In healthy animals under normotensive conditions (N), contracting skeletal muscle perfusion is regulated to maintain microvascular O2 pressures (Pmv) at levels commensurate with O2 demands. Hypovolemic hypotension (H) impairs muscle contractile function; we tested whether this condition would alter the matching of O2 delivery (Q̇o2) to O2 utilization (V̇o2), as determined by Pmv at the onset ofmuscle contractions. Pmv in the spinotrapezius muscles of seven female Sprague-Dawley rats (280 ± 6 g) was measured every 2 s across the transition from rest to 1-Hz twitch contractions. Measurements were made under N (mean arterial pressure, 97 ± 4 mmHg) and H (induced by arterial section; mean arterial pressure, 58 ± 3 mmHg, P < 0.05) conditions; Pmv profiles were modeled using a multicomponent exponential fitted with independent time delays. Hypotension reduced muscle blood flow at rest (24 ± 8 vs. 6 ± 1 ml−1·min−1·100 g−1 for N and H, respectively; P < 0.05) and during contractions (74 ± 20 vs. 22 ± 4 ml−1·min−1·100 g−1 for N and H, respectively; P < 0.05). H significantly decreased resting Pmv and steady-state contracting Pmv(19.4 ± 2.4 vs. 8.7 ± 1.6 Torr for N and H, respectively, P < 0.05). At the onset of contractions, H reduced the time delay (11.8 ± 1.7 vs. 5.9 ± 0.9 s for N andH, respectively, P < 0.05) before the fall in Pmv and accelerated therate of Pmv decrease (time constant, 12.6 ± 1.4 vs. 7.3 ± 0.9 s for N and H, respectively, P < 0.05). Muscle V̇o2 was reduced by 71% at rest and 64% with contractions in H vs. N, and O2 extraction during H averaged 78% at rest and 94% during contractions vs. 51 and 78% in N. These results demonstrate that H constrains the increase of skeletal muscle Q̇o2 relative to that of V̇o2 at the onset of contractions,leading to a decreased Pmv. According to Fick's law, this scenario will decrease blood-myocyte O2 flux, thereby slowing V̇o2 kinetics and exacerbating the O2 deficit generated at exercise onset.
- oxygen delivery
- oxygen utilization
- rat spinotrapezius muscle
in the muscles of healthy individuals, blood flow (Q̇) and therefore oxygen delivery (Q̇o2) increases rapidly at the onset of exercise. The Q̇o2 dynamics are so rapid that they either match or exceed the rate of O2 utilization (V̇o2) such that the microvascular pressure of O2 (Pmv) and the venous O2 content are either unchanged or may even increase above resting values for 10–15 s before they decrease exponentiallytoward lower steady-state values (3, 4, 19). Because Pmv represents the driving force for blood-muscle O2 movement, it is crucial to maintain Pmv as high as possible both across the rest-exercise transition and during the subsequent steady state.
In disease states such as diabetes (6) and chronic heart failure (CHF) (15), there is a mismatch of Q̇o2 and V̇o2 dynamics such that Pmv falls below values found in healthy muscles. Considering that V̇o2 = Do2 × (Pmv− mitochondrial Po2), where Do2 is the effective muscle O2 diffusing capacity, the reduced Pmv contributes to the slowed V̇o2 kinetics typical of these conditions (23, 49, 61). The ability to increase muscle Q̇o2 rapidly at the onset of contractions is dependent on the ability to increase the product of the pressure differential across the muscle (ΔP) and vascular conductance within the muscle vascular bed (i.e., ΔQ̇ = ΔP × Δvascular conductance). Impairment of either factor would be expected to compromise Q̇o2 unless there was a reciprocal change in its counterpart.
Numerous conditions exist that lead to profound systemic hypotension. These include physiological conditions, such as prolonged exercise leading to dehydration (12), bed rest (22), and microgravity during space flight (14), as well as pathological conditions, such as autonomic dysfunction (29, 58, 72), chronic fatigue syndrome (CFS) (48, 62), fibromyalgia (63), and cardiovascular diseases [e.g., mitral valve prolapse syndrome (9, 28), congestive heart failure (during exercise, Ref. 38), atrial fibrillation (1), hyponatremic CHF patients treated with captopril (45)]. In addition, a significant minority of otherwise healthy people experience occasional or frequent attacks of presyncope or syncope (41). All of these conditions are associated with fatigue and exercise intolerance, and it is probable that the associated hypotension impairs exercising muscle Q̇o2 and disrupts blood-muscle O2 transfer, in part, due to the reduced Pmv.
The purpose of the present investigation was to determine the effects of systemic hypovolemic hypotension induced by arteriosection (i.e., withdrawal of blood from the carotid artery) on the dynamic matching of Q̇o2 and V̇o2 at the onset of muscle contractions as assessed from the profile of Pmv as well as that of muscle blood flow and O2 uptake. We hypothesized that relative to the normotensive condition, hypotension would impair Q̇o2 at the onset of contractions such that Pmv would fall more rapidly and be reduced below its normotensive value across the rest-contractions transition and in the subsequent steady state.
MATERIALS AND METHODS
Six-month-old female Sprague-Dawley rats (280 ± 6 g, n = 14) were housed individually at 23°C and were maintained on a 12:12-h light-dark cycle. All rats were fed rat chow and water ad libitum. All procedures were approved by the Institutional Animal Care and Use Committee at Kansas State University.
Surgical preparation for Pmvmeasurements.
Rats were anesthetized with pentobarbital sodium (40 mg/kg ip, to effect). The carotid artery was cannulated using PE-50 tubing (Intra-Medic polyethylene tubing; Clay Adams, Sparks, MD). This provided a route of access for infusion of the phosphorescent probe at 15 mg/kg, monitoring of arterial blood pressure (Digi-Med BPA model 200, Louisville, KY), blood sampling, and blood withdrawal for induction of hypotension. Blood withdrawal for blood Po2 determination (Nova Stat Profile M, Waltham, MA), and hematocrit (Adams Micro-Hematocrit reader, Clay Adams, Parsipanny, NJ) was performed immediately after the stimulation periods. The left spinotrapezius was exposed as described previously (2, 15). Briefly, the skin and overlying fascia not intimately connected to the myocytes were removed carefully from the dorsal aspect of the muscle. Vascular and neural tissues that branch primarily from the scapular origin of the spinotrapezius were left undisturbed. Stainless steel electrodes were used to stimulate the muscle. The cathode was placed in close proximity to the motor point (0.5–1.0 cm caudal to the scapula), whereas the anode was sutured in place at the caudal edge of the muscle, near the fourth thoracic vertebra. Stimulation conditions (i.e., voltage and placement of electrodes) were maintained constant between stimulation periods (i.e., for normotensive and hypotensive conditions). The phosphor palladium meso-tetra-(4-carboxyphenyl)-porphyrin dendrimer (R2; Oxygen Enterprises, Philadelphia, PA) was infused at a dose of 15 mg/kg through the arterial cannula ∼15 min before each experiment.
The muscle was kept moist throughout the experiment using a Krebs-Henseleit bicarbonate-buffered solution equilibrated with 5% CO2/95% N2 at 38°C. The muscle was stimulated to contract at 1 Hz (∼4–6 V, 2.0-ms pulse duration, twitch contractions) for 4 min with a Grass S88 stimulator. After completion of the control stimulation period, ∼5 ml of blood (4.8 ± 0.3 ml) was withdrawn to induce a fall in mean arterial pressure (MAP) of ∼40 mmHg, and the rats were then allowed to stabilize for 30 min. This provided a rest period of at least 45 min between stimulation bouts to preclude “priming” effectson the muscle Pmv response (7). After the stabilization period, the muscle was stimulated for a second bout (parameters held constant),and Pmv was measured over the same portion of the muscle asduring the first protocol. Pmv measurements were recorded every 2 s throughout rest and both stimulation periods.
After completion of the experiment, each rat was euthanized with an overdose of anesthesia (pentobarbital sodium, 50 mg/kg ia).
Pmvmeasurements and calculations.
A PMOD 1000 frequency domain phosphorimeter probe (Oxygen Enterprises, Philadelphia, PA) was positioned ∼2 mm above the spinotrapezius, as described by Bailey et al. (2). A light guide contained within the probe focuses excitation light (524 nm) on the medial region of the exposed spinotrapezius (∼2.0-mm diameter, to ∼500 μm deep). The PMOD 1000 uses a sinusoidal modulation of the excitation light at frequencies between 100 Hz and 20 kHz, which allows phosphorescence lifetime measurements from 10 μs to ∼2.5 ms. In the single frequency mode, 10 scans (10 ms/scan for 100 ms total) were used to acquire the resultant lifetime of the phosphorescence (700 nm) and repeated every 2 s (for review, see Ref. 66). The phosphorescence lifetime was obtained computationally based on the decomposition of data vectors to a linearly independent set of exponentials (67).
The Stern-Volmer relationship allows the calculation of Pmv from a measured phosphorescence lifetime using the following equation (57): (1) where kQ is the quenching constant (mmHg/s) and t° and t are the phosphorescence lifetimes in the absence of O2 and at the ambient O2 pressure, respectively. For R2, with in vitro conditions similar to those found in the blood, kQ is 409 mmHg/s and t° is 601 μs (36). R2 is tightly bound to albumin in the plasma and is negatively charged. These properties, in combination with the extremely high albumin reflection coefficients in skeletal muscle (for review, see Ref. 50), ensure that the Po2 measurements emanate from the plasma within the microvasculature rather than the surrounding muscle tissue (47). Unlike near infrared spectroscopy, the Pmv signal is not subject to contamination from intramyocyte myoglobin. Moreover, Pmv represents the O2 pressure head for diffusive blood-myocyte O2 movement. The phosphorescence lifetime is insensitive to probe concentration, excitation light intensity, and absorbance by other chromophores in the tissue (57). The effects of pH and temperature are negligible within the normal physiological range, which was maintained herein (36, 46). In addition, as most of the O2 off-loading occurs on the linear portion of the O2 dissociation curve and all Pmv measurements are on that linear portion, it follows that Pmv is proportional to Q̇o2/V̇o2. There will, of course, be some uncertainties related to the precise position of the O2 dissociation curve and changes thereof along the intramuscular vascular tree.
Blood flow measurements.
Blood flow was determined in a second set of seven animals using the radionuclide-tagged microsphere technique (42). Initially, rats were anesthetized with pentobarbital sodium (40 mg/kg ip, to effect). Polyethlene catheters (PE-10 connected to PE-50) were emplaced in the right carotid and caudal (tail) arteries. The carotid artery catheter was advanced 2–3 mm rostral to the aortic valve and secured. The tail artery catheter was advanced toward the bifurcation of the descending aorta and secured. The carotid artery catheter was connected to a pressure transducer. Arterial blood pressure and heart rate were measured (Digi-Med BPA model 200). The tail artery catheter was connected to a 5-ml glass syringe, which was attached to a Harvard withdrawal pump (model 907, Cambridge, MA).
Blood flow was measured in the right (resting) and left (contracting) spinotrapezius muscles during normotensive and hypotensive conditions. Both spinotrapezius muscles were surgically exposed, with electrodes being placed on the left muscle. Microspheres were infused after 3 min of left spinotrapezius contractions for each condition with the right, quiescent spinotrapezius serving as the control (i.e., resting) condition. After the first microsphere was infused, hypotension was induced identically as that employed for phosphorescent quenching measurements (i.e., arteriosection of ∼5 ml blood), and the muscle and animal allowed ∼30 min to stabilize before the second microsphere label was infused. Two different microspheres (46Sc, 85Sr) with a diameter of 15 μm (New England Nuclear, Boston, MA) were infused in random order. Before infusion, the microspheres were agitated by sonication to suspend the beads and prevent clumping. Thirty seconds before infusion was initiated, blood withdrawal from the caudal artery at 0.25 ml/min was begun. The right carotid artery catheter was disconnected from the pressure transducer, and a specified microsphere (∼2.5 × 105 in number) was infused into the ascending aorta and flushed with saline to ensure clearance of the beads. Blood withdrawal from the caudal artery continued for 45 s after microsphere infusion.
After the final microsphere infusion, the rats were killed with an overdose of pentobarbital sodium (>80 mg/kg) via the right carotid artery catheter. After correct placement of the carotid catheter was verified, the left and right spinotrapezius and kidneys were removed. The radioactivity levels of the tissues were determined by a two-channel gamma scintillation counter (Packard Auto Gamma spectrometer, model 5230) set to record the peak energy activity of each isotope for 5 min. Total blood flow to each tissue was calculated by the reference sample method (27, 42) and expressed in milliliters per minute per 100 g of tissue. Adequate mixing of the microspheres was verified by demonstrating a <15% difference in blood flows to the right and left kidneys.
Calculated O2 uptake.
Resting and steady-state contracting muscle V̇o2 (V̇o2m) were estimated using the Fick equation assuming that Pmv is an appropriate analog for venous Po2 (40, 54). V̇o2m was estimated using arterial blood gases, hemoglobin content, Pmv, and measured blood flow and reported (in ml O2·min−1·100 g of tissue−1). Considering that no change was observed in muscle temperature or blood pH and the relatively light workload performed by the muscle, no appreciable shift in the O2 dissociation curve would be expected.
KaleidaGraph software (Kaleidagraph 3.5) was used to describe the time course of each Pmv response using an exponential function, following a time delay (TD): (2) where τ is the time constant of the response and ΔPmv is the difference between rest and steady-state contracting values.
When a marked biphasic Pmv response occurred (i.e., Pmv falling transiently to values below steady state) prior to the attainment of a steady state, a second exponential term was included in the model to reduce the residual sum of squares (3) where A1 (primary response) and A2 (secondary response) are the amplitudes of the two components of the response, respectively, τ1 and τ2 are the time constants, and TD1 and TD2 are the independent time delays of the respective responses. The mean response time (MRT; estimates the time to 63% of primary exponential response) was calculated as TD1 + τ1 and provides an index of the rapidity of the Pmv response after contraction onset.
For the normotensive condition, the single exponential with TD provided an excellent fit to the Pmv data at the onset of contractions as judged from 1) coefficient of determination (r2), 2) sum of the squared residuals (χ2), and 3) visual inspection of the raw data and the fit of the residual error to a linear model (4). The single exponential did not provide a good fit to the Pmv responses for the hypotensive condition; therefore, the more complex model with two exponentials (as described above), each with independent delays, was required to fit the Pmv response (15). A fall in Pmv below steady-state levels has been termed an “undershoot” (6, 15), and this was calculated as the difference between the minimum value for Pmv, which was typically at the nadir of the primary exponential fall in Pmv, and the subsequent Pmv steady-state value. To determine whether the Pmv undershoot was detectable statistically, we calculated the z-statistic to test the null hypothesis that the undershoot was not different from zero for the hypotensive condition (13). All model-dependent (e.g., τ, TD, MRT) and -independent (i.e., baseline, end-contracting Pmv) parameters and measurements, as well as blood flow and V̇o2m were analyzed with a one-way ANOVA. When appropriate, a Student-Newman-Keuls post hoc procedure was used to test for significance. In all instances, a significance level of P < 0.05 was accepted.
Arteriosection of ∼5 ml of blood (4.8 ± 0.3 ml) decreased MAP by ∼39%, resulting in a significantly reduced MAP (normotensive, 97 ± 4 mmHg, hypotensive, 58 ± 3 mmHg; P < 0.05) in concert with a mild bradycardia after blood withdrawal (normotensive, 366 ± 1 beats/min, hypotensive, 318 ± 9 beats/min; P < 0.05). There were no changes in MAP throughout the contractions in either group. Hematocrit was significantly reduced (20 ± 4%) after the blood withdrawal, and arterial Po2 was elevated by 20 ± 3% (P < 0.05). Representative Pmv responses to electrical stimulation and subsequent model fits for the spinotrapezius during normotensive and hypotensive conditions are illustrated in Fig. 1. As seen in Fig. 1 and presented below, the Pmv at rest and the response profile across the rest-contractions transition differed substantially between normotensive and hypotensive conditions.
Muscle blood flow.
As illustrated in Table 1, during normotensive contracting, there was a threefold higher blood flow, consistent with this stimulation intensity (4, 7). As observed with animals used for Pmv measurements, arteriosection resulted in a significant reduction in MAP as well as a mild bradycardia. Blood flow was reduced significantly with hypovolemic hypotension both at rest and during steady-state contractions (Table 1). In addition, muscle vascular conductance was reduced by ∼63% in quiescent muscle during hypotension. Moreover, in the contracting spinotrapezius, there was a strong tendency (42%; P = 0.07) for lower vascular conductance during hypotension (Table 1).
The precontracting, baseline Pmv was significantly lower in the hypotensive condition (Fig. 2). Likewise, the steady-state contracting value was significantly lower in the hypotensive compared with the normotensive condition (8.7 ± 1.6 vs. 19.4 ± 2.4 Torr, respectively; P < 0.05). The change in Pmv between baseline Pmv and the steady-state of the Pmv response, however, was not different between conditions (Fig. 2). In hypotension, but not normotension, Pmv values fell transiently below end-contracting values, thus undershooting the steady state (Fig. 1). The value of this undershoot in the hypotensive condition (3.7 ± 0.6 Torr) was significantly different from zero.
The single-exponential model provided an excellent fit to the Pmv data for the normotensive condition. In contrast, six of the seven responses in the hypotensive condition required the more complex two-component model to fit the data adequately. This resulted from the presence of the pronounced undershoot (i.e., transient decrease to a Pmv value below that present at the cessation of contractions) as noted above.
Hypovolemic hypotension resulted in a significant speeding of Pmv dynamics parameters (Fig. 3). Specifically, TD1 was reduced on average 50% (∼6 s) for the hypotensive condition, and τ was sped by ∼40% (Fig. 3). This resulted in a significantly reduced MRT in the hypotensive vs. normotensive condition (13.2 ± 1.5 vs. 24.4 ± 1.9 s, respectively; P < 0.05; Figs. 3 and 4). In addition, the rate of Pmv decline (ΔPmv/τ) was doubled during the on transition in the hypotensive condition (2.2 ± 0.4 vs. 1.1 ± 0.1 Torr/s for hypotensive vs. normotensive, respectively; P < 0.05). In the hypotensive, but not normotensive, condition, there was a secondary rise in Pmv that began 71 ± 17 s after the initiation of contractions (i.e., TD2). This secondary rise in Pmv in the hypotensive condition increased with a time constant (τ2) of 82 ± 47 s, and thus the steady-state Pmv was not approached until toward the end of the contractions bout.
Muscle O2 uptake.
During normotension, both resting and steady-state contracting V̇o2m were similar to those values calculated previously (5, 56). Specifically, the stimulation paradigm significantly increased muscle O2 uptake by ∼4.5-fold (Table 1). Interestingly, induction of hypotension resulted in a substantial reduction in resting (∼70%) and contracting (∼64%) spinotrapezius V̇o2 (P < 0.05). At rest, extraction increased from 51% during normotension to 78% during hypotension. During the steady-state of contractions, extraction was ∼78% during normotension and 94% during hypotension.
This investigation has demonstrated that systemic hypovolemic hypotension dramatically alters the matching of Q̇o2 to V̇o2 at rest and during contraction in skeletal muscle. Specifically, hypotension reduces vascular conductance both at rest and during the contracting steady state (Table 1). Moreover, O2 delivery was reduced to such a degree that the V̇o2 “reserve” (i.e., ability to increase fractional O2 extraction) was not adequate to prevent V̇o2 from falling. In addition, across the rest-contractions transition, hypotension accelerates the decline in Pmv, which falls to very low levels that are significantly below those observed in the normotensive state. Pmv reflects directly the Q̇o2-to-V̇o2 ratio (4, 5, 40); therefore, alterations in either Q̇o2 or V̇o2 that impact the Q̇o2-to-V̇o2 ratio will be reflected by changes in Pmv. Thus, consistent with a blunted Q̇o2 across the rest-contractions transition, hypotension induced a transient reduction in Pmv to below end-contracting (steady-state) values (Fig. 1). These low Pmv values would be expected to slow the dynamics of muscle V̇o2. Specifically, Fig. 5 demonstrates the perturbations in Q̇o2 and V̇o2 dynamics required to result in the observed Pmv profile. Consequently, blunted V̇o2 dynamics would increase the O2 deficit, exacerbating the breakdown of phosphocreatine (25, 70). Lowered intramuscular phosphocreatine concentrations are associated with increased glycolysis and greater dependence on finite glycogen stores, which will reduce exercise tolerance. Because hypotension is one consequence of multiple physiological and pathological conditions, such as autonomic failure and cardiac diseases, these results (i.e., reduced Pmv for transcapillary O2 flux) provide insight into the mechanism(s) of premature fatigue observed in these populations.
Hypotension and oxygen delivery.
With exercise in healthy normotensive individuals, there exists the potential for developing systemic hypotension due to the enormous capacity of skeletal muscle to augment vascular conductance (11, 16, 51). However, several mechanisms act in concert to divert flow from inactive regions (e.g., viscera) centrally for the purpose of maintaining or increasing MAP (16). Additionally, within muscle, there is a powerful sympathetic vasoconstrictor response that prevents excessive vasodilation of active or quiescent skeletal muscle (16). In this manner, MAP is defended and Q̇ to essential organs such as the brain, which has a very low tolerance to blood flow deprivation, is sustained (for review, see Ref. 16). Local skeletal muscle perfusion is also regulated in proportion to tissue metabolism (V̇o2; reviewed in Ref. 34), i.e., metabolic regulation of Q̇ such that O2 delivery in the steady state increases proportionally with V̇o2 demands. Indeed, under normal conditions at the onset of exercise, when muscle V̇o2 increases without discernible delay (i.e., <2 s) (5), there is a rapid increase in O2 delivery that matches or exceeds the increase in muscle V̇o2 (3, 4, 19) such that Pmv does not fall below baseline values (reflecting a decreased or constant arterial-to-venous O2 difference) for 10–20 s after the onset of contractions. This response maximizes the O2 driving pressure to facilitate blood-muscle O2 diffusion.
In hypovolemic hypotension, the body attempts to restore MAP by evoking a complex array of responses that includes augmented sympathetic activity (30), which induces a profound arteriolar and to a lesser extent a venular vasoconstriction (8), evidenced herein by a reduced resting Q̇ (Table 1). Enhanced sympathetic activation has been shown to blunt the early increase in Q̇o2 (first 15–20 s; phase 1) after the onset of contractions (65). This would markedly speed the Pmv kinetics and create an “undershoot” of the response (18). MAP remains relatively constant throughout the transition to exercise in healthy normotensive individuals (61, 65), and our results support the notion that maintenance of an adequate upstream pressure, i.e., MAP, is critical to the Q̇o2 response. The early rapid increase in Q̇o2 appears to be largely dependent on the increased ΔP due to the muscle pump effect (34), such that Q̇o2 increases at a faster rate than muscle V̇o2 (normotension) (5), whereas the subsequent phase of Q̇o2 depends on vasodilation and maintenance of ΔP. Therefore, hypotension will further impair the initial increase in Q̇o2 (due to decreased ΔP) as well as blunt the Q̇o2 response after 15–20 s (phase 2), as vasodilation must presumably overcome an enhanced neurally mediated vasoconstriction. The Pmv kinetics observed in our study during hypotension (rapid undershoot and subsequent slow increase to steady state) are consequent to a concomitant decrease in ΔP and vascular conductance (Table 1) that affects both phases of the Q̇o2 response such that Q̇o2 increases less compared with normotension and at a slower rate than muscle V̇o2 (Fig. 5). There is also evidence that low Q̇ conditions induce a red blood cell aggregation in venous vessels, and this may contribute to increased vascular resistance and further limit increases in Q̇ with hypotension (10). Consistent with this notion, the biphasic Pmv response (Fig. 1) present during hypotension is similar to that observed in skeletal muscle of rats with Type 1 diabetes (6) and also CHF (15), which shows very slow Q̇ and Q̇o2 dynamics (53).
Conditions in which hypotension and slowed muscle Q̇o2 could contribute directly to exercise intolerance include autonomic failure (58) and chronic fatigue syndrome. Individuals with chronic fatigue syndrome exhibit an impaired central (62) and muscle hemodynamic response to exercise (39), which mandates a greater breakdown of high-energy phosphates (see below). Moreover, as mentioned in the introduction, systemic hypotension and exercise intolerance are observed in several pathological conditions, including mitral valve prolapse (9, 28), CHF (38), and idiopathic chronic fatigue (37). In these conditions (i.e., systemic hypotension), when the individual attempts to exercise, there may be insufficient Q̇o2 across the rest-exercise transition to preserve the Pmv pressure head necessary to drive oxidative metabolism, which, once again, will necessitate incurring a larger O2 deficit and its associated intracellular consequences, possibly resulting in a reduced V̇o2.
Oxygen uptake and hypotension.
Because Pmv reflects the dynamic balance between Q̇o2 and V̇o2, the argument could be posited that the lower precontracting and steady-state values in hypotension (Fig. 2) are the result of an increased V̇o2 rather than a lower Q̇o2. However, as observed in Table 1, a reduced rather than increased V̇o2 was observed. There exists only a finite capacity to maintain V̇o2 in the face of falling O2 delivery, i.e., fractional O2 extraction can only increase to ≤100%. In the present investigation, O2 delivery during hypotension was reduced to such a degree that the ability to maintain V̇o2 was compromised (Table 1). Thus the contribution of ATP supplied via oxidative metabolism would be substantially reduced; therefore, reliance on alternative energy sources (e.g., anaerobic glycogenolysis) would be increased. Although not measured herein (see Experimental considerations below), fatigue would likely occur much more rapidly with a reduced V̇o2 in the hypotension condition, since we would not expect muscle ATP requirements during contractions per se to be altered with hypotension.
The functional significance of a reduced O2 availability (i.e., reduced Pmv) would be slowed V̇o2 kinetics (17, 26, 35) and, if of great enough magnitude, reduced steady-state V̇o2, although the ATP requirement for contractions would not be expected to change. However, the slowed V̇o2 kinetics would mandate a greater reliance on immediate (i.e., PCr breakdown) and anaerobic energy sources (i.e., glycogenolysis) to maintain energy output by the working muscle (70). This notion can be conceptualized by consideration of the overall equation for oxidative metabolism (24): (4) At a given ATP (thus V̇o2) demand, O2 availability (or pressure) at the mitochondria can modulate the concentrations of the other substrates (25, 69, 70). Kindig et al. (31) have demonstrated in single Xenopus laevis muscle fibers that intracellular Po2 falls more rapidly and to lower levels with a reduction in extracellular Po2 (broadly synonymous to decreased Pmv). Thus the hypotensive state in the present investigation resulted in a lowered Pmv, thereby reducing intracellular Po2; consequently, muscle ATP turnover could be maintained only at the expense of lowered intracellular energy levels (i.e., reduced [ATP]/[ADP][Pi], [NAD+]/[NADH], and [PCr], where brackets denote concentration) (see Eq. 4; Refs. 24, 70). Consistent with this notion, across the rest-to-exercise transient, skeletal muscle [PCr] does appear to fall more and end-exercise [ATP] is lowered in patients with CFS-induced hypotension (71). Because these patients do not exhibit either differences in muscle fiber characteristics (32) or abnormalities in resting glycolysis or oxidative metabolism (33), the greater decline in [PCr] and the lower end-exercise intracellular pH (33) can both be explained by the presence of a lower MAP and muscle vascular conductance and therefore an imbalance between Q̇o2 and V̇o2 requirements early during the exercise bout such that Pmv falls below those levels present in healthy individuals.
Recently, Haseler et al. (21) reported that reducing arterial O2 pressures [PaO2, via lowering the inspired fraction of O2 (FiO2)] does not alter initial PCr on kinetics to submaximal exercise in human gastrocnemius muscle vs. normoxia. However, with a reduced FiO2, there is an overall greater percent decrease in PCr (20, 52), suggesting a greater change in phosphorylation potential (i.e., [PCr/Pi]) to maintain a given cellular respiration rate (68, 69). In those studies, although there was a reduced PaO2, the ability for a compensatory increase in blood flow to buffer the reduced O2 content of arterial blood was not compromised (i.e., no large changes in MAP). However, in the present study where arterial O2 content was slightly decreased (due to reduced hematocrit), the ability to augment muscle blood flow, and therefore increase O2 delivery, was greatly diminished. Indeed, with a reduced Pmv(Fig. 1), cellular respiration rates were affected such that muscle O2 uptake at rest and during contractions was reduced significantly vs. that during normoxia (Table 1).
We chose to reduce MAP to ∼60 mmHg because 1) it has been demonstrated that this level of hypotension results in a large redistribution of blood flow, which maintains cerebral circulation (55), 2) there is no speeding of Pmv kinetics at the onset of contractions with MAP above 70 mmHg (stimulation parameters identical to those used herein; Behnke and Poole, unpublished observations), and 3) autoregulatory corrections in cerebral vascular resistance fail when MAP falls below 50 mmHg (64). The latter consideration would not affect the results of the present investigation because none of the animals had MAP values below ∼55 mmHg during hypotension.
Interestingly, in the present investigation, hypovolemic hypotension induced a modest bradycardia. This appears to contradict the expected baroreflex-mediated tachycardia; however, the bradycardia observed in the present study is not an unusual accompaniment to severe hemorrhage (44). This apparently paradoxical response results from activation of left ventricular C fibers (43).
Attachment of a force transducer to the intact spinotrapezius (in the in situ position) compromises the functional integrity of the muscle (alters muscle Q̇). In the present investigation, measurement of muscle fatigue would have been desirable. However, it is a reasonable supposition that in the hypovolemic condition, with a reduced ATP contribution via oxidative phosphorylation (Table 1) and an unchanged external ATP demand (stimulation parameters held constant), anaerobic and immediate energy reserves would be depleted more rapidly and the onset of fatigue accelerated.
In conclusion, both syncope and presyncope are found within the otherwise healthy population and hypotension is a common adjunct to multiple clinical conditions. The present investigation has determined that, during hypovolemic hypotension, a reduced microvascular Po2 (Pmv) is present within muscle at rest, across the transition to contractions, and in the subsequent steady state. These findings suggest that exercise intolerance and muscle dysfunction during hypotension can result from O2 exchange impairments resulting from an impaired diffusive O2 delivery and also a lowered intracellular Po2 consequent to this reduced Pmv.
This work was supported by National Institutes of Health Grants HLBI-50306, HL-69739 (to D. J. Padilla), HL-71270 (to B. J. Behnke), and AG-12298 (to T. I. Musch), National Aeronautics and Space Administration NNA04CC66G (M. D. Delp), and a Grant-in-Aid from the American Heart Association, Heartland Affiliate (D. C. Poole).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Copyright © 2006 the American Physiological Society