We tested the hypothesis that transmission of arterial pressure to brain tissue oxygenation is low under conditions of arterial pressure instability. Two experimental models of hemodynamic instability were used in healthy human volunteers; (1) oscillatory lower body negative pressure (OLBNP) (N = 8; 5 male, 3 female), and; (2) maximal LBNP to presyncope (N = 21; 13 male, 8 female). Mean arterial pressure (MAP), middle cerebral artery velocity (MCAv), and cerebral tissue oxygen saturation (ScO2) were measured non-invasively. For the OLBNP protocol, between 0 and -60 mmHg negative pressure was applied for 20 cycles at 0.05 Hz, then 20 cycles at 0.1 Hz. For the maximal LBNP protocol, progressive 5 min stages of chamber decompression were applied until the onset of presyncope. Spectral power of MAP, mean MCAv, and ScO2 were calculated within the VLF (0.04-0.07 Hz), and LF (0.07-0.2 Hz) ranges, and cross-spectral coherence was calculated for MAP-mean MCAv, MAP-ScO2, and mean MCAv-ScO2 at baseline, during each OLBNP protocol, and at the level prior to pre-syncope during maximal LBNP (sub-max). The key findings are (1) both 0.1 Hz OLBNP and sub-max LBNP elicited increases in LF power for MAP, mean MCAv, and ScO2 (p ≤ 0.08); (2) 0.05 Hz OLBNP increased VLF power in MAP and ScO2 only (p ≤ 0.06); (3) coherence between MAP-mean MCAv was consistently higher (≥0.71) compared with MAP-ScO2, and mean MCAv-ScO2 (≤0.43) during both OLBNP protocols, and sub-max LBNP (p ≤ 0.04). These data indicate high linearity between pressure and cerebral blood flow variations, but reduced linearity between cerebral tissue oxygenation and both arterial pressure and cerebral blood flow. Measuring arterial pressure variability may not always provide adequate information about the downstream effects on cerebral tissue oxygenation, the key end-point of interest for neuronal viability.
Introduction: Remote ischemic preconditioning (RIPC) is characterized by the cyclic application of limb blood flow restriction and reperfusion, and has been shown to protect the brain during a subsequent ischemic insult. Blood flow restriction exercise (BFRE) is a novel training modality that similarly combines bouts of blood flow restriction and reperfusion with low-intensity exercise and thus could potentially emulate the protection demonstrated by RIPC. One concern with clinical application of BFRE is the potential for an augmented exercise pressor reflex, resulting in an unsafe rise in sympathetic outflow. Due to the use of lower workloads, however, we hypothesized that BFRE would exhibit an attenuated increase in sympathetic outflow, mean arterial pressure (MAP), and brain blood flow. Methods: 11 subjects (6M/5F; age 28±2 yrs) underwent 2 leg press exercise interventions separated by ≥1 month: 1. BFRE - 220 mmHg thigh occlusion during 4 cycles x 5-min of exercise, consisting of 3 x 10 reps at 20% of 1 rep-max (1RM), and; 2. Conventional exercise (CE) - 4 cycles x 5-min of exercise, consisting of 3 x 10 reps at 65% 1RM without occlusion. 5-min of rest and reperfusion (for BFRE) followed each cycle. MAP, mean middle cerebral artery velocity (MCAv), cerebral oxygen saturation (ScO 2 ), and plasma norepinephrine concentrations ([NE]) were compared between trials. Results: MAP increased with exercise under both conditions (P<0.001) and was higher (P≤0.03) with CE vs. BFRE for each cycle (1. 107±3 vs. 103±2 mmHg, 2. 107±2 vs. 103±2 mmHg, 3. 107±2 vs. 103±2 mmHg, 4. 107±2 vs. 104±2 mmHg). MCAv and ScO 2 (N=10) increased over time (P<0.001) with no difference between conditions for MCAv (P=0.83), while ScO 2 was higher for CE vs. BFRE (65±1 vs. 62±1%, P=0.05). Plasma [NE] (N=7) increased over time for both conditions (P=0.08) and was higher for CE vs. BFRE (757±103 vs. 524±71 pg/ml, P=0.09). Conclusions: BFRE elicited an attenuated sympathetic response compared to CE as evidenced by lower MAP and plasma [NE]. The lower ScO 2 (i.e., increased oxygen extraction) and no difference in MCAv responses suggests greater cerebral metabolic demand with BFRE. These findings indicate that BFRE could be explored as an alternative to CE in the clinical setting such as stroke-rehabilitation.
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