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Changes in Respiratory Pattern Affect Dead Space/Tidal Volume Ratio during Spontaneous but not during Controlled Ventilation

A Study in Pediatric Patients

D. K. Rose, MD^*, and A. B. Froese, MD, FRCP (C)

* Research Fellow, Department of Anesthesia, University of Toronto, Hospital for Sick Children. Assistant Professor, Department of Anesthesia, University of Toronto, and Research Institute, Hospital for Sick Children.

Abstract

It is assumed in many gas exchange equations that dead space (V_D) is a constant fraction of tidal volume (V_T) during anesthesia despite variations in respiratory pattern. We tested this assumption by measuring V_D/V_T ratios, using samples of mixed expired gas and arterial blood in 20 healthy anesthetized children (aged 1 to 17 years), divided into three groups. Group 1 was intubated; ventilation was controlled; total minute ventilation was held constant; and frequency (f) varied from 10 to 30/minute. Group 2 was intubated; breathing was spontaneous; and pattern was altered from fast and shallow to slow and deep. Group 3 was given mask anesthesia; breathing was spontaneous; and V_D/V_T was measured at fast frequency only. The inter-relationships of dead space to tidal volume varied with the type of ventilation. In group 1 V_D increased in direct proportion to V_T (mean slope 0.25 ± 0.05; p < 0.001). In group 2, V_D was smaller and remained constant despite alterations in tidal volume (mean slope 0.09 ± 0.09; p > 0.05). Although V_D/V_T at the fast frequency was similar in the two groups, i.e., 0.35 in group 1 (f = 30) and 0.36 in group 2 (f = 35.6), differences became evident at slower frequencies. For a 50% reduction in frequency, V_D/V_T decreased only 15% during controlled ventilation, but 41% during spontaneous respiration. During mask anesthesia (group 3) V_D/V_T at a mean frequency at 29.4 was 0.55. For group 2 V_D per kilogram was related to age by the equation V_D/kg = 1.72–0.06 (age in years) ml at the fast frequency of 35.6 (n = 7; r = 0.73). The equation at a slower frequency of 23.2 was not significantly different. During mechanical ventilation the dead space volume was always larger than during spontaneous breathing and in addition was rate dependent. At f = 30, V_D/kg = 2.78–0.10 (age in years) ml (n = 7, r = 0.80); at f = 10, V_D/kg = 6.74–0.25 (age in years) ml (n = 7; r = 0.83). These measurements demonstrate that in children with spontaneous respiration, increases in tidal volume are distributed to perfused lung regions with a net decrease in V_D/V_T ratio. In contrast, with controlled ventilation wasted ventilation increases as V_T is increased with no net change in V_D/V_T ratio.

Key Words: VENTILATION: pediatric • ANESTHESIA: pediatric