, CRI, ≤0.3.
Figure 9 The Compensatory Reserve Index measured during
a 25-minute, graded-cycle ergometer exercise test performed
Case 8: Healthy Subject Undergoing Progressive at 100°F.
Central Hypovolemia With and Without Heat Stress
Tolerance to central hypovolemia was evaluated in a
healthy adult man following whole-body passive heat
stress induced by circulating hot water through a wa-
ter-perfused suit. The heat stress elicited an elevation of
approximately 1°C in core (intestinal) temperature. A
normothermic trial performed on a separate day was
conducted as the control condition. Tolerance to cen-
tral hypovolemia was quantified as the time to hemo-
dynamic decompensation during progressive LBNP. The
compensatory reserve and tolerance to reduced central
blood volume under normothermic and heat stress con-
ditions are illustrated in Figure 8. The subject’s average Each bar represents 30 minutes. Bar colors: green, Compensatory Re-
baseline CRI was approximately 0.83 for both experi- serve Index (CRI) >0.6; yellow, CRI ≤0.6 and >0.3; red, CRI ≤0.3. BL,
mental conditions. Heat exposure before LBNP reduced baseline; W, watts.
his CRI to approximately 0.5. Compared with normo-
thermia, heat stress reduced LBNP tolerance time by decreased gradually to an average of 0.60 during the
nearly 50% (from approximately 27 to 14 minutes) and 50W exercise level, to approximately 0.40 during the
maximal LBNP level from 90 to 60mmHg. 75W level, and to less than 0.30 during the 100W level.
Although restoration of compensatory reserve occurred
Case 9: Healthy Subject Performing Exercise upon cessation of exercise, it was more gradual and did
After a 5-minute baseline resting period of sitting in not return to baseline level.
an environment of 100ºF and 44% relative humidity, a
healthy 22-year-old woman (56.8kg) performed physi- Discussion
cal exercise at a pedaling rate of 60 rpm with consecu-
tive 5-minute work rates at 25, 50, 75, and 100W on a Dependence on traditional vital signs to identify hypo-
cycle ergometer, followed by a 5-minute recovery period. volemia in military-relevant medical and operational
Average steady-state oxygen requirement was 653mL/ scenarios is problematic because they do not directly re-
min (25W), 880mL/min (50W), 1,140mL/min (75W), flect the compensatory response to volume loss; instead,
and 1,380mL/min (100W). CRI values recorded every they are outcomes of compensation that only begin
minute are presented in Figure 9. CRI decreased slightly to be altered near the onset of physiologic failure. Al-
from 0.92 at baseline rest to an average of 0.80 dur- though standard vital signs are sensitive to volume loss,
ing the 5 minutes of the 25W exercise level. Following they are not specific and subsequently can be abnormal
11
a slight transient increase in CRI during the transition for a number of reasons. Diagnosis and early treat-
from 25W to 50W that coincided with deeper inspira- ment of reduced circulating blood volume associated
tion (i.e., increased tidal volume from 1.0 to 1.8 L), CRI with hemorrhage or dehydration are top priorities for

10 Journal of Special Operations Medicine Volume 16, Edition 1/Spring 2016