Chest Wall Compliance

Rationale. The high Ccw of infants influences FRC and ability to withstand respiratory loading. Certain disease states, e.g., neuromuscular disease, bony thoracic abnormalities, and in-tra-abdominal processes such as ascites, can alter Ccw.

Methods. Children greater than 7 years of age can probably understand the directions necessary for Ccw measurement as described for adults in Section 6 of this Statement, although passage of esophageal balloons in this age range is not routine. The measurement of passive Ccw in younger children and infants is technically difficult because of their inability to cooperate. To measure passive Ccw, the respiratory pump muscles must be inactive, or as close to inactive as possible. Several methods to achieve this have been described:

1. Passive Ccw has been measured under quasistatic conditions, during mechanical ventilation (MV) in preterm and full-term infants (32). Because during MV, pressure inside the chest wall is pleural pressure (approximated by an esophageal balloon or catheter), and pressure outside the chest wall is atmospheric (referenced to zero), the pressure difference across the chest wall is Pes. With an esophageal balloon in place, measurements of Pes at points of zero flow (end inspiration [Pi], following a 1.0-second end-inspiratory hold and end expiration [Pe]) are made, and the difference is divided into that breath's Vt. Chest wall compliance is then measured as: Ccw = VT/(Pi,es — PE,es).

2. Passive dynamic Ccw has been measured in spontaneously breathing sedated infants and toddlers (33). An anesthesia bag and mask is placed over the infant's mouth and nose, and gentle insufflations are given until the infant is no longer making respiratory efforts, as indicated by surface diaphragmatic EMG electrodes or by a conversion of Pes deflections from negative to positive during inspiration. With an esophageal catheter in place, CL is calculated as: Cl = VT/[(Pi,ao — Pi,es) — (PE,ao — PE,es)], where Pi,ao and PE,ao are pressures at the airway opening during inspiration and expiration, respectively.

3. Respiratory system compliance (Crs) is calculated as: Crs = VT/(Pi,ao — PE,ao), and Ccw is calculated as: 1/Ccw = 1/ Crs — 1/CL.

4. Passive static Ccw has been measured in preterm infants (32). After an end-inspiratory occlusion, Pes rises to a plateau in parallel with Pao, because the lung is at isovolume and transpulmonary pressure (Ptp) remains constant. After release of the occlusion, Pes decreases as lung volume decreases. Ccw is calculated as: Ccw = dV/(Pi,es — PE,es), where dV is the change in lung volume.

This technique can be performed in spontaneously breathing infants, and relies on an intact Hering-Breuer reflex causing relaxation of the chest wall after an end-inspiratory occlusion.

Advantages and disadvantages of Ccw measurements. Measurements of chest wall mechanics are useful in assessing overall respiratory function in infants, because the chest wall is highly compliant in this age range. This leads to inefficient chest wall motion and predisposes to fatigue. These tests should probably be performed only in the research setting, because of their technical difficulty and requirement for absolute chest wall muscle relaxation to achieve interpretable results.

Normal values and alterations in disease. Chest wall compliance has been measured in preterm and full-term infants, toddlers up to the age of 3 years, school age children and adolescents, and in young adults (see Table 1). Because different techniques have been used in each age range, it is difficult to compare estimates of Ccw with growth. Size correction is also important; as with Cl, Ccw must be corrected for resting lung volume, or, if that is not available, for body weight. Another way that Ccw can be compared between different age groups is by comparison with CL. The chest wall stiffens progressively with age. in preterm and full-term infants, Ccw is about three to six times greater than Cl (32-36); in school age children, Ccw is approximately twice CL (37); in adolescents and adults, Ccw is approximately equal to Cl and in the elderly, Ccw is approximately half Cl (38) (Table 1). Chest wall compliance is abnormally increased in infants and toddlers with neuromus-cular disease (39). In contrast, Ccw is abnormally decreased in adults with neuromuscular disease, possibly secondary to joint contractures due to years of low-VT breathing (40). Although Ccw is presumably diminished in restrictive chest wall disorders such as scoliosis and asphyxiating thoracic dystrophy, physiologic measurements are few. Likewise, the effects of abdominal disorders such as obesity, ascites, and prune belly syndrome on Ccw are poorly understood.

Clinical application. Measurement of Ccw is not performed in routine clinical practice, but has been reported in clinical research.

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