Ventilation studied with circulatory occlusion during two intensities of exercise

Gait variability is affected more by peripheral artery disease than by vascular occlusion

Background

Patients with peripheral artery disease with intermittent claudication (PAD-IC) have altered gait variability from the first step they take, well before the onset of claudication pain. The mechanisms underlying these gait alterations are poorly understood.

Aims

To determine the effect of reduced blood flow on gait variability by comparing healthy older controls and patients with PAD-IC. We also determined the diagnostic value of gait variability parameters to identify the presence of PAD.

Methods

A cross-sectional cohort design was used. Thirty healthy older controls and thirty patients with PAD-IC walked on a treadmill at their self-selected speed in pain free walking (normal walking for healthy older controls; prior to claudication onset for PAD) and reduced blood flow (post vascular occlusion with thigh tourniquet for healthy older controls; pain for PAD) conditions. Gait variability was assessed using the largest Lyapunov exponent, approximate entropy, standard deviation, and coefficient of variation of ankle, knee, and hip joints range of motion. Receiver operating characteristics curve analyses of the pain free walking condition were performed to determine the optimal cut-off values for separating individuals with PAD-IC from those without PAD-IC.

Results and discussion

Patients with PAD-IC have increased amount of variability for knee and hip ranges of motion compared with the healthy older control group. Regarding the main effect of condition, reduced blood flow demonstrated increased amount of variability compared with pain free walking. Significant interactions between group and condition at the ankle show increased values for temporal structure of variability, but a similar amount of variability in the reduced blood flow condition. This demonstrates subtle interactions in the movement patterns remain distinct between PAD-IC versus healthy older controls during the reduced blood flow condition. A combination of gait variability parameters correctly identifies PAD-IC disease 70% of the time or more.

Conclusions

Gait variability is affected both by PAD and by the mechanical induction of reduced blood flow. Gait variability parameters have potential diagnostic ability, as some measures had 90.0% probability of correctly identifying patients with PAD-IC.

Control of exercise hyperpnoea: Contributions from thin-fibre skeletal muscle afferents

NEW FINDINGS

What is the topic of this review? In this review, we examine the evidence for control mechanisms underlying exercise hyperpnoea, giving attention to the feedback from thin-fibre skeletal muscle afferents, and highlight the frequently conflicting findings and difficulties encountered by researchers using a variety of experimental models. What advances does it highlight? There has been a recent resurgence of interest in the role of skeletal muscle afferent involvement, not only as a mechanism of healthy exercise hyperpnoea but also in the manifestation of breathlessness and exercise intolerance in chronic disease.

ABSTRACT

The ventilatory response to dynamic submaximal exercise is immediate and proportional to metabolic rate, which maintains isocapnia. How these respiratory responses are controlled remains poorly understood, given that the most tightly controlled variable (arterial partial pressure of CO₂ /H⁺ ) provides no error signal for arterial chemoreceptors to trigger reflex increases in ventilation. This review discusses evidence for different postulated control mechanisms, with a focus on the feedback from group III/IV skeletal muscle mechanosensitive and metabosensitive afferents. This concept is attractive, because the stimulation of muscle mechanoreceptors might account for the immediate increase in ventilation at the onset of exercise, and signals from metaboreceptors might be proportional to metabolic rate. A variety of experimental models have been used to establish the contribution of thin-fibre muscle afferents in ventilatory control during exercise, with equivocal results. The inhibition of afferent feedback via the application of lumbar intrathecal fentanyl during exercise suppresses ventilation, which provides the most compelling supportive evidence to date. However, stimulation of afferent feedback at rest has no consistent effect on respiratory output. However, evidence is emerging for synergistic interactions between muscle afferent feedback and other stimulatory inputs to the central respiratory neuronal pool. These seemingly hyperadditive effects might explain the conflicting findings encountered when using different experimental models. We also discuss the increasing evidence that patients with certain chronic diseases exhibit exaggerated muscle afferent activation during exercise, resulting in enhanced cardiorespiratory responses. This might provide a neural link between the well-established limb muscle dysfunction and the associated exercise intolerance and exertional dyspnoea, which might offer therapeutic targets for these patients.

Gait variability patterns are altered in healthy young individuals during the acute reperfusion phase of ischemia-reperfusion

BACKGROUND

The role of ischemia reperfusion contributing to functional impairment in lower extremity peripheral arterial disease (PAD) patients has not previously been elucidated. The evaluation of gait variability patterns has proven useful in many pathologic populations. Therefore, the purpose of this study is to isolate and determine the specific effect of the acute reperfusion phase of ischemia-reperfusion on gait variability in young individuals with no vascular disease.

MATERIALS AND METHODS

Thirty healthy young individuals walked on a treadmill during baseline and the acute reperfusion phase of ischemia-reperfusion conditions while lower extremity joint kinematics were captured. Stride to stride variability was assessed using the largest Lyapunov exponent, approximate entropy, standard deviation, and coefficient of variation. Differences in gait variability between conditions were assessed using dependent t-tests.

RESULTS

The largest Lyapunov exponent values and approximate entropy values were significantly higher in the acute reperfusion phase of ischemia-reperfusion condition for the ankle, knee, and the hip. Coefficient of variation was significantly higher at the hip and standard deviation was higher at the knee and the hip during the acute reperfusion phase of ischemia-reperfusion condition.

CONCLUSIONS

The acute reperfusion phase of the ischemia-reperfusion cycle alters gait variability patterns at the ankle, knee, and the hip in healthy young individuals. Our findings indicate increased noise and irregularity of gait variability patterns post-ischemia. In young healthy individuals who do not have neuromuscular impairments, significant gait alterations are present during walking after a period of interruption of blood flow.

Skeletal muscle chemoreflex and pHi in exercise ventilatory control

To determine whether skeletal muscle hydrogen ion mediates ventilatory drive in humans during exercise, 12 healthy subjects performed three bouts of isotonic submaximal quadriceps exercise on each of 2 days in a 1.5-T magnet for 31P-magnetic resonance spectroscopy (31P-MRS). Bilateral lower extremity positive pressure cuffs were inflated to 45 Torr during exercise (BLPPex) or recovery (BLPPrec) in a randomized order to accentuate a muscle chemoreflex. Simultaneous measurements were made of breath-by-breath expired gases and minute ventilation, arterialized venous blood, and by 31P-MRS of the vastus medialis, acquired from the average of 12 radio-frequency pulses at a repetition time of 2.5 s. With BLPPex, end-exercise minute ventilation was higher (53.3 +/- 3.8 vs. 37.3 +/- 2.2 l/min; P < 0.0001), arterialized PCO2 lower (33 +/- 1 vs. 36 +/- 1 Torr; P = 0.0009), and quadriceps intracellular pH (pHi) more acid (6.44 +/- 0.07 vs. 6.62 +/- 0.07; P = 0.004), compared with BLPPrec. Blood lactate was modestly increased with BLPPex but without a change in arterialized pH. For each subject, pHi was linearly related to minute ventilation during exercise but not to arterialized pH. These data suggest that skeletal muscle hydrogen ion contributes to the exercise ventilatory response.

Dynamic exercise in man as influenced by experimental restriction of blood flow in the working muscles

The effects of reduced muscle perfusion pressure on dynamic exercise performance and cardiovascular and respiratory functions were investigated. Eight subjects were studied during supine cycle ergometry at stepwise increasing workloads until exhaustion with and without the legs exposed to a supra-atmospheric pressure of 50 mmHg (Leg Positive Pressure, LPP), a novel and convenient means of reducing the perfusion pressure in the working muscles. In the LPP condition exercise performance was reduced by 40% which, judging from assessments of perceived exertion, was due to premature muscle fatigue, indicating local or overall underperfusion of the working muscles. At any given work load, the arterial pressure response was considerably stronger during LPP than in the control condition. LPP also caused greater increases in blood lactate concentration and pulmonary ventilation, the differences from control increasing with the work load. Furthermore, the ventilatory equivalent for O2 at a given work load was markedly higher in the LPP than in the control condition, while exercise-induced decreases in end-tidal PCO2 were considerably exaggerated by LPP. The augmented pressor response during flow-restricted exercise, together with the strong ventilatory response which was out of proportion to overall O2 uptake, suggests increased activation of muscle chemoreflexes by accumulation of metabolic end products, the increased pressor response tending to reduce the local flow error in the working muscles.

Ventilatory control studied with circulatory occlusion during exercise recovery

Mechanisms involved in the control of pulmonary ventilation were studied in seven male subjects following 6 min of exercise on a cycle ergometer at 98w. Circulation to the legs was occluded by thigh cuffs (27 kPa) during the last 15 s of exercise and the subsequent 4 min of recovery. Respiratory gas exchange and the tidal partial pressures of O2 and CO2 were measured breath-by-breath. The results were compared to control studies without occlusion. There was a significant increase in both systolic and diastolic blood pressures during occluded recovery. Following occlusion systolic pressure remained elevated while diastolic pressure returned to control values. Occlusion during recovery caused hyperventilation during the first 1.5 min after exercise as evidenced by significantly higher VE/VCO2, VE/VO2, PETO2, and lower PETCO2. Following the release of the cuffs PETCO2, VE, VCO2, VO2, and heart rate all increased significantly above control values, while PETO2 decreased. PETCO2 rose abruptly 14.5 +/- 0.9 s after the release of the cuffs. Marked increases in VE and heart rate were seen, and occurred 30.8 +/- 1.5 s and 12.8 +/- 1.3 s, respectively, after cuff release. The 16.3 +/- 1.4 s lag between the increase in PETCO2 and VE after occlusion suggests that the ventilatory response to a sudden load of hypercapnic blood is not mediated by a pulmonary chemoreceptor. Other receptors, probably the peripheral chemoreceptors, appear to be responsible for hypercapnic hyperventilation.