Pulmonary impedance as an index of severity and mechanism of neonatal lung disease

Abstract

The measurement of resistive and elastic components of the respiratory system in neonates has been used to define disease severity and the response to therapy. The lung resistance RJ and dynamic lung compliance (C^Ldyn) partition the impediment to gas flow into two components, each of which may be altered independently. The concept of lung impedance (Z^Ldyn) is the combined effect of the elastic and resistive loads presented to the respiratory muscles, which determines the gas flow that will result from the pressures generated by the respiratory muscles. In a first order system where R^L and C ^Ldyn are single values (independent of volume or respiratory rate), is the vector sum of the reactive [1/(2πf_b × C^Ldyn)] and resistive (RJ components at the infant's breathing rate (f^b), if the transpulmonary pressure (P^tp) generated by the respiratory muscles during spontaneous respiration can be modeled mathematically by a sinusoidal function. Furthermore, the phase angle (θ) between the impedance and the resistive component will represent the relative magnitude of the resistive and reactive components. The validity of this model can be established by comparing the calculated θ to the observed temporal difference (measured θ) between the P_tp and flow derived from the polygraph tracing. This hypothesis was tested in 10 spontaneously breathing neonates with lung disease of differing etiology and severity. No significant difference was found between the measured and calculated θ values (mean difference, 1.2 ± 3.9). The Z^Ldyn value varied between 68 and 216 cmH²O/L/s; calculated θ varied between 71° and 33° (mean, 65° ± 5° in respiratory distress syndrome and 44° ± 9° in bronchopulmonary dysplasia) with flow always leading P_tp. These results validate the use of (Z^Ldyn) in spontaneously breathing infants to represent the magnitude of the respiratory load presented by the lung (not including the chest wall), while θ is related to the disease process. Pediatr Pulmonol. 1994; 17:41–49.