Abstract:BACKGROUND: Tolerance to central hypovolemia is dictated by exhaustion of the physiological capacity to compensate called the compensatory reserve. Such physiological compromise can have detrimental impact on performance in aerospace environments as well as survival from hemorrhage
on the battlefield. We induced central hypovolemia using progressively stepwise lower body negative pressure (LBNP) in women during various phases of the menstrual cycle to test the hypothesis that similar tolerance across all mens… Show more
“…Rather, as an exploratory aim of the parent projects, we sought to identify whether there was an interaction between the menstrual cycle phase and the administered analgesic on the variables of interest. Third, previous studies have shown that MSNA is higher in the midluteal phase of the menstrual cycle (38,39); however, such responses do not alter tolerance to LBNP (40). In the present study, the phase of the menstrual cycle on the days the trials were performed was not controlled.…”
Introduction
Trauma-induced hemorrhage is a leading cause of death in prehospital settings. Experimental data demonstrate that females have a lower tolerance to simulated hemorrhage (i.e., central hypovolemia). However, the mechanism(s) underpinning these responses are unknown. Therefore, this study aimed to compare autonomic cardiovascular responses during central hypovolemia between the sexes. We hypothesized that females would have a lower tolerance and smaller increase in muscle sympathetic nerve activity (MSNA) to simulated hemorrhage.
Methods
Data from 17 females and 19 males, aged 19-45, were retrospectively analyzed. Participants completed a progressive lower-body negative pressure (LBNP) protocol to presyncope to simulate hemorrhagic tolerance with continuous measures of MSNA and beat-to-beat hemodynamic variables. We compared responses at baseline, at two LBNP stages (40 mmHg and 50 mmHg), and at immediately before presyncope. In addition, we compared responses at relative percentages (33%, 66%, and 100%) of hemorrhagic tolerance, calculated via the cumulative stress index (i.e., the sum of the product of time and pressure at each LBNP stage).
Results
Females had lower tolerance to central hypovolemia (female: 561 ± 309 vs. male: 894 ± 304 min*mmHg [time*LBNP]; p = 0.003). At LBNP 40 mmHg and 50 mmHg, females had lower diastolic blood pressures (main effect of sex: p = 0.010). For the relative LBNP analysis, females exhibited lower MSNA burst frequency (main effect of sex: p = 0.016) accompanied by a lower total vascular conductance (sex: p = 0.028; main effect of sex).
Conclusions
Females have a lower tolerance to central hypovolemia, which was accompanied by lower diastolic blood pressure at 40 and 50 mmHg LBNP. Notably, females had attenuated MSNA responses when assessed as relative LBNP tolerance time.
“…Rather, as an exploratory aim of the parent projects, we sought to identify whether there was an interaction between the menstrual cycle phase and the administered analgesic on the variables of interest. Third, previous studies have shown that MSNA is higher in the midluteal phase of the menstrual cycle (38,39); however, such responses do not alter tolerance to LBNP (40). In the present study, the phase of the menstrual cycle on the days the trials were performed was not controlled.…”
Introduction
Trauma-induced hemorrhage is a leading cause of death in prehospital settings. Experimental data demonstrate that females have a lower tolerance to simulated hemorrhage (i.e., central hypovolemia). However, the mechanism(s) underpinning these responses are unknown. Therefore, this study aimed to compare autonomic cardiovascular responses during central hypovolemia between the sexes. We hypothesized that females would have a lower tolerance and smaller increase in muscle sympathetic nerve activity (MSNA) to simulated hemorrhage.
Methods
Data from 17 females and 19 males, aged 19-45, were retrospectively analyzed. Participants completed a progressive lower-body negative pressure (LBNP) protocol to presyncope to simulate hemorrhagic tolerance with continuous measures of MSNA and beat-to-beat hemodynamic variables. We compared responses at baseline, at two LBNP stages (40 mmHg and 50 mmHg), and at immediately before presyncope. In addition, we compared responses at relative percentages (33%, 66%, and 100%) of hemorrhagic tolerance, calculated via the cumulative stress index (i.e., the sum of the product of time and pressure at each LBNP stage).
Results
Females had lower tolerance to central hypovolemia (female: 561 ± 309 vs. male: 894 ± 304 min*mmHg [time*LBNP]; p = 0.003). At LBNP 40 mmHg and 50 mmHg, females had lower diastolic blood pressures (main effect of sex: p = 0.010). For the relative LBNP analysis, females exhibited lower MSNA burst frequency (main effect of sex: p = 0.016) accompanied by a lower total vascular conductance (sex: p = 0.028; main effect of sex).
Conclusions
Females have a lower tolerance to central hypovolemia, which was accompanied by lower diastolic blood pressure at 40 and 50 mmHg LBNP. Notably, females had attenuated MSNA responses when assessed as relative LBNP tolerance time.
“…Participants were recruited via flyers and word of mouth from the general Colorado Springs area. Female subjects were not taking birth control medication and were tested 5 ± 3 days after the onset of menses in order to control for the potential influence of variations in sex hormone concentration throughout the menstrual cycle on neural and hemodynamic responses to orthostatic stress (Usselman et al, 2016), although recent evidence suggests that cardiovascular responses and tolerance to central hypovolemia are not affected by menstrual cycle phase (Convertino et al, 2019). For a detailed review of the effects of sex hormones and the menstrual cycle upon hemodynamic responses and time to presyncope, the reader is referred to Goswami et al (2019).…”
We investigated the influence of caffeinated coffee consumption on cardiovascular responses and tolerance to central hypovolemia in individuals habituated to caffeine. Thirteen participants completed three trials, consuming caffeinated coffee, decaffeinated coffee or water before exposure to central hypovolemia via lower body negative pressure (LBNP) to pre syncope. Tolerance to central hypovolemia was quantified as cumulative stress index (CSI: LBNP level multiplied by time; mmHg × min). Prior to the consumption of caffeinated coffee, decaffeinated coffee, and water, heart rate (HR: 62 ± 10, 63 ± 9 and 61 ± 8 BPM, respectively), stroke volume (SV: 103 ± 23, 103 ± 17 and 102 ± 18 mL/beat, respectively), and total peripheral resistance (TPR: 14.2 ± 3.0, 14.0 ± 3.0, and 14.3 ± 2.7 mmHg/L/min, respectively), were not different between trials (all P > 0.05). Mean arterial pressure (MAP) increased following consumption of all drinks (Post Drink) (Caffeinated coffee: from 86 ± 8 to 97 ± 7; Decaffeinated coffee: from 88 ± 10 to 94 ± 7; and Water: from 87 ± 10 to 96 ± 6 mmHg; all P = 0.0001) but was not different between trials (P = 0.247). During LBNP, HR increased (P = 0.000) while SV decreased (P = 0.000) relative to post drink values and TPR as unchanged (P = 0.109). HR, SV, and TPR were not different between trials (all P > 0.05). MAP decreased at pre syncope in all trials (60 ± 5, 60 ± 7, and 61 ± 6 mmHg; P < 0.001). LBNP tolerance was greater following caffeinated coffee (914 ± 309 mmHg × min) relative to decaffeinated coffee and water (723 ± 336 and 769 ± 337 mmHg × min, respectively, both P < 0.05). Tolerance to central hypovolemia was greater following consumption of caffeinated coffee in habituated users.
“…48 It is unlikely that this difference is attributed to sex hormones, as menstrual cycle phase was found to be unrelated to LBNP tolerance. 49 Fig. 3Defining the compensatory reserve.…”
Section: The Compensatory Reserve As a Reflection Of Delivery Of Oxygenmentioning
The concept of prolonged field care (PFC), or medical care applied beyond doctrinal planning timelines, is the top priority capability gap across the US Army. PFC is the idea that combat medics must be prepared to provide medical care to serious casualties in the field without the support of robust medical infrastructure or resources in the event of delayed medical evacuation. With limited resources, significant distances to travel before definitive care, and an inability to evacuate in a timely fashion, medical care during exploration spaceflight constitutes the ultimate example PFC. One of the main capability gaps for PFC in both military and spaceflight settings is the need for technologies for individualized monitoring of a patient’s physiological status. A monitoring capability known as the compensatory reserve measurement (CRM) meets such a requirement. CRM is a small, portable, wearable technology that uses a machine learning and feature extraction-based algorithm to assess real-time changes in hundreds of specific features of arterial waveforms. Future development and advancement of CRM still faces engineering challenges to develop ruggedized wearable sensors that can measure waveforms for determining CRM from multiple sites on the body and account for less than optimal conditions (sweat, water, dirt, blood, movement, etc.). We show here the utility of a military wearable technology, CRM, which can be translated to space exploration.
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