Pressure Time Index of Inspiratory Muscles

Rationale. In several investigations, skeletal muscle fatigue was found to occur when a muscle generated more than 15% of its maximal force during sustained contraction. This work has led to the concept that a fatigue threshold exists, with fatigue occurring only when the level of pressure-time generated exceeds this threshold level. Additional work on this concept has shown that the fatigue threshold is higher during intermittent contractions and depends on the relative duration of contraction and relaxation. The same holds true for the in-spiratory muscles of subjects submitted to external inspiratory loads. The pressure-time index of the diaphragm is defined as

where Pdi is the mean transdiaphragmatic pressure generated per breath, Pdi,max is maximal transdiaphragmatic pressure, Ti is inspiratory time, and Ttot is total breath time. When breathing is accomplished predominantly with the diaphragm, the critical PTdi is 0.15-0.18. Below this threshold, breathing can be sustained for more than 1 hour without evidence of fatigue. Above this threshold, task failure occurs after a time limit that is inversely related to PTdi (29) (see Figure 2). In most situations in which inspiratory loads are applied, the spontaneous breathing pattern is characterized by predominant recruitment of inspiratory rib cage muscles other than the diaphragm, resulting in augmented rib cage expansion and abdominal paradox. Under these circumstances, the pressure-time index of the inspiratory rib cage muscles is defined as

where Ppl is mean pleural pressure generated per breath and Ppl,max is maximal pleural pressure (equivalent to MIP). With this breathing pattern, the critical PTrc is 0.30. Above this threshold, task failure occurs after a time limit that is inversely related to PTrc (30).

Methodology and equipment. The measurement of esoph-ageal pressure is required to compute PTrc, and the measurement of both esophageal and gastric pressures is required to compute PTdi. This is most commonly performed with balloon-catheter systems, as described in Pressure Measurements in Section 2 and in Figures 8 and 9 in Section 4 of this statement.

Advantages. In principle, PTdi and PTrc characterize the operational conditions of the inspiratory muscles with respect to their fatigue threshold. These indices may allow the assessment of the risk of fatigue before actual task failure occurs.

Disadvantages. The critical values of PTdi and PTrc were established in healthy subjects breathing against external loads. The critical thresholds may be different in clinical circumstances, in which a number of pathological factors (e.g., levels of tissue perfusion, presence of hypoxemia) may influence muscle performance. Second, pressure-time index assessment is dependent on accurate measurement of maximal muscle pressure generating capacity (i.e., the Pdi,max of the diaphragm, Ppl,max for the inspiratory rib cage musculature), which is often difficult in patients. Third, shortening velocity of muscle fibers strongly influences muscle energetics and the metabolic consequences of contraction. As a result, the critical PTdi and PTrc that can be tolerated are also a function of in-spiratory flow patterns, with lower values for these parameters at high inspiratory flows. Fourth, some clinical conditions (malnutrition, steroid myopathy) change muscle fiber populations, altering the relationship between muscle strength and fatigability. Conditions that result in a shift to a greater concentration of slow fibers in muscle may well result in a better tolerance of a given absolute level of the pressure-time index and an increase in the critical pressure-time index for the diaphragm and rib cage muscles. The critical pressure-time index in most patients remains to be measured.

Applications. The pressure-time index should be considered a conceptual framework within which to gauge the level of muscle function rather than an instrument for the clinical diagnosis of fatigue. Fatigue thresholds have been reported in patients with chronic obstructive pulmonary disease (31, 32); however, it remains largely untested in other pathologies.

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