Measures Of Respiratory Muscle Activity Used In Endurance Testing

Rationale

Many different kinds of tasks have been used to quantify the endurance properties of the respiratory muscles. Most often, endurance has been defined in terms of the ability to sustain a level of minute ventilation (ventilatory endurance) or a level of inspiratory and sometimes expiratory pressure. However, these simple measures often present limitations to evaluating the effect of the load on the respiratory muscles. From a muscle energetics viewpoint, the energy requirements of a working muscle (and therefore a rough estimate of its level of activation) are determined largely by the tension developed over time (i.e., tension-time product) and the rate of mechanical work being performed (W) (10, 11).

Pressure-Time Product

Methodology. Refee to Pressure Measurements in Section 2 of this Statement for spenific techniques for the measurement of pressure at the airway opening and esophageal, gastric, and transdiaphragmatic pressures. The pressure-time product (PTP) is the integration of respiratory pressure over time (i.e., f Pdt). It is common to express PTP over a 1-minute interval (i.e., units = pressure X time; e.g., cm H2O X minutes). The integration process can be performed by most medical amplifiers or digital computers, much like flow is integrated to obtain minute ventilation. If such techniques are used, assurances must be made that expiratory pressures during the expiratory phase, or inspiratory pressures generated due to chest wall elastic recoil, are excluded from the analysis of inspiratory PTP.

A common expression of the PTP is the mean pressure generated over an entire breath cycle (P) in Equation 1, in which

For example, if PTP is measured for a single breath period, then the sampling period would be total breath period (Ttot). A signal averaging circuit (available on most medical amplifiers for determining mean vascular pressure) can often be used to measure P directly, online. These are usually composed of "leaky integrators" with time constants of approximately 20 seconds. The analysis can also be done by digital computer or mechanical devices (12).

The P value calculated in Equation 1 can be measured at the mouth or airway opening if one wishes to estimate the average pressure generated by all the respiratory muscles working against an external load (i.e., Pmo). Alternatively, it can be measured using: transpulmonary pressure (Pl) for measurements of activity of the chest wall and its muscles against the lung and airways (2); transdiaphragmatic pressure (Pdi) for the activity of the diaphragm alone (1); or total respiratory muscle pressure (Pmus) for activity of the synergic respiratory muscles against the lung and rib cage (13).

When P is normalized to a fraction of the maximum in-spiratory pressure available, it is referred to as the pressure-time index (PTI). For example, for measurements of pressure at the mouth or airway opening, Equation 2 is

where PTImo is the pressure-time index measured at the mouth and PI,max is the maximum inspiratory pressure that can be generated at the mouth or airway opening (usually obtained for a period exceeding 1 second). For the PTI for the diaphragm (PTIdi), maximal transdiaphragmatic pressure (Pdi,max) is substituted for PI,max and Pdi is substituted for Pmo. Refer to Section 2 of this Statement for techniques of measuring PI,max and Pdi,max. Traditionally, the term tension-time index (TTI) has been applied to this measurement (1). From a physiologic viewpoint, TTI is the "ideal" variable, which is deterministic for a large number of relevant factors in muscle physiology, including muscle energetics and blood flow. However, for most experimental and clinical measurements for the respiratory system, the transduction of muscle tension into respiratory pressures is not straightforward. Therefore, to avoid misinterpretation of the data, it is recommended that PTI be substituted for TTI when pressure comprises the measured variable (see Section 5 of this Statement).

Advantages. Under conditions of relatively constant ventilation, respiratory muscle endurance (1), blood flow (14), and changes in oxygen consumption of the respiratory system

100%

Targel task

Task 50% Intensity

Targel task

Task 50% Intensity

Task failure at Tilm

"v. Peak incremental task intensity

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Maximum sustainable task -

^ Incremental task

Tlim

Figure 1. A schematic showing an idealized endurance curve for a given task (e.g., maximal voluntary ventilation) and its relationship to an endurance test (dotted line) as well as to an incremental loading test (thin stepped line). Tlim = Time limit a task can be endured before task failure.

Endurance Time

Figure 1. A schematic showing an idealized endurance curve for a given task (e.g., maximal voluntary ventilation) and its relationship to an endurance test (dotted line) as well as to an incremental loading test (thin stepped line). Tlim = Time limit a task can be endured before task failure.

(Vo2,rs) (15, 16) have been shown to be significantly correlated to changes in PTI (Figure 2). Furthermore, PTI is a parameter that describes the pressure-generating activity of the muscles, independent of a specific breathing rhythm, breathing frequency, or type of load within the experimental limits tested (1). Normalizing to maximum pressure can also be useful as a measure of the amount of pressure "reserve" utilized during contraction. For example, most normal subjects can sustain a PTIdi of up to approximately 0.18 (1) and a PTI for the chest wall muscles and the synergic inspiratory muscles of up to approximately 0.3 (2). These "critical" PTI values may be useful in estimating whether the muscles are undergoing contractions that are "likely" to lead to a loss of force, or fatigue (17, 18). However, critical PTI should be used with considerable caution, as it is highly likely that the critical PTI may vary somewhat across various pathological conditions. This has not been studied extensively. In addition, in clinical situations there is often some uncertainty regarding the accuracy of measurements of PI,max or Pdi,max used to calculate PTI (see Volitional Tests of Respiratory Muscle Strength in Section 2 of this Statement).

Disadvantages. When the level of ventilation increases at a constant PTP, the Vo2,rs is increased and endurance is reduced (15, 19). For example, in Figure 3A, when a subject is inspiring with a constant PTP (individual isopleths), increasing flow rates result in markedly increased oxygen consumption of the respirator system (Vo2,rs). Furthermore, when PTP is kept constant, increasing mechanical work rates of the respiratory system Wrs result in reduced inspiratory muscle endurance (Figure 3B). Therefore, when the tasks involve high levels of ventilation, as may occur during exercise, during ventilatory endurance measurements, or in patients with high or changing ventilatory requirements, the various measures of pressure over time (i.e., PTP, P, and PTI) become less predictive as global measures of the activity or endurance of the muscles. under these conditions, the mechanical work rate (Wrs), discussed below, begins to take on a greater significance (19). As shown in Figure 3C, when ventilation is allowed to vary over a wide range of PTP, Wrs becomes highly predictive of the Vo2,rs and therefore the energy utilization of the respiratory muscles.

Another illustration of these points is that the critical PTI for the respiratory muscles working synergically can vary from 0.12 to 0.4, depending on the particular pattern of ventilation, particularly when inspiratory flows and timing are varied over a wide range (13, 20). Nevertheless, under most testing conditions, when ventilation remains relatively low and constant, and duty cycle is kept within a range that is normally seen during spontaneous ventilation (i.e., 0.3-0.5), measures of PTP (alternatively, PTI or P) are still the most predictive global measure of respiratory muscle activity available.

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