Patients with Parkinson's disease may have more difficulty performing repetitive motor acts than single motor acts because of bradykinesia and skeletal muscle rigidity. We thought that repetitive ventilatory tasks might be similarly limited and that this dysfunction would likely contribute to respiratory muscle fatigue. We studied 9 patients with Parkinson's disease who had no evidence of restrictive or obstructive lung disease and 5 normal age-matched control subjects who performed repetitive, forced inspiratory resistive-loaded tasks. The time a given mean airway opening pressure could be sustained, the incremental oxygen cost of breathing, and the work rate of breathing (W) were measured. Although maximal static inspiratory pressures were comparable in both groups, 8 of the 9 patients could not sustain as high a W in the resistive-loaded tasks as could the normal control subjects (41.0 +/- 23.0 versus 67.7 +/- 29.1 J/min; mean +/- SD, p less than 0.01) and the efficiency of breathing was reduced (2.0 +/- 0.8 versus 3.8 +/- 1.4%; p less than 0.01). These findings are similar to derangements of task performance by peripheral skeletal muscle groups in Parkinson's disease.

When a subject breathes against an inspiratory resistance, the inspiratory pressure, the inspiratory flow, and the lung volume at which the breathing task takes place all interact to determine the length of time the task can be sustained (Tlim). We hypothesized that the mechanism actually limiting tasks in which these parameters were varied involved the rate of energy utilization by the inspiratory muscles. To test this hypothesis, we studied four experienced normal subjects during fatiguing breathing tasks performed over a range of pressures and flows and at two different lung volumes. We assessed energy utilization by measuring the increment in the rate of whole body O2 consumption due to the breathing task (VO2 resp). Power and mean esophageal pressure correlated with Tlim but depended also on lung volume and inspiratory flow rate. In contrast, VO2 resp closely correlated with Tlim, and this relationship was not systematically altered by inspiratory flow or lung volume. The shape of the VO2 resp vs. Tlim curve was approximately hyperbolic, with high rates of VO2 resp associated with short endurance times and lower rates of VO2 resp approaching an asymptotic value at high Tlim. These findings are consistent with a mechanism whereby a critical rate of energy utilization determines the endurance of the inspiratory pump, and that rate varies with pressure, flow, and lung volume.

Skeletal muscle adapts differently to training with high forces or with high velocities. The effects of these disparate training protocols on the inspiratory muscles were investigated in ten healthy volunteers. Five subjects trained using high force (pressure) loads (pressure trainers) and five trained using high velocity (flow) loads (flow trainers). Pressure training entailed performing 30 maximal static inspiratory efforts against a closed airway. Flow training entailed performing 30 sets of three maximal dynamic inspiratory efforts against a minimal resistance. Training was supervised and carried out 5 days a week for 6 weeks. Inspiratory flow rates and oesophageal pressure-time curves were measured before and after training. Peak inspiratory pressures during maximal static and dynamic efforts and peak flows during the maximal dynamic efforts were calculated. The time-to-peak pressure and rate of rise in peak pressure during maximal static and dynamic manoeuvres were also calculated before and following training. Maximal static pressure increased in the pressure training group and maximal dynamic pressure increased in the flow training group. Both groups increased the rate of pressure production (dP/dt) during their respective maximal efforts. The post-training decrease in time-to-peak pressure was proportionately greater in the flow trainers than in the pressure trainers. The differences in time-to-peak pressure between the two groups were consistent with the different effects of force and velocity training on the time-to-peak tension of skeletal muscle.

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