Interpretation of Pulmonary Function Tests A Practical Guide, 3. ed

9. Maximal Respiratory Pressures

In some clinical situations, evaluation of the strength of the respiratory muscles is very helpful. The strength of skeletal muscles, such as those of the arm, is easily tested by determining the force that they can develop, as by lifting weights. In contrast, the strength of the respiratory muscles can be determined by measuring the pressures developed against an occluded airway.

9A. PHYSIOLOGIC PRINCIPLES

Some basic muscle physiology is rEV1ewed here to aid in determining the best way of estimating the strength of respiratory muscles. Muscles, when maximally stimulated at different lengths, exhibit a characteristic length-tension behavior, as depicted in Figure 9-1. The greatest tension developed by the muscle occurs when it is at its optimal physiologic length. Less tension is developed at other lengths. To apply this concept to respiratory muscles, volume can be thought of as equivalent to length, and pressure as equivalent to tension. The expiratory muscles (chest wall and abdominal muscles) are at their optimal lengths near total lung capacity. Figure 9-2 shows, as expected, that the highest expiratory pressures are generated near total lung capacity. The subject blows as hard as possible against an occluded airway. Conversely, near residual volume, the inspiratory muscles (primarily the diaphragm) are at their optimal lengths. Near residual volume, they develop the most negative pressure when the subject is sucking against an occluded airway. Therefore, the maximal strength of the expiratory muscles is measured near total lung capacity and that of the inspiratory muscles is measured near residual volume.

FIG. 9-1. Classic length-tension behavior of striated muscle. L max is the length at which maximal tension can be developed.

FIG. 9-2. Maximal respiratory pressure that can be developed statically at various lung volumes (vital capacity, VC). Expiratory pressures are positive, and inspiratory pressures are negative. Total lung capacity is at 100% VC and residual volume at 0% VC.

9B. MEASUREMENT TECHNIQUES

The classic dEV1ce used for these measurements is shown in Figure 9-3. It consists of a hollow stainless-steel tube to which are attached negative- and positive-pressure gauges. The distal end of the tube is occluded, except for a 2-mm hole. Modern equipment has electronic pressure transducers connected to computer processors. Function is the same, but is less obvious on physical inspection.

Maximal expiratory pressure (PEmax) is measured as follows. The subject inhales maximally, holds the foam rubber mouthpiece firmly against the mouth, and exhales as hard as possible. Several reproducible efforts are obtained, and the highest positive pressure maintained for 0.8 second is recorded. Foam rubber mouthpiece held firmly against the lips is used rather than a standard snorkel-type mouthpiece because at high pressures (150 cm H2O or more) air leaks around a conventional mouthpiece, the buccal muscles not being strong enough to maintain a tight seal. A nose clip is not usually required except for certain patients with severe muscle weakness.

Maximal inspiratory pressure (Pimax) is measured by having the subject exhale to residual volume, hold the tubing against the lips, and suck as hard as possible. Again, the greatest negative pressure sustained for 0.8 second is recorded. The small 2-mm hole at the distal end ensures that the dEV1ce is measuring the pressure developed in the lung by the inspiratory muscles. Without it, if the subject closes the glottis and sucks with the cheeks, a very large negative pressure can be developed. The leak prevents this from happening because the pressure produced by sucking with a closed glottis decreases rapidly and cannot be sustained. To ensure accuracy, the subject must exert maximal effort. Herein lies a shortcoming of the test. These efforts can be uncomfortable. Some subjects are unable, or unwilling, to make such effort. Determined coaching by the technician is essential.

FIG. 9-3. Classical instrument used to measure maximal static expiratory and inspiratory pressures. The expiratory gauge measures 0 to +300 cm H₂O, and the inspiratory gauge 0 to -300 cm H₂O. Gauges are alternately connected to the cylinder by a three-way stopcock, as indicated by the arrows on the right-hand gauge (A). The side view (B) shows the small 2-mm hole at the distal end of the metal tube.

TABLE 9-1. Normal values for maximal respiratory pressures, by age

Age (yr)

PEmax, maximal expiratory pressure; Pimax, maximal inspiratory pressure. ‘Numbers represent mean ±2 standard dEV1ations.

9C. NORMAL VALUES

Normal values obtained from a motivated group of 60 healthy male and 60 healthy female subjects are listed in Table 9-1. As anticipated from Figure 9-2, PEmax is roughly double the Pimax. Male subjects developed greater pressures than female subjects, and both sexes had a decline in pressure with age, except for inspiratory pressures in male subjects.

9D. INDICATIONS FOR MAXIMAL PRESSURE MEASUREMENTS

1. In patients with neuromuscular disease who have dyspnea, measurement of respiratory muscle strength is a more sensitive test than spirometry or maximal voluntary ventilation [1]. We studied 10 patients with early neuromuscular diseases (amyotrophic lateral sclerosis, myasthenia gravis, and polymyositis). Eight of the 10 had considerable dyspnea, but only 2 had a significant reduction in the vital capacity (77%). Five had a reduced maximal voluntary ventilation (73%). However, nine patients had significant reductions in PEmax (47% predicted) and Pimax (34% predicted). In the early stages, dyspnea was best explained by a reduction in respiratory muscle strength at a time when the strength of other skeletal muscles was little impaired. Table 9-2 lists some neuromuscular conditions in which respiratory muscle weakness has been encountered.

TABLE 9-2. Neuromuscular disorders associated with respiratory muscle weakness

2. It is useful to measure respiratory muscle strength in the cooperative subject with an isolated, unexplained decrease in the vital capacity or maximal voluntary ventilation. Such decreases could be early signs of respiratory muscle weakness and could explain a complaint of dyspnea. Other conditions in which muscle weakness has been documented are lupus erythematosus, lead poisoning, scleroderma, and hyperthyroidism.

PEARLS: An effective cough is generally not possible when maximal expiratory pressure is less than 40 cm H2O.

Unexplained fainting may be due to cough syncope in the subject with severe chronic bronchitis. Sustained airway pressures of more than 300 cm H₂O have been measured in this condition during paroxysms of coughing. Such pressures are sufficient to reduce venous return and thus cardiac output, leading to syncope, occasionally even when the subject is supine.

3. Measurement of respiratory muscle strength in the intensive care unit has been used as an assessment of readiness to wean from mechanical ventilation. A pressure transducer can be connected to the 15-mm adapter on the endotracheal tube. If testing is performed for patients who are not intubated (as a measure of risk of respiratory failure in patients with respiratory muscle weakness), it is important to have the small leak in the dEV1ce described in section 9B.

When maximal respiratory pressures are used for assessment of weaning potential, an inspiratory pressure greater than -20 cm H₂O and an expiratory pressure greater than +50 cm H₂O have been identified as predictive of the ability to wean most patients from ventilatory support. However, use of a single factor in deciding about weaning potential is not encouraged. It must be kept in mind that the ability to breathe unassisted depends on the balance between the capacity of the respiratory muscles to perform work and the workload imposed on the respiratory muscles by the chest wall and lungs.

REFERENCE

1. Black LF, Hyatt RE. Maximal static respiratory pressures in generalized neuromuscular disease. Am Rev Respir Dis 103:641-650,1971.