Work Rate of the Respiratory System

Methodology. Generally, the ventilatory work rate (power output) of the respiratory system (Wrs) refers to mechanical work performed by the respiratory muscles against the lungs and chest wall during ventilation. It is calculated as the integration of the appropriate measures of pressure X volume (see Assessment of the Function of the Active Chest Wall: Campbell Diagram in Section 6 of this Statement). In this discussion, we will use Wrs to also include the work rate performed by the respiratory system against any external loading device. Work rate is expressed in joules per minute (1 J = 1 kPa ■ 1 L; 1 kPa = 10.2 cm H2O). The complete measurement of work of breathing against the lung and chest wall, for both inspiration and expiration, is complex, largely because components involving movement and distortion of the chest wall are difficult to quantify without relatively sophisticated analyses. However, in many cases, measuring the work performed against an external load (Wext) may provide sufficient information for purposes of respiratory muscle endurance testing.

If a subject is breathing against an external load and ventilation remains near spontaneous levels during loading, the rate of work performed against the lung and chest wall remains relatively unchanged from normal breathing. Therefore, any "changes" in Wrs can be attributed largely to changes in the work performed against the external load, or Wext. For example, if a subject were breathing against an in-spiratory resistive load, Wext would be directly proportional to changes in Pmo because (Equation 3)

where Vi = inspiratory minute ventilation. Equation 3 emphasizes one of the reasons why measures of the pressure-time product are so powerful in predicting endurance and changes in energy consumption during external loading. If Vi stays constant, changes in Pmo become the sole determinant of changes in Wext.

Figure 2. Three important physiologic variables that are directly related by the pressure-time index of the diaphragm (PTIdi). (A) Endurance time (Tlim) of the diaphragm in human subjects. The critical PTIdi of approximately 0.18 refers to the maximum PTIdi that is sustainable for a period longer than 2 hours. Values above 0.18 result in fatigue and task failure; redrawn by permission from Reference 1. (B) Diaphragmatic blood flow is affected by the PTIdi such that above a critical level, increases in PTIdi result in reductions in blood flow; redrawn by permission from Reference 14 (data on dogs). (C) Oxygen consumption of the respiratory system (VO2,rs) increases as a function of PTIdi. Measures above 0.2 are difficult to measure in the steady state because of fatigue. Redrawn by permission from Reference 16.

Figure 3. (A) Effects of inspiratory flow rate on the oxygenconsumption of the respiratory muscles (Vo2,rs) at constant pressure-time products (isopleths). Reprinted by permission from Reference 15 (data on humans). (B) Effects of changes in respiratory work rate on endurance time (Tlim) at constant pressure-time product. Reprinted by permission from Reference 19. (C) Effects of respiratory work rate on Vo2,rs when pressure-time product is allowed to vary. Reprinted by permission from Reference 15.

Figure 3. (A) Effects of inspiratory flow rate on the oxygenconsumption of the respiratory muscles (Vo2,rs) at constant pressure-time products (isopleths). Reprinted by permission from Reference 15 (data on humans). (B) Effects of changes in respiratory work rate on endurance time (Tlim) at constant pressure-time product. Reprinted by permission from Reference 19. (C) Effects of respiratory work rate on Vo2,rs when pressure-time product is allowed to vary. Reprinted by permission from Reference 15.

The use of Equation 3 eliminates the necessity of performing complex integrations of individual pressure-volume loops for each breath, which are required for more sophisticated estimates of the total Wrs, discussed below. Therefore, it is possible to measure changes in Wext, online, with digital or electronic multiplication of Pmo and Vi.

An additional component of Wext occurs from gas compression (expiration) or decompression (inspiration) when large pressures are generated in the airways (15). During inspiratory loading, this gas decompression can account for as much as 0.4 L of displaced tidal volume in normal subjects, elevating the work of breathing by as much as 50%. If thoracic volume is measured in a volume displacement box, this additional volume is measured directly. However, it can also be calculated. Appropriate equations for adjustment of Vi for gas decompression depend on the nature of the loading device (15, 19, 21). For example, if a threshold loading device is used, in which inspiratory pressure generation is approximately constant, the inspired minute ventilation can be adjusted appropriately by adding the decompression volume calculated in the following way:

where AVt,i is the additional inspiratory tidal volume in liters (due to gas decompression) that must be added for each breath in the calculation of inspiratory minute ventilation (Vi) in Equation 3. Vt,i is the inspired tidal volume (before gas decompression); FRC is the functional residual capacity in liters (measured independently); Pbs is body surface pressure (usually atmospheric); PH2O is water vapor pressure at body temperature; and Pmo is the threshold loading pressure at the end of an inspiration. Of course, all pressures in Equation 4 must be of the same units (e.g., mm Hg or kPa). For measures of ventilatory endurance, or when there are changing levels of ventilation, a significant portion of the work being performed by the respiratory muscles is done against the resistive and elastic properties of the lung and chest wall. Therefore, accurate estimates of total Wrs must include these measurements. The work rate against the lung and chest wall is most often obtained by the Campbell method (22), which requires the use of an esophageal balloon for estimating pleural pressure and measurement of a relaxation-pressure-volume curve for the lung and chest wall. The original Campbell method (22) is somewhat tedious to apply practically for routine clinical endurance measurements. Equipment is now available to perform the calculations automatically by computer; but even with computerized techniques, examination of the breath-by-breath pressure-volume loops is required. For relevant discussions of the appropriate use of the Campbell method and the Campbell pressure-volume diagram, refer to reviews (23-25).

Advantages. As discussed previously, when ventilatory flow rate increases, total Wrs becomes an increasingly important determinant of both energy consumption of the muscles and endurance time (Figure 3). For ventilatory endurance testing, measurements of Wrs overcome the problems of variability in lung and chest wall impedance between subjects and in the same subjects over time. Such changes in lung mechanics are inevitable in patients who may have wide diurnal variations and fluctuations over more extended time periods. Therefore, measurement of Wrs may be necessary to draw appropriate conclusions regarding the endurance properties in various patient groups. To a large extent, these studies have yet to be systematically performed.

Whether Wrs or P should be chosen as the primary global measure of respiratory muscle activity for endurance testing cannot be stated with certainty at this time. It would be ideal if a comprehensive relationship between Wrs, P, Vo2,rs, and endurance for the respiratory muscles could be derived for all loading conditions. From an energetics standpoint, the relationship between them is roughly described for the inspiratory muscles by Equation 5:

where Ers is the efficiency of the inspiratory muscles and Pmus is the mean respiratory muscle pressure per breath (15). Equation 5 suggests that if one knew Ers in a given subject, as well as Vi, the energetics and presumably the endurance of the respiratory muscles could be predicted. Unfortunately, Ers is not particularly constant at different relative velocities of muscle shortening (24) or at differing ventilations, depending on the way breaths are performed (21), making this ideal difficult to obtain.

Disadvantages. The largest disadvantage of monitoring Wrs during endurance measurements is the complexity of its accurate measurement and analysis. This is not true, however, for the component of Wrs that comes from Wext. Furthermore, after decades of studies regarding the work of breathing, there are portions of chest wall movement and distortion that remain elusive and difficult to quantify under loading conditions. As shown in Figure 4, distortions of the chest wall are commonly seen as an adaptive response to external loading (26). Distortions are also seen during maximum ventilatory maneuvers (27). Finally, the simple measurement of the relaxation pressure-volume curve is not easy to obtain in many patients because of the requisite for complete muscle relaxation (28, 29).

Finally, one component of P that may be important in determining endurance characteristics, and that is not directly related to Wrs, involves the influence of developed pressure on blood flow during contraction. For example, as Pdi increases, blood flow to the diaphragm is limited, presumably by the relationships between tissue pressure and vascular conductance (14, 30) (Figure 2B). Because sustainable task intensities may in part reflect a balance of energy utilization and supply, it is likely that the influence of P on muscle perfusion has an independent effect on endurance that cannot be fully accounted for by its mathematical contribution to Wrs or its energetic contribution to Vo2,rs.

0 0

Post a comment