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1Pulmonary Function and Clinical Exercise Physiology Unit, Division of Respiratory Diseases and 3Division of Cardiology, Department of Medicine, Federal University of Sao Paulo, São Paulo, São Paulo, Brazil; and 2Cardiopulmonary Laboratory, Nucleus of Research in Physical Exercise, Federal University of São Carlos, São Paulo, Brazil
Submitted 26 December 2007 ; accepted in final form 24 March 2008
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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%) in deoxy-hemoglobyn, oxi-Hb ([O2Hb]), tissue oxygenation index, and total Hb ([HbTOT], an index of local blood volume) in the vastus lateralis were measured by near infrared spectroscopy. In addition, QT was monitored by impedance cardiography and arterial O2 saturation by pulse oximetry (SpO2). There were significant improvements in exercise tolerance (Tlim) with PAV. Blood lactate, leg effort/Tlim and dyspnea/Tlim were lower with PAV compared with sham ventilation (P < 0.05). There were no significant effects of RM unloading on systemic O2 delivery as QT and SpO2 at submaximal exercise and at Tlim did not differ between PAV and sham ventilation (P > 0.05). Unloaded breathing, however, was related to enhanced leg muscle oxygenation and local blood volume compared with sham, i.e., higher [O2Hb]% and [HbTOT]%, respectively (P < 0.05). We conclude that RM unloading had beneficial effects on the oxygenation status and blood volume of the exercising muscles at similar systemic O2 delivery in patients with advanced CHF. These data suggest that blood flow was redistributed from respiratory to locomotor muscles during unloaded breathing.
blood flow; cardiac failure; cardiovascular physiology; hemodynamics; muscle; oxygen consumption
More recently, much emphasis has been paid to the constraining role of the mechanical-ventilatory responses on exercise capacity in CHF. Several authors, for instance, found that patients with more advanced CHF typically have a hyperventilatory response to exercise that holds important clinical and prognostic implications (3, 24). These patients may also present with decreased respiratory muscle (RM) strength and endurance (23, 30). In addition, the work of breathing can be augmented during exercise because of increases in duty cycle (by decreasing expiratory time) and reductions in lung compliance (1). It is not surprising, therefore, that selected strategies aimed to unload or train the RM have successfully improved exercise tolerance in CHF patients (11, 31, 32, 39, 52).
In this context, it is intuitive to consider that enhanced exercise capacity after RM unloading probably results from a better balance between local O2 demand and supply with beneficial effects on the resulting dyspnea (31, 35, 42, 54). An important contributory explanation, however, might be related to blood flow redistribution from respiratory to locomotor muscles (14, 20, 21). There are several lines of evidence suggesting that increased fatiguing contractions and accumulation of metabolites in the inspiratory and expiratory muscles may activate unmylenated type IV phrenic afferents, increasing sympathetic vasoconstrictor activity via a supraspinal reflex with the purpose to redirect blood flow from peripheral to ventilatory muscles (4, 21, 33, 43, 45, 48, 49). Indirect support to this hypothesis in humans has been given by O'Donnell and coworkers (39) who found that the improvement in exercise endurance with pressure support ventilation was associated with less exertional leg discomfort in patients with CHF. In addition, skeletal muscle perfusion has been shown to be selectively reduced as whole body exercise progresses in some patients with CHF (34, 53); this effect, however, was not found with small muscle mass exercise where the ventilatory requirements are considerably lower (27). There is, therefore, a sound rationale underpinning the notion that respiratory and peripheral muscles could actively compete for the available cardiac output (QT), especially in pathological conditions where the ability to further increase O2 delivery is impaired. To the best of the authors' knowledge, however, no previous study has specifically looked at the potential effects of RM unloading during exercise on blood perfusion of the working peripheral muscles in humans with CHF.
Therefore, the primary objective of this study was to investigate whether locomotor muscle perfusion (as estimated by muscle oxygenation and blood volume measured by near infrared spectroscopy) would be significantly improved with RM unloading during high-intensity exercise in patients with advanced CHF.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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This was a prospective, randomized, and controlled study involving nine nonsmoking men recruited from the CHF Outpatient Clinic of our institution. Patients were required to have an established diagnosis of CHF for at least 6 mo, three-dimensional echodopplercardiography showing left ventricle ejection fraction <35%, New York Heart Association classification score III, and no hospitalization in the preceding 6 mo. The patients were on a stable phase of the disease, and they have been treated with standard medications for CHF (Table 1). Patients were excluded from study if they had clinical and/or functional evidence of obstructive pulmonary disease (forced expiratory volume in one second/forced vital capacity <70%), exercise-induced asthma, unstable angina or significant cardiac arrhythmias, and myocardial infarction within the previous 6 mo. No patient had been submitted to cardiovascular rehabilitation in the preceding year. Patients gave a written informed consent, and the study protocol was approved by the Institutional Medical Ethics Committee.
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After a ramp-incremental cardiopulmonary exercise test on a cycle ergometer, the patients performed, on a separate day, a high-intensity constant work rate (CWR) trial test at 70–80% peak work rate (WR) to individually select the flow (FA) and volume (VA) assist levels of proportional assisted ventilation (PAV) (see below). At a subsequent experimental visit, the patients undertook, 1 h apart, two CWR at the previously defined WR to the limit of tolerance (Tlim). During these tests, they were randomly assigned to receive sham ventilation or the preselected levels of PAV.
Noninvasive Positive Pressure Ventilation
PAV was applied via a tight-fitting facial mask with pressure levels being delivered by a commercially available mechanical ventilator (Evita-4; Draeger Medical, Lübeck, Germany). PAV provides ventilatory assistance in terms of FA (cmH2O·l–1·s–1) and VA (cmH2O/l) that can specifically unload the resistive and elastic burdens, respectively. PAV was set individually on a preliminary visit for each patient using the "run-away" method (6, 60). Initially VA and FA were set at the minimum values of 2 cmH2O/l and 1 cmH2O·l–1·s–1, respectively. Next, VA was increased slowly by 2 cmH2O/l until run-away was demonstrated. This phenomenon is identified when the inspiratory phase of the ventilator extends well beyond the inspiratory time observed at lower levels and goes into the neural expiration (60). Afterward, FA was set by increasing its value by 1 cmH2O·l–1·s–1 until run-away occurred, with VA being kept fixed at 2 cmH2O/l. These settings were applied during exercise at the same WR selected for the experimental visit with FA and VA values being adjusted for patient comfort to avoid run-away during exercise (see RESULTS for the actual values).
Noninvasive sham ventilation was applied by the same equipment using the minimal inspiratory pressure support (5 cmH2O of inspiratory pressure support and 2 cmH2O of positive end-expiratory pressure) to overcome the resistance of the breathing circuit, as informed by the ventilator manufacturer. The patients and the accompanying physician were unaware of the ventilation strategy applied by a trained physiotherapist. This was accomplished by visually isolating the ventilator and its monitor from them.
Measurements
Pulmonary function tests. Spirometric tests were performed by using the CPF System (Medical Graphics-MGC, St. Paul, MN) with airflow being measured by a calibrated Pitot tube (PreVent Pneumotach). Carbon monoxide diffusing capacity was measured by the modified Krogh technique. Total lung capacity (TLC, liters) and residual volume (RV, liters) were obtained by constant-volume, different-pressure body (Elite System; MGC). Maximum inspiratory and expiratory pressures (cmH2O) were measured at RV and TLC, respectively (MVD300; MVD, Porto Alegre, Brazil). Predicted values for pulmonary function tests were those proposed for the adult Brazilian population (36, 37). In addition, a blood sample was drawn from the radial artery according to standard guidelines with the sample being immediately analyzed for arterial partial pressure for O2 and CO2 (mmHg) (ABL77; Radiometer, Copenhagen, Denmark).
Exercise test.
Symptom-limited exercise tests were performed using a calibrated computer-based exercise system (CardiO2 System; MGC). The tests were performed on an electronically braked cycle ergometer (model 400; Corival, Lode, The Netherlands) at 60 revolutions/min (rpm), and they were preceded by an unloaded baseline pedaling at 0 watts for 2 min. The following data were recorded as a mean of 15 s: O2 uptake (
O2, ml/min), CO2 output (ml/min), minute ventilation (l/min), respiratory rate (rpm), and tidal volume (liters). Peak O2 was the highest value found at exercise cessation (38). Subjects were also asked to rate their "shortness of breath" and "leg effort" at exercise cessation using the 0–10 Borg's scale.
Oxyhemoglobin saturation (SpO2, %) was continuously determined by pulse oximetry (POX 010–340; Mediaid, Torrance, CA) with a finger probe in the right index. To improve signal stability, the probe was fixed to the finger with tape and covered with an optically dense sheet. Another similar oximeter with a probe in the left index was used to compare the values recorded in the cardiopulmonary exercise system. After the experiments, the recorded values were revised for aberrant data points. From these data, arterial O2 content (CaO2) was estimated as CaO2 (ml/100 ml) = 1.39[Hb] x SpO2, where [Hb] is hemoglobin concentration.. Capillary samples were also collected from the ear lobe for blood lactate measurements (mMol/l) at exercise cessation (Yellow Springs 2.700 STAT plus; YSI).
In the maximum exercise test, the rate of power increment was individually selected (usually 5–10 W/min) to provide an exercise duration of 8–12 min. The CWR exercise tests were performed at 70–80% of the previously determined peak WR to the Tlim (min) or 20 min, whichever was reached first. Tlim was defined as the time point in which the patients signaled to stop exercising or could not maintain the required pedaling rate for 10 s despite being encouraged by the investigators.
Skeletal muscle oxygenation.
Skeletal muscle oxygenation profiles of the left vastus lateralis were evaluated with a commercially available near infrared spectroscopy (NIRS) system (Hamamatsu NIRO 200; Hamamatsu Photonics). The theory of NIRS has been described in detail elsewhere (7, 15, 47). The intensity of incident and transmitted light was recorded continuously and, along with the relevant specific extinction coefficients, used for online estimation of the changes () from baseline of the concentrations of oxy[hemoglobin + myoglobin] ([O2Hb]), deoxy[hemoglobin + myoglobin] ([HHb]), and total [hemoglobin + myoglobin] ([HbTOT]). The last variable has been used as an index of local blood volume and muscle vasodilation in patients with CHF ([HbTOT]= [O2Hb] + [HHb]) (51, 55). From these values, an additional index of muscle oxygenation was calculated [tissue oxygenation index (TOI) = 100 x ([O2Hb]/[HbTOT])]. Because of the uncertainty of the differential pathlength factor (DPF) for the quadriceps, we did not use a DPF in the present study. Therefore, the values were recorded as a delta () from baseline in units of micromolar per centimeter. To improve intersubject comparability, [O2Hb] and [HHb] values were expressed as the percentage of the maximal value determined on at least two preexercise maximal voluntary contractions (MVCs) performed after the system had been zeroed. This was achieved by having the subject exert the maximum volitional effort against the cycle ergometer pedal fixed at 90° in relation to the ground with active encouragement from the investigators. We used the maximum observed value provided that it did not differ >10% from the lower value and that a clear steady-state value was observed for at least 10 s. Although previous studies have used the maximum deoxygenation provided by cuff-induced ischemia (12, 19, 29), pilot experiments in these patients showed that a clear steady state in the NIRS signals was not observed in most subjects even after long periods of ischemia (up to 8 min). In addition, we found that the intrasubject coefficient of variation (CV) for [HHb] was substantially lower in response to MVC (5.3%) than the cuff technique (12.7%). Considering, however, that the highest values of [HbTOT] were not obtained during the MVCs or cuff-induced ischemia, we normalized this variable to the highest value found on early recovery.
In the present study, a set of optodes was placed at the belly of the vastus lateralis midway between the lateral epicondyle and greater trochanter of the femur. Care was taken to obtain the local skinfold thickness (Harpender caliper), since the signal amplitude and reliability deteriorate substantially with thicker subcutaneous fat layers (15). Therefore, if the subject skinfold thickness exceeded 20 mm, a secondary place was chosen in the same muscle group. The optodes were housed in an optically dense plastic holder, thus ensuring that the position of the optodes, relative to each other, was fixed and invariant. The optode assembly was secured on the skin surface with tape and then covered with an optically dense, black vinyl sheet, thus minimizing the intrusion of extraneous light and loss of NIRS light. The thigh, with attached optodes and covering, was wrapped with a purpose-made, elastic bandage to minimize movement of the optodes. We obtained the test-retest CV for each of the studied variables, with actual values being 8.7% for [O2Hb], 4.1% for [HHb], 4.3% for [HbTOT], and 6.5% for TOI.
To test the sensitivity of the main NIRS variables to improved muscle perfusion, four patients performed an exercise bout at 70–80% peak WR on a separate day. In this test, a cuff was placed around the proximal thigh, and its pressure was increased to 30 mmHg above the arterial systolic pressure after 2 min of exercise. After stabilization of the signals, the highest [HHb] value was considered to represent maximal deoxygenation ("100%"); similarly, the lowest [O2Hb] and [HbTOT] values during occlusion were assumed to represent the "zero" point. The pressure was then subsequently reduced every 30 s to progressively lower values until a pressure of 30 mmHg below the arterial systolic pressure was reached (see RESULTS for findings).
Central hemodynamics.
QT (l/min) and stroke volume (SV, ml) were measured by impedance cardiography (model PF-05; PhysioFlow, Manatec, France). These data were also used to estimate (est) O2 delivery (DO2est = QT x CaO2est, l/min). The PhysioFlow device and methodology have been described elsewhere (8). Before each test, verification of the correct signal quality was performed by visualizing the electrocardiogram (ECG) tracing and its first derivative (dECG/dt) and the impedance waveform (Z) with its first derivative (dZ/dt) (8). We calculated the CV for QT and SV during exercise in a subgroup of patients (n = 4) (3.3 and 4.1%, respectively). The sensitivity of the system to small changes in QT was tested during a series of four constant WR exercise tests in these patients. This was performed by comparing the measured QT values with those expected from the submaximal QT-O2 relationship described in previous studies with similarly severe patients (
4.5–5) (50, 51). As shown in Fig. 1, the system was sensitive to detect small changes in QT (100 ml/min) with acceptable accuracy (within ± 10% for all readings).
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The SPSS version 13.0 statistical software was used for data analysis (SPSS, Chicago, IL). Results were summarized as means ± SD or median (range) for changes in Tlim. To contrast within-subject exercise responses at Tlim, paired t or Wilcoxon tests were used as appropriate. Repeated-measures ANOVA with Bonferroni correction for multiple comparisons was used to compare the physiological responses to exercise with PAV and sham ventilation at selected time points, taking into consideration the sequence of treatment. The level of statistical significance was set at P < 0.05 for all tests.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Resting characteristics are presented in Table 1. Etiology of CHF was predominantly nonischemic, and patients had moderate to severe left ventricle dysfunction. Patients had decreased RM strength and lung diffusing capacity compared with normal predicted values (36, 37). In relation to maximal exercise tolerance, they presented with moderate reductions in peak O2 with five patients being classified as Weber's class II and four patients as class III (56). Peak heart rate and chronotropic stress were markedly reduced, since all patients were receiving β-blockers for at least 6 mo. Although median scores of breathlessness and leg effort did not differ at exercise cessation, seven out of nine patients reported leg discomfort as the main limiting symptom (Table 2).
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The actual values for volume and flow assist during PAV were as follows: 5.7 ± 1.5 H2O/l and 3.1 ± 1.1 cmH2O·l–1·s–1, respectively. Repeated-measures ANOVA revealed that there were no significant effects of treatment sequence on Tlim (P > 0.05). There was a significant improvement in the tolerance to CWR exercise with PAV [median (range) = 290 s (109–729 s) vs. 366 s (120–1,200 s), P < 0.01 (Fig. 2)]. These changes corresponded to a percent variation of 38.4% (10.0–95.1%). In a subgroup of patients (n = 4), we found that the test-retest differences for Tlim were within 10% for all subjects (ranging from 3.2 to 9.1%). In addition, although no patient reached the time limit of 20 min with sham ventilation (see METHODS), the tests were finished at this time point by the investigators in two patients when using PAV (Fig. 2). There were, however, no significant change in O2 at Tlim (Table 3).
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There were no significant effects of PAV on the cardiovascular responses compared with sham. As shown in Table 4 and Fig. 3, A and B, QT and SV were similar during both exercise tests at Tlim and at submaximal exercise. There was also no significant change in SpO2 with PAV compared with sham ventilation (Table 4): the median (range) of the intrasubject difference in SpO2 between the two interventions throughout exercise was 0% (–2 to 2%). Consequently, estimated systemic O2 delivery (DO2est = QT x CaO2est) remained unchanged with PAV at isotime and Tlim (Table 4). In similarity, there were no treatment effects on mean blood pressure; consequently, total vascular resistance did not vary with PAV (P > 0.05; Table 4).
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As described in METHODS, we tested the response characteristics and sensitivity of the key NIRS variables to improved muscle perfusion induced by the release of cuff-induced ischemia in a subgroup of patients (n = 4). We found that enhanced muscle perfusion was associated with a linear decrease in [HHb] that was accompanied by greater increases in [O2Hb], i.e., [HbTOT] increased substantially with improved muscle perfusion (Fig. 4).
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[HHb]% and a concomitant (but not proportional) decrease in [O2Hb]%. Consequently, TOI [100 x ([O2Hb]/[HbTOT])] declined with exercise progression. As expected, [HbTOT]%, an index of local blood volume (23), increased with exercise (Fig. 5). Unloaded breathing was associated with a pattern of changes similar of that found with increased muscle perfusion (Fig. 4). Therefore, PAV was associated with lower [HHb]% values, which were accompanied by a blunted decrease in [O2Hb]%, i.e., TOI was improved with PAV. Moreover, [HbTOT]% was significantly increased with PAV (Table 4 and Fig. 5). Consistent with these data, leg effort/Tlim and lactate/Tlim were lower with PAV than sham (Tables 3 and 4, respectively).
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[HHb]% but higher [O2Hb]% values compared with sham ventilation in all subjects. In addition, there were no significant test-retest PAV-sham differences in [HHb]% (–16.1 ± 8.3 vs. –18.9 ± 8.5%, respectively) and [O2Hb]% (31.7 ± 10.1 vs. 28.5 ± 8.3%, P > 0.05). |
DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Several aspects make this an innovative investigation. Specifically, this is the first study to evaluate the effects of RM unloading on peripheral muscle oxygenation in humans with advanced CHF. In addition, we followed noninvasively the determinants of systemic O2 delivery, i.e., QT and CaO2. The main findings of the present study can be summarized as follows: 1) enhanced exercise tolerance with PAV was accompanied by a significant improvement in appendicular muscle oxygenation and blood volume, and 2) there were no discernible changes in the central hemodynamic adjustments or CaO2. These data might indicate that RM unloading allowed a fraction of the limited QT to be redirected from respiratory to locomotor muscles with beneficial effects on energy supply to the working peripheral muscles.
Peripheral-RM Competition for Blood Flow in CHF
There is a main hypothesis to explain the increased muscle oxygenation with RM unloading in our study (Fig. 5 and Table 4). In normal subjects, Dempsey and coworkers (2, 14, 20, 21, 33, 43, 45, 48, 49, 57) have provided convincing evidence that increased RM work was causally related to lower perfusion of the leg muscles at maximum exercise. They postulated that a RM fatigue-induced metaboreflex would increase sympathetic vasoconstrictor outflow, thereby decreasing the locomotor muscle perfusion. Although these studies did not actually determine whether blood flow to the RM was increased, it is conceivable that part of the QT has been redirected to the RM, i.e., a "stealing" effect (14).
Based on the main findings of the present investigation, we postulate that, at near maximal exercise, when the ability to further increase QT is likely to be limited in CHF patients, PAV was instrumental in reducing diaphragmatic fatigability and, probably, the accumulation of by-products that could activate the local metaborreceptors. Therefore, lower limb sympathetic outflow may have been reduced with consequent improvements in local oxygenation and local blood volume (see Methodological Considerations for further elaboration). These data are consistent with those recently reported by Miller et al. (33) who found that PAV increased limb blood flow even during mild intensity exercise in a canine model of CHF. Although more direct evidence of this phenomenon should be sought (e.g., by looking at diaphragmatic and leg perfusion under loaded and unloaded conditions), the present study seems to constitute the first experimental evidence that the stealing phenomenon can occur in humans with CHF.
We found that whole body O2 did not decrease as would be expected from a lower work of breathing with respiratory unloading. However, this interpretation is hampered by the fact that increasing and decreasing the work of the RM may promote opposite changes in limb blood flow (and muscle O2). In fact, Harms et al. (20) found that limb O2 was higher with RM unloading. Therefore, any decrease in RM O2 with unloaded breathing in the present study may have been cancelled out by increases in appendicular muscle O2.
Methodological Considerations
We found that RM unloading was associated with increased O2 availability to the working peripheral muscles (higher [O2Hb]% and TOI) and improved local blood volume ([HbTOT]%) as determined by NIRS (Table 4 and Fig. 5). This technique provides noninvasive and continuous monitoring of relative concentration changes in deoxy-, oxy, and total hemoglobin/myoglobin in the muscle microvasculature (small arterioles, capillaries, and venules) (5, 7, 9, 12, 13, 15, 16, 19, 28). In this context, it should be recognized that NIRS does not actually measure muscle blood flow. However, [HHb]% provides a reliable index of local O2 extraction, reflecting the O2-to-muscle blood flow relationship (12, 13, 16, 19, 22, 26, 58). Grassi et al. (18), for instance, demonstrated that when venous outflow is isolated (e.g., dog gastrocnemius) the O2 extraction kinetics are remarkably similar to those found for [HHb] in exercising humans. Therefore, if the constant metabolic demands of the peripheral muscles (O2) could be met by higher rates of convective O2 delivery during PAV compared with sham, fractional O2 extraction ([HHb]%) was expected to be reduced (Table 4 and Fig. 5) (25). This pattern of changes is consistent with our findings showing a linear decrease in [HHb]% (and increase in [O2Hb]% and [HbTOT]%) with higher muscle perfusion (Fig. 4).
The NIRS technology used in this study (constant-wave spectroscopy) assumes a constant reduced scattering coefficient (µ's) that has been shown to overestimate the changes in NIRS variables during exercise (17). However, we scaled the changes during exercise to the inflection induced by preexercise MVCs, minimizing the error in absolute changes incurred by assuming constant µ's. Another assumption inherent to NIRS is that the relative contributions of arterial and venous blood to the signals remain constant from rest to exercise and recovery. Although this contention remains untested in skeletal muscle during exercise, there is no a priori reason to believe that RM unloading would selectively change the relative contribution of individual components of the measured signals. Moreover, this is expected to have a minor effect since the microvascular volume is predominantly (85%) composed of capillaries (41). Finally, NIRS is not able to differentiate between the signal attenuation due to Hb and myoglobin. However, this confounding effect has been estimated to 10% of the whole signal (29), a noncritical issue in this crossover study.
Study Limitations
First, it should be recognized that PAV was not associated with a substantial improvement in exercise tolerance in some of our patients (Fig. 2). Therefore, the beneficial effects of RM unloading were probably not critical to enhance the tolerance to heavy-intensity exercise in some of these patients with advanced CHF. Second, impedance cardiography was used to monitor the central hemodynamics. Although there are some concerns about the absolute accuracy of this technique during exercise, the relative changes (Table 4) have been found to provide acceptable estimates of the changes induced by exercise in patients with CHF (10, 44). In fact, we found that the technique was responsive to small changes in the metabolic demand, as demonstrated by the QT-O2 relationship in response to a series of CWR exercise tests (Fig. 1). Third, we did not obtain ethical approval to perform direct measurements of work of breathing (e.g., using esophageal balloons) in these patients. Therefore, we are uncertain about how much the work of breathing was effectively diminished with PAV in individual patients. Finally, we did not address whether the RM unloading would improve peripheral muscle oxygenation during moderate exercise in which the ventilatory demands are considerably lower (46, 57).
Conclusions
We conclude that RM unloading (PAV) increased O2 availability and blood volume in the lower limbs during CWR exercise of heavy intensity in patients with advanced CHF. These changes were not related to increased systemic O2 delivery (QT x CaO2), suggesting that a fraction of the QT had been redistributed from respiratory to locomotor muscles as a consequence of PAV-induced reductions in blood flow requirements of the RM.
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GRANTS |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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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.
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REFERENCES |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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O2 and leg blood flow during submaximal exercise. J Appl Physiol 87: 643–651, 1999.This article has been cited by other articles:
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  P. Laveneziana, D. E. O'Donnell, D. Ofir, P. Agostoni, L. Padeletti, G. Ricciardi, P. Palange, R. Duranti, and G. Scano Effect of biventricular pacing on ventilatory and perceptual responses to exercise in patients with stable chronic heart failure J Appl Physiol, May 1, 2009; 106(5): 1574 - 1583. [Abstract] [Full Text] [PDF]
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) (repeated-measures ANOVA).