Effects of rate and amplitude of breathing on respiratory system elastance and resistance during growth of healthy children.

Intrinsic properties of lung and chest wall tissues can lead to breathing rate (frequency [f]) and amplitude (tidal volume [VT]) dependence of respiratory system resistance (R) and elastance (E). To explore these dependencies on R and E within physiological limits of tidal volume and breathing frequencies during early childhood. we measured airway opening pressure (Pao) and flow (V'ao) in 15 anesthetized, paralyzed, intubated, and mechanically ventilated healthy children (age 1 day to 72 months; weight 2.5-21 kg) at multiple combinations of VT (6, 10, and 14 mL/kg) and frequency (10, 20, and 30 breaths/min). In each instance, R and E were estimated by multiple linear regression applied to the tracheal pressure, flow, and volume (V), assuming a simple series R-E model. R decreased substantially with increasing frequency and weight (Wt), but was unaffected by changes in VT (R = 764Wt(-0.91) x f(-0.57)). E decreased sharply with increasing Wt, was lower at higher VT, and was slightly, yet significantly, increased at higher frequency (E = 2,905Wt(-1.38) x VT(-0.18) x f(0.11)). Such frequency dependence of R and E is consistent with stress adaptive, or viscoelastic, properties of respiratory tissues. The small V dependence of E is similar to that observed in other species under healthy conditions and presumably reflects the combined nonlinear pressure-volume relationships of the healthy parenchymal and chest wall tissues. Lack of VT dependence of R at high inspiratory flow rates suggests that turbulent flows are either not an important form of energy dissipation in the lower airways of children or they are counterbalanced by a decrease in tissue damping at high VT. The above regression models represent the first attempt to quantify simultaneously the separate effects of lung growth as well as rate and amplitude of breathing on R and E. Similar equations based on a larger sample of healthy subjects can provide normative R and E values for comparison with mechanically ventilated children with lung disease.

A first-pass cost analysis of propofol versus barbiturates for children undergoing magnetic resonance imaging.

Intravenous (IV) propofol was compared with IV thiopental/pentobarbital as a sedative for children undergoing magnetic resonance imaging (MRI) of the brain or spine. Fifty-eight outpatients (aged 11 mo to 6 1/2 yr, ASA grade I and II) were enrolled in the study and randomized to two groups. After IV cannulation, Group I received IV propofol (1-2 mg/kg), followed immediately by a propofol infusion (75-100 micrograms.kg-1.min-1). Group II received IV thiopental (1-3 mg/kg) followed by a pentobarbital bolus (2-3 mg/kg). Supplemental thiopental doses (1-2 mg/kg) were administrated to maintain adequate sedation. Discharge time and postanesthesia recovery scores were determined by an independent blinded observer. Time of recovery to full consciousness in Group I was significantly less than in Group II (19 +/- 7 min vs 35 +/- 20; P < 0.005). Time to discharge was also significantly less in Group I (24 +/- 6 min vs 40 +/- 11; P < 0.05). A preliminary cost analysis was applied to the clinical data obtained and to a theoretical model of a pediatric MRI center. Cost analysis of anesthesia services revealed added drug costs ($1600.76 per year for the propofol group) but significant savings of postanesthesia care unit (PACU) nursing time ($5086.67 per year). Outcomes such as patient morbidity and technical quality of the MRI scans did not differ significantly between the two groups. In conclusion, analysis of the clinical data suggests that propofol may be more suitable than barbiturates for children undergoing outpatient procedures despite its higher price.