Relaxation Rate

Rationale. On cessation of contraction, skeletal muscles relax at a rate determined by their relative proportion of fast and slow fibers. When muscles fatigue, their relaxation rate declines as a result of a slower uptake of calcium previously released from the sarcoplasmic reticulum. During various types of intermittent contractions, the rate of decay of Pes and of Pdi reflects the relaxation rate of inspiratory muscles and of the diaphragm, respectively. When fatigue is induced by breathing against external loads, the inspiratory muscle relaxation rate falls early and then stabilizes, following a time course similar to that of the change in electromyogram (EMG) power spectrum. Thus, relaxation rates typically decline before the occurrence of muscle failure at about the same rate as the center frequency of EMG does. On cessation of loading, the relaxation rate recovers quickly and reaches baseline values within 5 to 10 minutes (42, 43).

Methodology. The relaxation rate of Pes or Pdi can be measured during intermittent contractions against loads (42), during sniffs with airway occlusion (43) or without airway occlusion (44), and during phrenic nerve stimulation (43). The most useful and simple maneuver is the unoccluded sniff, which is easy to perform for most subjects and provides large and consistent changes in relaxation rate after fatigue (45).

Standard balloon-catheter systems are used to measure Pes and gastric pressure (Pga), from which Pdi is obtained. The maximal relaxation rate (MRR) of Pes or Pdi is calculated as the first derivative of pressure with respect to time (dP/dt) over the first half of the relaxation curve. This is obtained by drawing a tangent to the steepest portion of the pressure curve. Because the MRR increases with the amplitude of the pressure swing, it is usual to normalize the MRR and to express it as a percentage of the pressure fall in 10 milliseconds (42, 45). When the natural logarithm of pressure is plotted as a function of time, a straight line appears over the lower 6070% portion, indicating a monoexponential decay. The reciprocal of the slope of this line represents the time constant (t) of this exponential decay, which may be used as another measure of muscle relaxation, usually expressed in milliseconds (42, 45) (see Figure 3). Thus, a slower muscle relaxation is documented by a decline in the MRR and by an increase in t. In-spiratory muscle relaxation rate can be assessed in a less invasive manner by measuring the MRR of nasopharyngeal or mouth pressure during sniffs with balloons positioned in these locations (44). An entirely noninvasive measure of inspiratory muscle MRR can be obtained by using SNIP (46). These less invasive techniques have been validated only in normal subjects. Transmission of brief pressure swings from the alveoli to the upper airways is likely to be dampened in patients with abnormal lung mechanics.

Equipment. The pressure measurement system required for sniffs is described in Section 2 of this Statement. Analysis of the MRR is best done with a computer program (46).

Advantages. The measurement of inspiratory muscle relaxation rate is relatively simple and requires minimal cooperation from subjects. The sniff maneuver is easily performed by most subjects and patients and does not need to be perfectly "maximal," provided that the MRR is expressed as percentage pressure fall per 10 milliseconds. Sniffs should, however, be performed as near to the maximal as possible, because the MRR is effort dependent below 60% of maximal pressure (47). The muscle relaxation rate slows at an early stage during fatiguing contractions and may therefore indicate that inspira-tory muscle fatigue is incipient.

Disadvantages. The relationship between changes in relaxation rate and force loss during fatigue is not understood. For instance, the degree of force loss does not correlate with the changes in relaxation rate and therefore cannot be inferred from this parameter (45, 47). The rapid recovery of this index with rest also poses practical problems of measurement in clinical settings. The range of normal values for the MRR and t is wide, with overlap between fresh and fatigued states. Serial measurements are thus required to detect the onset of in-spiratory muscle fatigue in an individual (45). Finally, some clinical conditions (e.g., asthma) have been reported to elicit activation of inspiratory muscles during expiration (postin-spiratory inspiratory muscle activity). Under such circumstances, persistent activation of some muscles or portions thereof would

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Figure 3. The time constant of respiratory muscle relaxation, t, as a function of breath number during a series of breathing trials against respiratory loads. Each curve represents the results from a single breathing trial, with individual points on each curve representing relaxation time constants calculated on single breaths during the trial. Each trial is annotated with the pressure-time index (TTdi, equivalent to the pressure-time index using the nomenclature in the present document) achieved during the trial. During a trial with a high pressure-time index of 0.4 (uppermost curve), relaxation slowed dramatically, with a large increase in the relaxation time constant, t, within a few breaths. In contrast, with less intense respiratory loading, that is, lower pressure-time indices, there was less slowing of relaxation and smaller increases in the relaxation time constant. At the lowest pressure-time index (0.15, lowest curve), there was essentially no slowing of relaxation and no increase in the time constant during the breathing trial. Reprinted by permission from Reference 42.

be expected to alter measured relaxation rates, distorting the relationship of alterations in the MRR to cellular events and to the development of muscle fatigue.

Applications. Measurement of the relaxation rate can be used with confidence as an early sign of fatigue in subjects subjected to high external inspiratory loads (42-45) or to highlevel hyperpnea (46). The inspiratory muscle relaxation rate can also be used to detect fatiguing contractions during exercise in patients with chronic obstructive pulmonary disease (48) or during weaning trials from mechanical ventilation (49). Because interpretation of changes in relaxation rate is not straightforward, this test can be considered useful only for clinical research.

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