Ventilatory Endurance Tests Rationale

The goal of ventilatory endurance testing is to define the maximum sustainable ventilation (MSV), usually expressed as a fraction of maximal voluntary ventilation (MVV). The time duration needed to define "sustainable" is a topic of some controversy and varies with the specific technique described below. As shown in Table 1, normal subjects can sustain ventilations ranging from 60 to 80% of MVV. Therefore, with submaximal exercise, it is probably rare that any normal individuals ever exceed their MSV, because maximum exercise ventilations average approximately 61 ± 14% of MVV in the normal population (31). In some athletes, ventilation is maintained near the sustainable level of sedentary subjects. For example, elite cross-country skiers can sustain ventilation averages during exercise in excess of 100 L/minute, or approximately 61% of their predicted MVV for periods of 30 to 85 minutes (32), with little or no evidence of fatigue. However, the baseline MVV in these athletes is frequently elevated above normal, and unlike normal subjects, they can sustain 86-90% of MVV for 4 minutes, presumably because of their extreme conditioning. In a clinical setting, the measurement of ventila-tory endurance takes on a much greater importance because patients with chronic lung disease or perhaps heart failure (4) may progress to a condition in which exercise is limited by their ability to sustain ventilation. The ventilatory endurance test is a measure of both inspiratory and expiratory muscle endurance.

Methodology

Early techniques for measuring MSV required repeated trials of MVV with gradually decreasing levels of ventilation, until an MSV could be determined (33). These have generally been found to be exhaustive and time-consuming, rendering them largely impractical for most clinical investigations. However, more recent methods have been developed that make the procedure more practical to perform, requiring only 10-25 minutes/test (4, 34, 35).

For all measurements of MSV in obstructed patients, it is recommended that the test be preceded by administration of a nebulized bronchodilator. This may be particularly useful if ventilatory endurance is to be repeated at different times, for example, before and after rehabilitation, to reduce inherent variability in airway resistance.

The test begins with the routine measurement of a 12-sec-ond MVV, using the same equipment employed for the MSV test. Protocols for technique and reproducibility of MVV, which meet American Thoracic Society (ATS) criteria, are available (36, 37). Accurate MVV measurements are critically important for interpretation of MSV. There are two primary techniques for acquiring MSV, the maximum effort technique and the maximum incremental technique, as discussed below.

The maximum effort technique requires subjects to target a ventilation of approximately 70-90% of their MVV (7, 34), using visual feedback from a spirometer or an oscilloscope (Figure 5). Sometimes, one or two short practice trials are used to determine the starting target ventilation. During the first 2-5 minutes, the target ventilation is adjusted up or down to a level slightly lower than the subject's maximum effort. The subject is then continually encouraged to meet the target for the next 8 minutes. There are some studies that have described measuring only a 4-minute MVV as an indicator of endurance

Figure 4. Chest wall distortion during inspiration, shown as rib cage (solid symbols) and abdominal movement (open symbols), with progressive increases in inspiratory mechanical loads. The y axis represents the percentage of tidal volume excursion in which the abdominal compartment is moving "in" during inspiration ("abdominal paradox") and the rib cage compartment is distorted "out" during inspiration, along a path that is greater than its expected path of expansion. Reprinted by permission from Reference 26.

(32). Although a potentially useful and practical approach, insufficient data are available to evaluate whether this provides a sufficient estimate of sustainable ventilation. In all studies, it is necessary to control end-tidal carbon dioxide (PetC02) during the test, usually by adjustment of the carbon dioxide fraction (Fco2) in the rebreathing dead space. The average ventilation achieved over the last minute is consid ered to be the MSV. It is not routine to measure the Wrs or Vo2,rs, but there are rational advantages in doing so, as discussed below.

There is no standardized equipment available for measuring ventilatory endurance. However, the system used should have the following capabilities: (1) provide for maintaining isocapnia during hyperpneic maneuvers; (2) have a low impedance to air flow, which meets accepted standards for spirometry such as ATS criteria (e.g., < 2.5 cm H2O/L/second to 14 L/second) (36); (3) provide reasonable humidification of the inspired air; and (4) provide real-time visual feedback of ventilation. Mechanical systems that approach these criteria have been described in the literature (5, 7, 34, 38), one of which is illustrated in Figure 5. Care must be taken if pneumo-tachographs are used for ventilation measurements, to ensure that they are linear over the range of measured flows, that their electronic drift is compensated for, and that they do not contribute significantly to the resistance of the system. If the pneumotachograph is in the patient line, appropriate compensations should be made for changes in gas viscosity due to supplemental oxygen if it is used.

The maximum incremental technique is a newer procedure for obtaining an estimate of MSV. It uses 10% incremental increases in target ventilation, every 3 minutes, beginning at 20% of MVV until the subject cannot sustain the target ventilation for the last 3-minute period (4, 35) (MSV is calculated from the last 10 breaths of the last minute of the highest target ventilation). This technique, which resembles an incremental exercise test, was demonstrated to result in MSV measure-

TABLE 1. PREDICTED VALUES FOR MAXIMUM SUSTAINABLE VENTILATION/MAXIMUM VOLUNTARY VENTILATION

Subject Age

Subject Sex

MSV/MVV*

Author (Ref.)

(yr)

No. of Subjects

(No. F/M)

(%)

Keens(7)

26 ± 2

16

16/0

60 ± 8

Keens(7)

31 ± 1

14

0/14

62 ± 9

Leith and Bradley (5)

31 ± 3

12

1/11

80 ± 6

Bai (39)

31 ± 3

5

0/5

75 ± 4

Belman and

67 ± 4

25

14/9

63 ± 11

Gaesser (69)

(estimate)

Mancini (4)

50 ± 1

8

1/7

55 ± 9

Definition of abbreviations: F = female; M = male; MSV = maximum sustainable ventilation; MVV = maximum voluntary ventilation. * Results represent means ± SD.

Definition of abbreviations: F = female; M = male; MSV = maximum sustainable ventilation; MVV = maximum voluntary ventilation. * Results represent means ± SD.

ments nearly identical to those that could be attained by traditional approaches, and was well tolerated by subjects (4).

The importance of sustaining a maximum ventilation for the three or more minutes at the end of the test should be emphasized. Presumably, during this period, fatigue of the respiratory muscles is progressing rapidly because it is a period of maximum effort following a relatively long period of "near-maximum effort." Presumably this results in a decay of ventilation to a near sustainable level.

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