Reduced Defense of Central Blood Volume During Acute Lower Body Negative Pressure–Induced Hypovolemic Circulatory Stress in Aging Women

Physiology of Human Hemorrhage and Compensation

Hemorrhage is a leading cause of death following traumatic injuries in the United States. Much of the previous work in assessing the physiology and pathophysiology underlying blood loss has focused on descriptive measures of hemodynamic responses such as blood pressure, cardiac output, stroke volume, heart rate, and vascular resistance as indicators of changes in organ perfusion. More recent work has shifted the focus toward understanding mechanisms of compensation for reduced systemic delivery and cellular utilization of oxygen as a more comprehensive approach to understanding the complex physiologic changes that occur following and during blood loss. In this article, we begin with applying dimensional analysis for comparison of animal models, and progress to descriptions of various physiological consequences of hemorrhage. We then introduce the complementary side of compensation by detailing the complexity and integration of various compensatory mechanisms that are activated from the initiation of hemorrhage and serve to maintain adequate vital organ perfusion and hemodynamic stability in the scenario of reduced systemic delivery of oxygen until the onset of hemodynamic decompensation. New data are introduced that challenge legacy concepts related to mechanisms that underlie baroreflex functions and provide novel insights into the measurement of the integrated response of compensation to central hypovolemia known as the compensatory reserve. The impact of demographic and environmental factors on tolerance to hemorrhage is also reviewed. Finally, we describe how understanding the physiology of compensation can be translated to applications for early assessment of the clinical status and accurate triage of hypovolemic and hypotensive patients. © 2021 American Physiological Society. Compr Physiol 11:1531-1574, 2021.

Cytochrome P450 Epoxygenase 2J2 Protects Against Lung Ischemia/Reperfusion Injury by Activating the P13K/Akt/GSK-3-β/NF-kB Signaling Pathway During Deep Hypothermic Low Flow in Mice

BACKGROUND

Cytochrome P450 epoxygenase 2J2 (CYP2J2) metabolizes arachidonic acid to epoxyeicosatrienoic acids, which exert anti-inflammatory effects and alleviate oxidative stress in the cardiovascular system. Our previous work revealed that CYP2J2 is expressed in pulmonary artery endothelial cells. It was therefore hypothesized that CYP2J2 overexpression may prevent lung ischemia/reperfusion injury (LIRI) in 3-week-old C57BL/6 mice during deep hypothermic low flow (DHLF). This study aimed to establish whether CYP2J2 protects against LIRI and the mechanisms of CYP2J2 overexpression during DHLF in mice. The aim of this study was to explore the effects of DHLF on lung tissue in mice and to find out the regularity of this process, so as to provide theoretical data for lung tissue protection in children undergoing this process in clinic.

METHODS

A 3-week-old C57BL/6 mouse model was used to mimic LIRI conditions during DHLF by clamping the left pulmonary artery and left main bronchus for 120 min, followed by reperfusion for 2 h. The body temperature of the mice was maintained between 18°C and 19°C to induce DHLF.

RESULTS

During DHLF, lung ischemia/reperfusion increased the left lung wet/dry weight, the left lung weight/body weight ratio, the protein concentration in bronchoalveolar lavage fluid, and the concentration of proinflammatory mediators in the lungs, including interleukin (IL)-1, IL-8, and necrosis factor (NF)-α, and decreased the concentration of the anti-inflammatory mediator IL-10. Furthermore, activation of NF-κB p65 and degradation of IKBα were remarkably increased in lung tissues after ischemia/reperfusion. The CYP2J2 overexpression group showed the opposite results (P < 0.05), and p-Akt1 and p-GSK-3β expression were significantly higher in the CYP2J2 overexpression group (P < 0.05). Moreover, the changes in IL-1, IL-8, tumor necrosis factor-α, IL-10, p-Akt1, p-GSK-3β, NF-κB p65, and IKBα were reversed in the Akt1 gene heterozygous knockout group, and lung damage was significantly higher in the Akt1 gene heterozygous knockout group than in the CYP2J2 overexpression group. CYP2J2 overexpression can protect against LIRI, whereas Akt1 gene heterozygous knockout in mice can abolish this protective effect.

CONCLUSIONS

CYP2J2 overexpression can protect against LIRI by activating the P13K/Akt/GSK-3β/NF-kB signaling pathway during DHLF. Thus, changing CYP2J2 expression can be a novel strategy for the prevention and treatment of LIRI during DHLF.

Age-Dependent Association of Occult Hypoperfusion and Outcomes in Trauma

BACKGROUND

Occult hypoperfusion (OH), or global hypoperfusion with normal vital signs, is a risk factor for poor outcomes in elderly trauma patients. We hypothesized that OH is associated with worse outcomes than shock in both young and elderly trauma patients.

METHODS

We conducted a single-center cohort study of adult (16 years or older) trauma patients from 2016 to 2018 with base excess measured on arrival. Perfusion states were defined as shock if heart rate was >120 beats/min or systolic blood pressure was <90 mmHg; OH if base excess was < -2 mmol/L, heart rate was <120 beats/min, and systolic blood pressure was >90 mmHg; and normal for all others. Patients were stratified as young (younger than 55 years) or elderly (55 years or older). Bayesian regression was used to assess the relationship between arrival perfusion state and mortality or serious complication.

RESULTS

Of 3,126 included patients, 808 were elderly. Rates of shock (33% and 31%) and OH (25% and 23%) were similar in young and elderly patients, respectively. OH on arrival was associated with higher odds of mortality or serious complication than normal perfusion, regardless of age group. Compared with shock, OH was associated with an odds ratio of 1.21 (95% CI, 0.97 to 1.52, posterior probability 96%) for poor outcomes in elderly patients and an odds ratio of 0.52 (95% CI, 0.42 to 0.65, posterior probability <1%) for poor outcomes in younger patients. Findings were similar on sensitivity analysis, excluding shock patients with base excess ≥ -2 mmol/L.

CONCLUSIONS

In elderly but not younger patients, OH is associated with worse outcomes than shock. Although shock parameters might need to be redefined in elderly patients, more attention is necessary for the diagnosis and treatment of all hypoperfused states in this age group.

Slower lower limb blood pooling in young women with orthostatic intolerance

NEW FINDINGS

What is the central question of this study? Orthostatic stress is mostly caused by venous blood pooling in the lower limbs. Venous distension elicits sympathetic responses, and increased distension speed enhances the cardiovascular response. We examine whether lower limb blood pooling rate during lower body negative pressure is linked to orthostatic intolerance. What is the main finding and its importance? A similar amount of blood was pooled in the lower limb, but at a slower rate in women who developed signs of orthostatic intolerance. The difference in blood pooling rate increased with orthostatic stress and was most prominent at a presyncope-inducing level of lower body negative pressure. The findings have implications for the pathophysiology as well as treatment of orthostatic intolerance. Vasovagal syncope is common in young women, but its aetiology remains elusive. Orthostatic stress-induced lower limb blood pooling is linked with central hypovolaemia and baroreceptor unloading. Venous distension in the arm elicits a sympathetic response, which is enhanced with more rapid distension. Our aim was to study both the amount and the speed of lower limb pooling during orthostatic stress and its effects on compensatory mechanisms to maintain cardiovascular homeostasis in women with orthostatic intolerance. Twenty-seven healthy women, aged 20-27 years, were subjected to a lower body negative pressure (LBNP) of 11-44 mmHg. Five women developed symptoms of vasovagal syncope (orthostatic intolerant) and were compared with the remaining women, who tolerated LBNP well (orthostatic tolerant). Lower limb blood pooling, blood flow and compensatory mobilization of venous capacitance blood were measured. Lower body negative pressure induced equal lower limb blood pooling in both groups, but at a slower rate in orthostatic intolerant women (e.g. time to 50% of total blood pooling, orthostatic intolerant 44 ± 7 s and orthostatic tolerant 26 ± 2 s; P < 0.001). At presyncope-inducing LBNP, the mobilization of venous capacitance blood was both reduced (P < 0.05) and much slower in orthostatic intolerant women (P = 0.0007). Orthostatic intolerant women elicited blunted arterial vasoconstriction at low-grade LBNP, activating only cardiopulmonary baroreceptors, while orthostatic tolerant women responded with apparent vasoconstriction (P < 0.0001). In conclusion, slower lower limb blood pooling could contribute to orthostatic intolerance in women. Mobilization of venous capacitance blood from the peripheral to the central circulation was both slower and decreased; furthermore, reduced cardiopulmonary baroreceptor sensitivity was found in women who developed orthostatic intolerance. Further studies including women who experience syncope in daily life are needed.

Precordial vibrations provide noninvasive detection of early-stage hemorrhage

Graded lower-body negative pressure was used to create a hemodynamic response similar to hemorrhage. Echocardiogram measurements showed a maximal reduction of 32.4% in stroke volume. Analysis of systolic time intervals, such as pre-ejection period and left ventricular ejection time (LVET), derived from a seismocardiogram (SCG), were demonstrated to be more sensitive in detection of early-stage hemorrhage compared with pulse pressure, heart rate, and the amplitude features extracted from SCG. In particular, the LVET and pre-ejection period/LVET features, extracted from SCG, were significantly different between, and correlated with, the different stages of lower-body negative pressure (r = 0.9 and 0.88, P < 0.05), for 32 subjects. These results suggest a portable, cost-effective solution for identification of mild or moderate hemorrhage using accelerometers.

Decreased circulatory response to hypovolemic stress in young women with type 1 diabetes

OBJECTIVE

Diabetes is associated with hemodynamic instability during different situations involving acute circulatory stress in daily life. Young men with type 1 diabetes have been shown to have impaired circulatory response to hypovolemic stress. The effect of type 1 diabetes on cardiovascular response to hypovolemia in young women is unknown, however.

RESEARCH DESIGN AND METHODS

Lower body negative pressure of 30 cm H2O was used to create rapid hypovolemic stress in 15 young women with type 1 diabetes (DW) and 16 healthy women (control subjects [C]). Compensatory mobilization of venous capacitance blood (capacitance response) and net fluid absorption from tissue to blood were measured with a volumetric technique. Overall cardiovascular responses and plasma norepinephrine levels were measured.

RESULTS

Capacitance response was reduced (DW, 0.67 ± 0.05; C, 0.92 ± 0.06) and developed slower in DW (P < 0.01). Capacitance response was further reduced with increasing levels of HbA1c. Fluid absorption was almost halved in DW (P < 0.01). The initial vasoconstrictor response was reduced and developed slower in DW (P < 0.05). Arterial vasoconstriction was further reduced in the presence of microvascular complications (P < 0.05).

CONCLUSIONS

DW present with decreased and slower mobilization of venous capacitance blood and decreased net fluid absorption from tissue to blood during hypovolemic circulatory stress. Collectively, this indicates that DW are prone to hemodynamic instability, especially in the presence of microvascular complications and poor glycemic control.

Thirst response to acute hypovolaemia in healthy women and women prone to vasovagal syncope

The present study measured self-perceived thirst and plasma angiotensin II (ATII) concentrations during graded hypovolaemic stress, induced by lower body negative pressure (LBNP), to elucidate the dependence of thirst on haemodynamics. A total of 24 women aged between 20 and 36 (mean age, 23) years rated their thirst on a visual analogue scale, graded from 0 to 100, when LBNP of 20, 30 and 40 mmHg was applied. Half of the women had a history of vasovagal syncope (VVS). The results showed that the thirst score increased three-fold when LBNP was applied, from 11 (median; 25th-75th percentiles, 9-25) to 34 (27-53; P<0.001). The women in the VVS group had twice as great an increase as those without a history of VVS (P<0.02). The plasma ATII concentration increased significantly in response to LBNP, both in the VVS group and in the control group, but the changes did not correlate with thirst. Application of LBNP decreased systolic and mean arterial pressures, cardiac output and pulse pressure (P<0.001 for all), but thirst correlated only with increase in heart rate and, independently, with reduction of mean arterial pressure. In conclusion, thirst and ATII increase in response to hypovolaemic stress, but are not statistically related. The haemodynamic parameter that was most strongly related to thirst was tachycardia.