Celia J. Lanteri

Respiratory mechanics during mechanical ventilation: A model study on the effects of leak around a tracheal tube

Air leak around a tracheal tube (TT) during mechanical ventilation is likely to occur during the inspiratory phase because airway pressure is high for a prolonged period. The presence of a leak may introduce errors in measurements of respiratory mechanics made at the airway opening. If so, respiratory mechanics can be measured more accurately when data are collected during the expiratory phase of ventilation. We examined this phenomenon in a lung model. When a leak was introduced into the model, simulating a leak around the TT, the leak occurred predominantly during the inspiratory phase of respiration. As the magnitude of the leak increased, the overestimation of resistance progressively increased, when calculated from pressure and flow measured at the airway opening. A large leak (38%) resulted in an overestimation of respiratory system resistance by 51% and an underestimation of elastance (Ers) by 23% when calculated from the entire ventilatory cycle. However, there was no under‐ or overestimation in Rrs when calculated from the expiratory phase only, and ERS was overestimated by only 6.1%. Varying peak inspiratory pressure, end‐expiratory pressure, and expiratory time did influence the effect of leak, however, respiratory mechanics could still be calculated accurately from the expiratory phase under these conditions. We conclude that measurements of lung mechanics from the expiratory phase is a promising approach to dealing with the problem of measuring respiratory mechanics in mechanically ventilated infants with leaks around the tracheal tube. Pediatr. Pulmonol. 1997; 24:423–428.

Influence of inertance on respiratory mechanics measurements in mechanically ventilated puppies

The complete equation of motion for a single compartment model (SCM) includes an inertance term to describe pressure changes in phase with acceleration, as well as terms for resistance and elastance. Inertance has traditionally been excluded from the model when measuring respiratory mechanics at conventional ventilatory frequencies in mature respiratory systems. However, this omission has been questioned recently for measurements of respiratory mechanics in intubated infants where higher ventilation frequencies and smaller tracheal tubes are the norm. We investigated 1) the significance of inertance in an immature respiratory system during mechanical ventilation, and 2) the effect of omitting it from the model on estimates of respiratory mechanics. Six anesthetised, paralysed and mechanically ventilated puppies (2.6–3.9 kg) were studied. A SCM, including an inertance term was fitted to measurements of flow and airway opening (PAO) or transpulmonary (PTP) pressure using multiple linear regression to estimate respiratory system and lung resistance (RRS, RL), elastance (ERS, EL) and inertance (IRS, IL) respectively, at various ventilation frequencies (0.2–2 Hz). Data obtained at each ventilation frequency were also fitted with a similar model without the inertance term.

Differing patterns of mechanical response to direct fetal hormone treatment

A single combined intramuscular dose of betamethasone and l-thyroxine (T4) or placebo was injected into the shoulder of fetal lambs 48 hours prior to delivery at days 121 (n = 14), 128 (n = 25) or 135 (n = 20) of gestation. Respiratory mechanics were calculated using multiple linear regression analysis. Both respiratory system resistance (RRS) and elastance (ERS) decreased approximately 4 fold between gestational days 121 (D121) and 135 (D135). Both variables were also reduced by hormone treatment. Reduction in ERS was due to a reduction in both lung (EL) and chest wall (EW) components. In absolute terms EW decreased with gestational age; however, EW as a proportion of total elastance (% EW) increased. Inclusion of a volume-dependent elastance term in the multiple linear regression model enabled us to separate total elastance into volume-independent (E1) and volume-dependent (E2V) components. E1 decreased almost 8-fold compared with only a 2.5-fold fall in E2V between D121 and D135. %E2, the proportion of ERS which is volume-dependent and which provides an index of overventilation, doubled over this time period. Hormone treatment affected E1 and E2V components equally hence %E2 was not altered. Both excised lung volume and end expiratory alveolar volume increased with gestational age and with treatment. The response to treatment was qualitatively similar at each of the gestational ages examined, however, for all mechanics variables, except resistance and E1, the magnitude of response to treatment was significantly smaller in D135 animals compared with other age groups.

The effect of nedocromil sodium on histamine responsiveness in clinically stable asthmatic children

A double-blind, randomized, placebo-controlled, parallel-group study was undertaken in 120 clinically stable asthmatic children (aged 6-19 years) to determine the ability of 8 weeks of treatment with nedocromil sodium (4 mg, three times a day) to reduce the level of histamine responsiveness. Despite the subjects being clinically stable and reporting few asthma symptoms, approximately one-third had abnormal pulmonary function on enrollment into the study. Statistically significant increases in pulmonary function were seen in the group treated with nedocromil sodium but not in the control group (p = 0.01). Furthermore, 52% of the individuals with abnormal pulmonary function returned to normal following treatment with nedocromil sodium compared to 11% of those with abnormal pulmonary function who received placebo. However, there were no differences in the level of histamine responsiveness between the two treatment groups at baseline or after 4 or 8 weeks of treatment with nedocromil sodium or placebo. These data suggest that the level of histamine responsiveness is not intimately related to the level of asthma control in children.

Lack of vagal influence on pulmonary visco-elasticity in puppies

We recently demonstrated that most of the change in lung function following inhaled histamine in 8-10 week old puppies was due to changes in the tissue visco-elastic properties and argued that vagally-mediated reflexes may be responsible for this phenomenon (Sly and Lanteri, J. Appl. Physiol. 1990; 68: 1562-1567). To test this hypothesis we compared 5 puppies treated with inhaled lignocaine or bilateral cervical vagotomy did not alter baseline pulmonary mechanics and there were no differences in the response to histamine challenge between the three groups. These results show that vagal reflexes, mediated via the central nervous system, are not involved in the response of the pulmonary tissues to inhaled histamine, but do not exclude the possible involvement of local axonal reflexes.