Integrated Pump Function Thoracoabdominal Motion

Rationale. The respiratory muscles (diaphragm and intercos-tals) act on the rib cage to effect respiratory motion and ventilation. Rib cage motion can be taken as an index of intercostal muscle action, while AB motion can be taken as an index of diaphragmatic descent. Thus thoracoabdominal motion (TAM) provides a visual index of respiratory muscle function.

Methods. The most widely used method to assess TAM is respiratory inductive plethysmography, although strain gauges and magnetometers are also used. Respiratory inductive plethys-mography uses thin cloth bands that are placed around the RC and aB. A wire is sewn into the bands, and when an oscillatory current is applied, changes in the cross-sectional area of the chest wall are reflected as changes in the electric inductance of the wires, displayed as a change in voltage output. Respiratory inductive plethysmography can be used to quantitate asyn-chrony by measuring a phase angle, ( (( = 0°, synchronous breathing; ( = 180°, paradoxic breathing), between the RC and AB compartments. The calculation of ( does not require calibration for volume. Respiratory inductive plethysmography can also be used to quantitate the relative contribution of the RC and AB to Vt; this does require calibration for volume. Several techniques have been described for volumetric calibration. The isovolume, least mean squares, and quantitative diagnostic calibration techniques are described elsewhere (77-81) (see Devices Used to Monitor Breathing in Section 6 of this Statement).

Advantages and disadvantages of respiratory inductive pleth-ysmography. Respiratory inductive plethysmography is valuable as a direct, noninvasive measure of chest wall motion and an indirect measure of respiratory muscle function. Being an indirect measure, however, chest wall motion can reflect events other than respiratory muscle function. Chest wall motion is a final common pathway of integrated respiratory system output; it can be influenced by underlying lung mechanics and Ccw. Thus, asynchronous chest wall motion can represent neuromuscular weakness (82-84), fatigue (85), high Ccw (86), abnormally low Cl or high lung resistance (87, 88), upper airway obstruction (89), effects of anesthesia (90), or a combination of two or more of these. Its major weakness is thus that it is a nonspecific indicator, and abnormal chest wall motion must be interpreted in the context in which it occurs.

Calibration of respiratory inductive plethysmography for volume is difficult in infants, especially in preterm infants with highly compliant chest walls and paradoxic chest wall motion. The highly compliant infant chest wall probably invalidates the assumption of two degrees of freedom (RC and AB) that is the basis for most respiratory inductive plethysmography calibration equations. Furthermore, phase angles may be difficult to interpret if breathing does not approximate a sinusoidal pattern (91).

5.2

70

21.7

0.10

5.2

59

25.6

0.11

4.3

97

14.7

0.07

4.3

77

18.9

0.09

3.7

105

13.0

0.06

3.7

98

Normal values and alterations in disease. in infants, the high Ccw predisposes to asynchronous chest wall motion. Thus, normal preterm infants display asynchronous chest wall motion during NREM sleep (86). Normal full-term infants display synchronous RC-AB motion in NREM sleep (^ = 8 ± 7° [mean ± SD]) (84), but may display asynchronous motion in REM sleep (21). The time spent with paradoxic inward RC movement during REM sleep decreases with advancing age; paradoxic, RC motion is present during nearly 100% of the REM time in the newborn, but only ~ 10% of the REM time by 3 years of age (21). For these reasons, it should be noted whether the infant is in the quiet awake state, or in clinically determined quiet or active sleep, when performing measurements of TAM.

Normal adults and children have synchronous breathing between the RC and AB compartments (92, 93).

As far as neuromuscular disease is concerned, assessment of TAM can pinpoint the site of weakness: paradoxic inward motion of the RC during inspiration indicates intercostal muscle weakness, whereas paradoxic inward motion of the AB during inspiration indicates diaphragmatic weakness (82, 83). Such chest wall asynchrony disappears during MV (94, 95), and may indicate respiratory muscle rest.

Turning to chronic airflow obstruction, in adults, abnormalities of diaphragmatic function accompany chronic hyperinflation (96). Similar problems occur in infancy and childhood. Preterm infants develop chest wall asynchrony and decreased minute ventilation when breathing through inspira-tory resistive loads (8); it is not clear whether this represents respiratory muscle fatigue or simply high Ccw in the face of increasing negative intrathoracic pressure. infants with bron-chopulmonary dysplasia display asynchronous RC-AB motion. The degree of asynchrony is proportional to the degree of abnormality in lung compliance and resistance (88).This improves on administration of aerosolized bronchodilators (87). upper airway obstruction can likewise cause thoracoabdomi-nal asynchrony (89).

Clinical application. Measurements of TAM have been made in clinical research studies as described. By differentiating the summed RC and AB signals, tidal flow-volume curves have been analyzed (97). Measurements of TAM are also used in routine clinical practice, particularly for detecting upper airway obstruction during sleep studies. They are also used in infant monitoring devices to detect apnea.

The following two methods of analyzing integrated respiratory pump function have limited clinical applications at present and are primarily techniques under investigation.

1. Diaphragmatic movement (real-time ultrasonography): Diaphragmatic excursions have been measured in full-term infants by real-time ultrasonography (98). Only the right hemidiaphragm is accessible by this technique, through the ultrasonographic window provided by the liver; the left hemidiaphragm is obscured by stomach and bowel gas. The posterior aspect of the diaphragm moves to a greater extent than the anterior aspect, perhaps because of the effect of the larger posterior area of apposition to the inner chest wall. Diaphragmatic excursions are lessened, and the diaphragm moves more uniformly, in infants who are paralyzed and undergo MV. Similar observations (4) led to the conclusion that, unlike the piston-like motion of the adult diaphragm, the infant diaphragm has a bellows-like motion. Normal ultrasonographic values for diaphragmatic excursion in infants and adults have been presented (4, 99101); there is a wide degree of variation between the infant studies, suggesting that, at least for now, clinical application is limited (Table 4).

2. Phrenic nerve stimulation (see Stimulation Tests in Section 3 of this Statement): Transcutaneous phrenic nerve stimulation has been applied to children in the clinical setting; to date phrenic nerve latency has been studied as a way of assessing phrenic nerve damage in, for example, patients recovering from cardiac surgery, but no studies of diaphragmatic fatigue have been performed. Normal phrenic nerve latency is on the order of 4.5-6.5 milliseconds. There is less than 10% day-to-day variability of this measurement (102). Phrenic nerve latency time diminishes slightly with age, by 0.5 to 1 millisecond between birth and 10 years (103). Delays of 2 milliseconds or longer may indicate phrenic nerve damage (102). Magnetic cervical stimulation may be an alternative, painless way to stimulate the phrenic nerves (104).

Anterior magnetic phrenic nerve stimulation is an established technique in adults (see Section 2 of this Statement). Experience with the technique in children is limited but the method can be used to assess diaphragm function in neonates in the intensive care unit (109).

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