This Section of the Statement has explored the available techniques to assess respiratory muscle endurance. The measurements and techniques include the following:
1. Pressure-time product (PTP): The integration of inspira-tory pressure swing over time. Pressure can be esophageal, Pdi, or mouth pressure (if an external resistance is added to the circuit). PTP of the expiratory muscles can also be measured. If pressure is normalized to a fraction of the maximum pressure, the value obtained is the pressure time index (PTI). A PTI of 0.15-0.18 is the upper limit that can be sustained indefinitely by the diaphragm in humans and as high as 0.3 for the rib cage muscles. The PTI thresholds are lower if inspiratory flow is high.
2. Work of breathing: Calculated by the integration of pressure on tidal volume, measures work against an external in-spiratory or expiratory load and is a useful test for measuring endurance as a function of workload. Values of work of breathing relate well to oxygen consumption over a wide range of ventilations. This measurement is limited to respiratory research and could benefit from computerized equipment to facilitate measurement and analysis in the clinical setting.
3. Ventilatory endurance tests: Maximal sustainable ventilation (MSV) expressed as a percentage of 12 seconds of maximum voluntary ventilation. Two techniques are available to determine MSV: the maximum effort technique (the subject seeks to sustain ventilation at a target level of 70-90% MVV for 8 minutes) and the maximum incremental technique (starting at 20% MVV, the target ventilation is increased by 10% every 3 minutes). There are limited normal data for MSV and these show considerable variability. Each laboratory should develop its own normal values. MSV can be difficult to interpret (e.g., in patients with COPD). The incremental technique may prove to be of value in the clinical setting. To date, most studies of venti-latory endurance have been undertaken within a research context.
4. Endurance of external loads applied to the airway: The external load can be resistive (the pressure required depends on flow), elastic (pressure depends on tidal volume), threshold (finite pressure required to open the valve, which is independent of flow and volume), or an isoflow load (flow rate held constant). The most widely used technique is that of threshold loading. Either the maximum sustainable threshold load or the maximum incremental threshold load can be measured. The incremental threshold loading test, which uses the same principles as an incremental exercise test, is the most commonly undertaken, and there are limited normal data available. It is not clear to what extent the test reflects respiratory muscle strength rather than endurance.
5. Repeated maximum inspiratory pressures: In this test the subjects undertake 18 repeated PI,max maneuvers, each effort lasting 10 seconds with a 5-second rest between contractions. Pressure drops to 87% of PI,max in young normal subjects over the run. Equipment is simple, and only a manometer and stopwatch are required. Few data from studies in patients are available.
6. Maximal sustainable isoflow: In this test the subject breathes against a high impedence (air tank) providing a constant flow (1 L/second). The subject develops maximal pressure at a TI/Ttot of 0.40. Maximum pressure declines exponentially to a sustainable level of 61%, yielding a PTI of 0.18. This technique has not yet been tested in large populations. It has potential as a method of training the inspiratory muscles as well as documenting their endurance.
7. Endurance of the diaphragm: This has been studied in normal subjects by measuring Pdi and TI/Ttot, which were kept constant by following a pattern of pressure and timing displayed on an oscilloscope. A PTI of 0.20-0.30 resulted in task failure at an earlier time. The technique was developed as a physiologic study designed to measure the use of TTIdi as a parameter to evaluate the development of diaphragm fatigue.
Of the tests of ventilatory endurance available, the most promising, in a clinical context, appears to be the maximum incremental ventilation test. To specifically assess the endurance of the inspiratory muscles, relatively independently of lung and chest wall mechanisms, the incremental threshold loading test appears to be most useful.
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