Endurance To External Loads Rationale

When an external mechanical load is applied to the airway opening, the respiratory muscles must generate an additional pressure to overcome the impedance of the load. The external load can be one of several types: (1) a flow resistive load, in which the pressure required of the muscles is dependent on the flow rate across the resistance. Flow resistive loads can be linear or nonlinear depending on whether they produce laminar or turbulent flow; (2) elastic loads, in which the pressure required of the muscles is dependent on lung volume. The higher the tidal volume, the higher the pressure required. Such loads are flow independent; (3) threshold loads, in which a finite pressure is required to open a valve that allows flow to occur. Therefore, the pressure required of the muscles at the airway opening is relatively constant, independent of both volume and flow. Threshold loads result in contractions that are similar to isotonic contractions; or (4) isoflow loads, in which the flow rate and therefore, the rate of inflation is held constant and the pressure generated against the flow is a measured output variable. Isoflow loads are similar in concept to "isokinetic" contractions of limb muscles, in which velocity of shortening is held constant.

To conduct a respiratory muscle endurance test with an external load requires setting the task that the subject must perform against the load. For example, the subject may be asked to breathe normally or to breathe with a set breathing pattern or with a specific muscle configuration. Different ways of contracting against the load result in markedly different measures of endurance, reemphasizing the importance of the concept of task specificity.

The advantage of using externally applied loads is that it is much easier to control the relevant variables during the test. It is even possible to design tests that are specific to the diaphragm (1) or the rib cage muscles (2, 43). Generally, these tests require large developed pressures against normal or relatively modest changes in ventilatory requirements. such conditions are similar to those of weight lifting, with relatively low velocities of shortening. In contrast, measures of ventilatory endurance, described previously, are more like activities of running with large velocities of shortening and participation by a large number of synergic muscle groups. Interestingly, measurements of endurance to high inspiratory resistive loads appear to be more a reflection of rib cage muscle endurance than diaphragm endurance (44). Therefore, the exact extent to which measurements of endurance to high external loads apply to ventilatory endurance or to clinically relevant conditions such as exercise has not been well defined.

A large number of devices and techniques have been developed to measure endurance of external loads. The most common is the use of orifice-type flow resistance applied to the in-

Figure 6. Relationships between power output, ventilation, and endurance time (Tlim). Solid line: Maximum voluntary ventilation. Dotted line: Calculated work rate or power output of the respiratory muscles. Redrawn by permission from Reference 33.

spiratory circuit (20, 45). Excellent studies can be performed with flow resistances, but because the pressure load seen by the respiratory muscles depends on the developed flow, the technique requires visual feedback of some form of ventilation, preferably the flow rate. Therefore, for practical reasons, flow resistances have largely been replaced in most clinical laboratories by threshold loading devices or other techniques discussed below. The techniques below have generally been used to measure inspiratory muscle endurance.

Maximum Sustainable Threshold Loading

Methodology. Nickerson and Keens (46) developed a method in which endurance times are measured in response to gradually decreasing threshold pressures, starting near PI,max. They described one of the first threshold loading devices that was relatively flow independent. The test usually begins with a careful measurement of PI,max. sequential Tlim measurements are then made, beginning at approximately 90% of Pi,max and decreasing in increments of 5%. Subjects are allowed to rest between each measurement for approximately 10 times the length of Tlim. No attempt is made to control the breathing pattern. Task failure is determined at each load by the inability to maintain ventilation against the load, resulting in the subject coming off the mouthpiece. other definitions of task failure define a point at which a subject is unable to generate the threshold pressure or a target flow for three consecutive breaths (47). The first pressure that can be sustained for more than 10 minutes is considered the sustainable inspiratory pressure (SIP). The SIP is determined by averaging the pressures over the last 20 breaths.

The original Nickerson and Keens (46) threshold loading device has never been available commercially, but is made of a simple plunger, with leaded rings added to the inside of the chamber to weight the valve. A more modern version is illustrated in Figure 7. There is a linear relationship between increases in weight and the pressure required to lift the plunger. The original device used a plunger, seated with a 1-in. o ring onto a 45° surface (46). Larger o rings result in more flow independence but less stability. Even small changes in the size of the contact circumference and the precision of the seating can have large effects on the weight/pressure relationship. Therefore, each homemade valve requires independent additional supports for the plunger, which improve its stability (48), and the use of standardized, commercially available valve mechanisms (nondisposable positive end-expiratory pressure valves), which improve the pressure-flow characteristics (Figure 7)

(49). Some commercially available spring-loaded threshold valves do not have the pressure range necessary for testing endurance in most patients.

Normal values. As with most respiratory muscle endurance testing techniques, normal values have not yet been developed. For example, the influence of stature, age, and sex is not described and the numbers of subjects have been low. Nickerson and Keens (46) tested 15 normal individuals ranging from 5 to 75 years of age. The 12 adults could maintain a mean ± SD SIP of 82 ± 22 cm H2O, or 71 ± 10% of Pi,max. On a second trial, in 12 subjects, both Pi,max and SIP increased by approximately 10%, while the relationship of SIP/ PI,max remained constant. Somewhat different results were found by Martyn and coworkers (50) when using the method of Nickerson and Keens (46). They found that the SIP was 52 ± 6% of PI,max on the first trial. However, when subjects were asked to repeat the loads that they had previously failed, they were able to increase their SIP to 77 ± 6% of PI,max (50).

Advantages. The attraction of the technique of Nickerson and Keens (46) has been that it provides a method for evaluating global respiratory muscle endurance in a one-session test, much like a pulmonary function test. There were no previous studies that defined a technique to establish sustainable pressure in a practical setting. Furthermore, the test is noninvasive and is tolerated relatively well, the equipment required is inexpensive and does not require a great deal of training or coordination for the subject, and the results are relatively independent of the mechanics of breathing because minute ventilation increases minimally.

Disadvantages. It is clear that subjects will adjust their breathing pattern as they attempt to breathe against any kind of large mechanical load, and they will learn to do this over time (51). This effect may have been underestimated by Nick-erson and Keens (46) as discussed by Martyn and coworkers

(50). Relatively small changes in duty cycle (52), inspiratory flow rate (20, 52), and tidal volume (13) can have relatively large effects on endurance measures. Therefore, it would seem appropriate to control the pattern of contraction against the load during the test. However, it is likely that a naive subject will be able to achieve longer Tlim values when allowed to breathe spontaneously. Artificially imposing a breathing pattern may not be appropriate for body size, vital capacity, or CO2 production. Furthermore, chest wall configuration, and therefore respiratory muscle recruitment, are quite different when inspiring against "target" respiratory patterns, when agonists and antagonists are simultaneously recruited (52), as compared with spontaneous or maximum uncontrolled inspirations (13). Nevertheless, the effects of the pattern of contraction and recruitment on Tlim result in an inherent measurement variability between subjects and in the same subject over time (50). It is likely that this proble m could be overcome to some extent by measuring Pmo and Wext, because they are likely to be the most dominant determinants of Tlim, regardless of the pattern of breathing. However, this has not been measured systematically in available clinical studies using maximum sustainable threshold loading.

Having subjects begin with endurance trials at the highest pressure loads can be exhausting, uncomfortable, and time-consuming for the patient. The test generally requires a minimum of 2 hours, as was originally described (46).

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