Chronic ventilatory effects of diazepam and barbiturates in conscious cats

Expiratory braking defines the breathing patterns of asphyxiated neonates during therapeutic hypothermia

Introduction

Although neonatal breathing patterns vary after perinatal asphyxia, whether they change during therapeutic hypothermia (TH) remains unclear. We characterized breathing patterns in infants during TH for hypoxic-ischemic encephalopathy (HIE) and normothermia after rewarming.

Methods

In seventeen spontaneously breathing infants receiving TH for HIE and in three who did not receive TH, we analyzed respiratory flow and esophageal pressure tracings for respiratory timing variables, pulmonary mechanics and respiratory effort. Breaths were classified as braked (inspiratory:expiratory ratio ≥1.5) and unbraked (<1.5).

Results

According to the expiratory flow shape braked breaths were chategorized into early peak expiratory flow, late peak expiratory flow, slow flow, and post-inspiratory hold flow (PiHF). The most braked breaths had lower rates, larger tidal volume but lower minute ventilation, inspiratory airway resistance and respiratory effort, except for the PiHF, which had higher resistance and respiratory effort. The braked pattern predominated during TH, but not during normothermia or in the uncooled infants.

Conclusions

We speculate that during TH for HIE low respiratory rates favor neonatal braked breathing to preserve lung volume. Given the generally low respiratory effort, it seems reasonable to leave spontaneous breathing unassisted. However, if the PiHF pattern predominates, ventilatory support may be required.

GB toxicity reassessed using newer techniques for estimation of human toxicity from animal inhalation toxicity data: New method for estimating acute human toxicity (GB)

Estimated human inhalation toxicity values for Sarin (GB) were calculated using a new two independent (concentration, exposure time), one dependent (toxic response), non-linear dose response (toxicity) model combined with re-evaluated allometric equations relating to animal and human respiration. Historical animal studies of GB toxicity containing both exposure and fractional animal response data were used to test the new process. The final data set contained 6621 animals, 762 groups, 37 studies and 7 species. The toxicity of GB for each species was empirically related to exposure concentration (C; mg m(-3)) and exposure time (T; min) through the surface function Y = b0 + b1 Log10C + b2 Log10T or Y = b0 + b2 Log10C(n)T where Y is the Normit, b0, b1 and b2 are constants and n is the 'toxic load exponent' (Normit is PROBIT - 5). Between exposure times of 0.17 and 30 min, the average value for n in seven species was 1.35 +/- 0.15. The near parallel toxic load equations for each species and the linear relationship between minute volume/body weight ratio and the inhalation toxicity (LCt50) for GB were used to create a pseudo-human data set and then an exposure time/toxicity surface for the human. The calculated n for the human was 1.40. The pseudo-human data had much more variability at low exposure times. Raising the lower exposure limit to 1 min, did not change the LCt50 but did result in lower variability. Raising the lower value to 2 min was counterproductive. Based on the toxic load model for 1-30 min exposures, the human GB toxicities (LCt01, LCt05, LCt50 and LCt95) for 70 kg humans breathing 15 l min(-1) were estimated to be 11, 16, 36 and 83; 18, 25, 57 and 132 and 24, 34, 79 and 182 mg x min m(-3) for 2, 10 and 30 min exposures, respectively. These values are recommended for general use for the total human population. The empirical relationships employed in the calculations may not be valid for exposure times >30 min.

Allometric respiration/body mass data for animals to be used for estimates of inhalation toxicity to young adult humans

The relationship between body weight (BW) and respiratory minute volume (V(m)) was reviewed by collecting a database from the literature. The data were separated into anaesthetized and non-anaesthetized groups. Only young adult terrestrial mammals were included in the final data set. This database is the largest to be reported to date, is the first to separate the anaesthetized and non-anaesthetized groups and is matched to the target population of young, fit adult humans. The data set of non-anaesthetized animals contained 142 studies representing 2616 animals and 18 species from mice at 12 g body weight to horses and a giraffe at ca. 500 kg body weight. Analysis of the data indicated a power law (allometric) relationship between the minute volume and body weight. The resulting allometric equations for the empirical relationship between minute volume and body weight are: log(10)V()(m)= -0.302 + 0.809 log(10)BW and V(m) = 0.499 BW(0.809)where V(m) is the minute volume (l min(-1)) and BW is the body weight (kg). From these equations, a minute volume of 15.5 lmin(-1)was obtained for a 70 kg human in the same physiological and/or emotional state as the animals. The results of the analyses were compared to other empirical studies in the literature, the more recent of which also indicated a scaling factor of 0.8. The relationship between minute volume and body weight is recommended for use in estimating the inhalation toxicity to young adult humans (military personnel), because this is the first study to use a large database focused exclusively upon non-anaesthetized young adult terrestrial mammals.

Effects of chronic administration of phenobarbital in low doses on control of breathing in the cat

The ventilatory response to CO hypoxia (FICO = 0.0025), hypercapnia, and hypoxia was studied in a group of intact, conscious cats before and during chronic administration of phenobarbital (60 mg/day). It was found that the ventilatory response to hypercapnia or hypoxia was not significantly modified during phenobarbital administration. However, the ventilatory response to CO hypoxia was markedly blunted with phenobarbital: the initial ventilatory inhibition was still observed but the subsequent hyperventilation was delayed. Furthermore, the tachypnea was less intense and the decrease in tidal volume was smaller during CO hypoxia. In addition, the behavioral reactions which usually accompanied the tachypnea were attenuated and unconsciousness was often noted. These results indicate that the modifications caused by phenobarbital do not result from a general inhibition of the respiratory control network but rather from a selective inhibition of the structures rostral to the brain stem. As a consequence, the behavioral reactions and resulting respiratory activation were attenuated. Since the hypoxic tachypnea and behavioral reactions are observed only during central hypoxemia, it is concluded that arterial chemoreceptor afferents normally inhibit the supra-pontine structures which are otherwise stimulated by central hypoxemia.