Dietz, Niki M.

Abstract Introduction Detecting and estimating blood volume loss are important for the diagnosis and management of life-threatening hemorrhage. There is a direct relationship between percent blood volume loss (BVL%) and falling in stroke volume (SV); furthermore, there is a direct relationship between diminishing SV and the corrected flow time of the carotid artery (ccFT). Therefore, we hypothesized that change in ccFT (ccFTΔ) would directly relate with BVL% in a phlebotomy paradigm. Materials and Methods A prospective, convenience sample of healthy volunteers was studied. Volunteers underwent blood donation protocol in a physiology lab. A wireless, wearable Doppler ultrasound system monitored ccFT during blood donation. Traditional vital signs were also measured throughout. The relationship between traditional vital signs as well as ccFTΔ were compared to BVL% using 2 mixed linear regression models. Results Data from 20 subjects and blood donations comprising 46,123 cardiac cycles are included. On average 413.5 ± 39 mL of blood were drawn representing 8.6 ± 1.6% BVL%. There was a significant and strong linear relationship between BVL% and ccFTΔ during blood draw with the regression slope demonstrating that for every 1 ms reduction in ccFT there was a 0.4% reduction in blood volume. Conclusions In healthy subjects and when blood loss is the sole mechanism for falling SV, we estimate that for every 1 ms reduction in ccFT, there is an approximate 0.4 BVL%. Validation of these results in patients with traumatic hemorrhage is underway.

The key vascular signals regulating muscle blood flow during exercise are unclear. We tested the hypothesis that in young adults, leg blood flow (LBF) during exercise would be reduced independently by pharmacological inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX). In 13 young adults, we measured heart rate (ECG), blood pressure (femoral artery and vein), blood gases, and venous oxygen saturation (indwelling oximeter) during steady‐state dynamic (60/min) knee extension exercise. LBF was determined by Doppler ultrasound of the femoral artery. Exercise (20W) increased leg vascular conductance (LVC) from 4± 1 to 33± 3 ml/min/100mmHg. The NOS inhibitor L‐NAME reduced LVC by ∼10% to 30± 1. Infusion of the COX inhibitor, ketoralac (n=9), increased LVC from 30± 1 to 34± 2. L‐NAME infusion reduced venous oxygen saturation from 48 to 38%, suggesting greater extraction to maintain oxygen consumption. Control intra‐arterial infusions of acetylcholine (ACH) and nitroprusside (NTP) tested inhibitor efficacy. The change in LVC to two doses of ACh was blunted 53 and 15%, by L‐NAME+ketorolac infusion. In contrast, the LVC responses to NTP were unaltered. These data implicate a modest role for NO‐mediated leg exercise hyperemia in adults and a potentially modest LBF restraint by vasoconstrictor prostaglandins. Funded by the NIH HL‐46493 and the Minnesota Obesity Center.

The augmented skeletal muscle vasodilation during hypoxic exercise is due, in part, to an intensity dependent contribution of beta‐receptor activation. We tested the hypothesis that an adenosine mediated vasodilation increases with hypoxic exercise intensity. Subjects (N=10) breathed hypoxic gas to titrate arterial O 2 saturation to 80% while remaining normocapnic (re‐breath system). Consecutive bouts of forearm exercise (10% and 20% of maximum) were performed under normoxia and hypoxia during phentolamine (alpha‐receptor antagonist) alone and the combination of phentolamine / aminophyline (adenosine receptor antagonist). Exogenous adenosine administered before and after aminophyline established effective receptor blockade. Forearm vascular conductance (FVC; ml/100mmHg/min) was calculated from blood flow (ml/min) and blood pressure (mmHg). During hypoxic exercise, the rise in FVC (Δ from baseline) during phentolamine was 314±30 and 470±37 (10% and 20% respectively). ΔFVC was similar at both 10% (349±45; P=0.3) and 20% (536±47; P=0.2) exercise during hypoxia with combined phentolamine / aminophyline. This occurred despite significant reduction in ΔFVC to exogenous adenosine following phentolamine / aminophyline (P<0.03). Our data suggests that in the absence of overlying vasoconstriction, adenosine does not contribute to the augmented hypoxic exercise hyperemia in humans.

The vascular effects of prostacyclin and contribution of endothelial derived nitric oxide (NO) to prostacyclin mediated dilation are unknown in aging. A pilot study was performed in healthy older subjects (n=6, 61–73 years old) and younger controls (n=6, 19–45 years old) that were matched for BMI. Subjects underwent brachial artery catheterization and received escalating doses of epoprostenol (PGI 2 2.5, 5, 10 ng/100ml forearm tissue/min) followed by sodium nitroprusside (SNP 0.25, 0.5, 1 mcg/100ml forearm tissue/min) after PGI 2 washout. SNP and PGI 2 were repeated after NO synthase inhibition with N G ‐Monomethyl‐L arginine acetate (L‐NMMA 1mg/min after 50mg load). Using venous occlusion plethysmography, forearm vascular conductance (FVC) was calculated using mean arterial pressure (MAP), as FVC= (FBF/MAP X 100). Percent change in FVC from baseline after PGI 2 was lower in the aging individuals (47, 160, 227 % change FVC) compared to younger controls (85, 218, 327 %change FVC) (p=0.039), L‐NMMA reduced baseline FVC similarly, and the dilator response to PGI 2 was no longer significant between groups. SNP effects were not significant between groups and unaffected by L‐NMMA. This data suggests a reduced effect to prostacyclin in the aging individual may be due to a reduction in the contribution of endothelial derived NO. Support provided by HL‐46493 and UL1‐RR24150.

Heart failure (HF) patients have reduced cardiac reserve and increased work of breathing (WOB)., Increased demand for locomotor muscle blood flow (i.e. activities of daily living) may result in competition between respiratory and locomotor vascular beds. We hypothesized that HF patients would demonstrate improved locomotor blood flow with respiratory muscle unloading during moderate activity. We studied 10 HF patients (EF=30±8%) and 10 controls during two 10 min. cycling sessions at 60% VO 2 max. Session 1 (S1): 5 min of normal breathing (NB) and 5 min respiratory muscle unloading with a ventilator. Session 2 (S2): 5 min NB and 5 min of respiratory muscle loading with inspiratory resistance. Measurements included: leg blood flow (LBF), cardiac output (CO), and inspiratory pleural pressure time integral as an index of the WOB. During S1, WOB was reduced similarly in both groups (HF: 50±60%; CTL: 50±56%, p<0.05 for both). In HF, CO was reduced (9.6±1.4 vs 8.2±2.1 L/min, p<0.05) and LBF increased (4.9±2.6 vs 6.2±2.9 L/min, p<0.01). In CTL, there was no change in CO (14.7±3.1 vs 14.8±5.0 L/min) or LBF (10.9±5.6 vs 10.3±5.2 L/min). During S2, WOB was increased similarly in both groups (HF: 111±75%, CTL: 110±85%, p<0.01 for both). In HF, there was no change in CO (10.0±1.3 vs 10.3±2.5 L/min) or LBF (5.0±1.8 vs 4.9±1.4 L/min). In CTL, CO increased (15.4±4.3 vs 16.9±4.7 L/min, p<0.01) with no change in LBF (10.7±5.3 vs 10.3±5.6 L/min). These data suggest that the respiratory muscles of HF patients preferentially recruit blood flow away from the locomotor muscles during moderate intensity, whole body exercise.

Aging humans exhibit lower exercise blood flow (exercise hyperemia), but the vascular mechanisms mediating this remain unknown. In aging humans, agonist infusions suggest endothelial dysfunction, due in part to less nitric oxide (NO) and prostaglandin (PG) mediated vasodilation. Notably, in young adults inhibition of NO or PGs decreases exercise hyperemia by 20% and 12%, respectively. We tested the hypothesis that aging humans would exhibit an attenuated reduction in exercise hyperemia following inhibition of NO or PGs, due to less vasodilation from these signals. Eleven older males and females (55–76 years) performed dynamic forearm exercise for 20 minutes. Forearm blood flow (FBF) was measured beat‐to‐beat with Doppler ultrasound while saline or drugs were infused sequentially via brachial artery catheter in the exercising forearm. After achieving steady state exercise (min 5), L‐NAME (25 mg) was infused for 5 minutes to inhibit NO synthase. After 2 more minutes of exercise (saline), ketorolac (6 mg) was infused for 5 minutes to inhibit PGs, followed by 3 more minutes of exercise with saline. L‐NAME reduced exercise hyperemia by 13–18% in older subjects, whereas ketorolac had no net effect on blood flow. In the reverse drug order (n=6), ketorolac did not effect FBF, but L‐NAME reduced FBF by 10–13%. Our results suggest that in aging humans, the contribution of NO to exercise hyperemia may be modestly reduced, but the role of PGs mediating vasodilation is completely lost. Loss of vasodilating PGs along with diminished NO may contribute to lower blood flow in aging humans. Grant support: HL‐69692, HL‐46493, and GCRC RR‐00585.

Hypercapnic cerebrovascular reactivity is decreased in obstructive sleep apnea and congestive heart disease patients. Ventilatory response to hypercapnia and/or hypoxia is blunted in apnea divers, whereas cerebrovascular reactivity to hypercapnia is presently unknown. We hypothesized that cerebrovascular reactivity to hypercapnia is increased in elite apnea divers, compared to control subjects, as a part of the O 2 conserving mechanism to maintain adequate brain O 2 supply. In the present study we determined cerebrovascular reactivity by measuring change in middle cerebral artery velocity per mmHg change in P ET CO 2 in response to two hyperoxic hypercapnia rebreathing maneuvers in seven elite apnea divers and seven control subjects. Ventilatory response to hypercapnia was blunted in divers compared to controls. Hypercapnia‐induced ventilation increase was significantly lower in divers due to lower respiratory frequency. Cerebrovascular reactivity was unchanged in both groups (3.7±1.4 vs 3.4±1.3 %/mmHg P ET CO 2 in divers and controls) and the same was found for cerebral vascular resistance and a time for middle cerebral artery velocity recovery to baseline value. Results indicate that the regulation of the cerebral circulation in response to hypercapnia shows no difference between divers and controls serving as protective mechanism against chronic intermittent cerebral hypoxia/hypercapnia. This study was supported by Croatian Ministry of Science, Education and Sports.

An open-circuit (OpCirc) acetylene uptake cardiac output (Q˙t) method was modified for use during exercise. Two computational techniques were used. OpCirc1 was based on the integrated uptake vs. end-tidal change in acetylene, and OpCirc2 was based on an iterative finite difference modeling method. Six subjects [28–44 yr, peak oxygen consumption (V˙o2) = 120% predicted] performed cycle ergometry exercise to compareQ˙t using OpCirc and direct Fick methods. An incremental protocol was repeated twice, separated by a 10-min rest, and subsequently subjects exercised at 85–90% of their peak work rate. Coefficient of variation of the OpCirc methods and Fick were highest at rest (OpCirc1, 7%, OpCirc2, 12%, Fick, 10%) but were lower at moderate to high exercise intensities (OpCirc1, 3%, OpCirc2, 3%, Fick, 5%). OpCirc1 and OpCirc2 Q˙t correlated highly with Fick Q˙t( R2= 0.90 and 0.89, respectively). There were minimal differences between OpCirc1 and OpCirc2 compared with Fick up to moderate-intensity exercise (<70% peakV˙o2); however, both techniques tended to underestimate Fick at >70% peakV˙o2. These differences became significant for OpCirc1 only. Part of the differences between Fick and OpCirc methods at the higher exercise intensities are likely related to inhomogeneities in ventilation and perfusion matching ( R2= 0.36 for Fick − OpCirc1 vs. alveolar-to-arterial oxygen tension difference). In conclusion, both OpCirc methods provided reproducible, reliable measurements ofQ˙t during mild to moderate exercise. However, only OpCirc2 appeared to approximate FickQ˙t at the higher work intensities.

Venous occlusion plethysmography is a simple but elegant technique that has contributed to almost every major area of vascular biology in humans. The general principles of plethysmography were appreciated by the late 1800s, and the application of these principles to measure limb blood flow occurred in the early 1900s. Plethysmography has been instrumental in studying the role of the autonomic nervous system in regulating limb blood flow in humans and important in studying the vasodilator responses to exercise, reactive hyperemia, body heating, and mental stress. It has also been the technique of choice to study how human blood vessels respond to a variety of exogenously administered vasodilators and vasoconstrictors, especially those that act on various autonomic and adrenergic receptors. In recent years, plethysmography has been exploited to study the role of the vascular endothelium in health and disease. Venous occlusion plethysmography is likely to continue to play an important role as investigators seek to understand the physiological significance of newly identified vasoactive factors and how genetic polymorphisms affect the cardiovascular system in humans.