Maximum Sustainable Isoflow

Methodology. The isoflow method allows subjects to inspire with Pi,max against a device that provides a constant inspiratory flow rate to the lungs (14, 64) (Figure 8). In this way, it resembles the repeated Pi,max technique but the lungs are inflated and the inspiratory muscles are allowed to shorten at a relatively constant rate. The method was modeled after isoki-netic testing devices commonly used in limb muscle evaluation. Visual feedback of inspiratory pressure, over time, is provided from an oscilloscope. Breathing pattern is generally set such that the subjects hyperventilate during the test. The in-spiratory airflow is humidified, and PETCO2 is maintained at eucapnia with supplemental CO2. For routine measurements, inspiratory flow is maintained, at approximately 1 L/second, inspiratory time at 1.5 seconds and total breath period at 3.5 seconds (duty cycle = 0.42). Many other breathing patterns have been used with this technique (14, 64, 65); however, for normal subjects this pattern has been shown to be well tolerated. Subjects continue to inspire maximally with each breath for 10 minutes. Airway opening pressures generally decline exponentially during this period until a "sustainable" pressure is obtained (Figure 8). Using curve-fitting techniques, it has been shown that sustainable pressures in normal subjects can be calculated within 5% with only 5 minutes of endurance testing (64).

To roughly calculate the additional inspiratory pressure used to overcome lung and chest wall impedance, the isoflow apparatus can be modified to inflate the subject's lungs during complete relaxation (13). This additional positive pressure can be added to active inspiratory pressures developed during each breath to estimate the total inspiratory muscle pressure (Pmus).

The isoflow apparatus consists of a large and well-regulated pressure source providing inspiratory flow across an extremely high resistance (13, 64). The pressure drop across the resistance is so high (8,000 to 14,000 cm H2O) that any additional inspiratory pressures developed by the subject at the airway opening have negligible effects on flow rate. Flow is

TABLE 2. PREDICTED VALUES FOR INCREMENTAL THRESHOLD LOADING*

Age

Subjects No.

Ppeak/Pi,max*

Author (Ref.)

(yr)

(M/F)

(%)

PTipeak

Notes on End Point

Martyn (50)

33 ± 2

14 (9/5)

88 ± 10

NR

Ppeak determined from highest load over 1 min

McElvaney (53)

31 ± 5

10 (5/5)

Trial 1: 84 ± 17 Trial 2: 87 ± 21

~ 0.22 ± 0.07 ~ 0.25 ± 0.08

Highest Ppeak tolerated for full 2 min

Morrison (56)

67 ± 4

8 (5/3)

80 ± 17

~ 0.32 ± 0.12

Highest Ppeak tolerated for full 2 min

Eastwood (57)

30 (28-41)*

Trial 3: ~ 94 ± 21

NR 0.26 ± 0.11

Highest Ppeak tolerated for 30 s

Definition of abbreviations: F = female; M = male; NR = not reported; Pi,max = maximum Inspiratory pressure; Ppeak = peak threshold pressure achieved during incremental loading under conditions stated in Notes on End Point column; PTIpeak = peak pressure-time index achieved during incremental loading under conditions stated in Notes on End Point column.

* Results represent means ± SD.

* Pi,max was measured at residual volume.

Definition of abbreviations: F = female; M = male; NR = not reported; Pi,max = maximum Inspiratory pressure; Ppeak = peak threshold pressure achieved during incremental loading under conditions stated in Notes on End Point column; PTIpeak = peak pressure-time index achieved during incremental loading under conditions stated in Notes on End Point column.

* Results represent means ± SD.

* Pi,max was measured at residual volume.

initiated by negative mouth pressures of —2 to —3 cm H2O and turned off at +2 to +3 cm H2O by an electrical triggering circuit. Subjects are protected from the high-pressure source by breathing from a nonrebreathing valve, which will ensure that flow bypasses the mouth if there is no active inspiration. End-tidal C02 is monitored continuously, and additional CO2 is bled into the inspiratory line to maintain PetC02 at eucapnia. This technique has been used primarily to measure inspiratory muscle endurance.

Normal values. Normal values have not been well described over a wide range of subjects. However, in 15 normal subjects (8 males and 7 females; age, 26 ± 6 years) with breathing patterns described above, the peak airway pressure dropped to 70 ± 7% of their initial pressures (measured with inspiratory flow of 1 L/second), and 61 ± 12% of Pi,max by the end of 10 minutes of repeated contractions (64). The sustainable PTI with the pattern of contraction described above was 0.18 ± 0.04. There is a small but significant training effect between the first and fourth trials with the procedure (64).

Advantages. The isoflow technique has the advantage that most of the important parameters influencing respiratory endurance measurements are controlled. For example, PetC02 (and therefore arterial oxygen saturation), breath timing, in-spiratory flow, and tidal volume are fixed. Furthermore, lung and chest wall mechanics can be accounted for at a first approximation (13). An additional strength is the fact that it is possible to measure the inspiratory muscle strength under similar conditions used in the endurance test. This avoids the difficulty of comparing pressure measurements under static contractions (Pi,max) with contractions under dynamic conditions, where changes in length and velocity of contraction affect pressure development (13, 64). Furthermore, because subjects are performing maximal contractions, the fatigue process develops rapidly and the sustainable pressures can be obtained in a few minutes of testing. The decay of inspiratory pressure over time is an additional variable that can be helpful in distinguishing effects on the fatigue process, independent of sustainable pressure development (61). The test is noninvasive and is tolerated well by naive subjects.

Disadvantages. As yet, the isoflow technique has not been used to test patient populations and therefore its utility has not been determined in the clinical setting. it has, however, been shown to be useful for studying mechanisms of fatigue (13, 59, 61, 65). One potential problem with applying the tech nique on patients is the difficulty with imposing the same breathing pattern used on normal subjects. For example, normal subjects have relatively high ventilatory requirements during the test to assure maintenance of ETCO2, whereas patients with lung disease may not be able to physically perform such high levels of ventilation. Furthermore, the method also depends on subject cooperation, and one cannot be certain of the relative contributions of the rib cage or the diaphragm during contractions. Finally, the equipment used for the isoflow technique is not available commercially, although it is not particularly ex-

Figure 8. (A) Equipment used for the isoflow loading device. The tanks and regulators on the left provide a high-pressure source through an extremely high resistance. Flow is activated by a pressure-triggering device. The oscilloscope provides visual feedback, so that the subjects can perform maximally. CO2 is maintained at a constant value by supplementing the inspiratory gas. (B) A typical endurance curve from an isoflow test. Open circles: The peak pressure developed during inspiration. Open triangles: The average pressure generated during inspiration per breath. Reprinted by permission from Reference 64.

Figure 8. (A) Equipment used for the isoflow loading device. The tanks and regulators on the left provide a high-pressure source through an extremely high resistance. Flow is activated by a pressure-triggering device. The oscilloscope provides visual feedback, so that the subjects can perform maximally. CO2 is maintained at a constant value by supplementing the inspiratory gas. (B) A typical endurance curve from an isoflow test. Open circles: The peak pressure developed during inspiration. Open triangles: The average pressure generated during inspiration per breath. Reprinted by permission from Reference 64.

pensive to create from basic equipment in most physiology laboratories.

In summary, at the present time, for clinical applications, the most promising and practical technique for evaluating the endurance qualities of the global inspiratory muscles against external loads appears to be the incremental threshold loading technique, originally described by Martyn and coworkers (50) and refined by later studies (8, 53, 56, 57). However, because of the uncertainties with regard to its specificity for endurance, we recommend that the term "maximum incremental performance" be used to describe its outcome measures until further information is available. The usefulness of the technique will be advanced by careful quantification of Pmo, Wrs, and Vo2,rs during the tests and by consistent recording of the maximum values that can be maintained for the full 2-minute increments. Further work needs to be done to define predicted values in the normal population.

other methods described here are of considerable value under experimental conditions and should be considered as options, particularly for specific experimental designs where careful control of the variables is critical. Whenever possible, with any of these techniques, the goal should be to define the sustainable level of P and Wrs. Further studies in patient populations will be required to determine the comparative usefulness of these techniques in a clinical environment.

All of the external loading techniques are more likely to reflect the endurance qualities of the rib cage muscles as compared with the diaphragm (44, 57, 66). This should be kept in mind with regard to their clinical implications.

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