A new method of evaluating the chemosensitivity of the respiratory center in children.

Decreased ventilatory response to carbon dioxide is often present in lung disease. This can be due to a reduction in the output of the respiratory center or an inability of the respiratory pump to respond to a normal output because of the size or impedence of the pump. To separate these mechanisms we have measured the isometric force developed by the respiratory muscle during brief airway occlusion, by measuring the pressure generated at 100 msec (Pm100). We studied 43 subjects ranging in age from 6 to 50 years, and nine neonates. We found a linear rise of Pm100 with rising PAco2 during carbon dioxide rebreathing maneuvers. Our results also show that although there are wide variations in slopes of Pm100 to carbon dioxide tension (SPm100/PAco2) between individuals, in a given subject this slope remains constant even following repeated studies. It also remains constant for age and size, indicating that the neuromechanical output of the respiratory apparatus does not change with growth. In contrast the ventilatory response either assessed as Ve/PAco2 or Vt/PAco2 depended on age and lung size. The results suggest that throughout growth, comparison of Pm100/PAco2 and Ve/PAco2 can distinguish between abnormalities of neuromuscular output from other causes of ventilatory impairment.