Breathing, locomotion and everything in-between. Proceedings of a symposium held at the 2nd International Congress of Respiratory Science. Bad Honnef, Germany. August 9-13, 2009

In the chick embryo at day 3, gas exchange occurs by diffusion and oxygen consumption (V?_O2) does not depend on the cardiovascular convection of O₂. Whether or not this is the case in hypoxia is not known and represents the aim of the study. The heart of chicken embryos at 72 h (stage HH18) was filmed through a window of the eggshell by a camera attached to a microscope. Stroke volume was estimated from the changes in heart silhouette between systole and diastole. V?_O2was measured by a closed system methodology. In normoxia, a decrease in temperature (T) from 39 to 31 °C had parallel depressant effects on V?_O2and HR. At 39 °C, a progressive decrease in O₂ lowered V?_O2; HR was maintained until the O₂ threshold of ～ 15%. In severe hypoxia (4% O₂) V?_O2and HR were, respectively, ～ 12% and ～ 62% of normoxia. At 32 °C the hypoxic threshold for HR was significantly lower. During constant hypoxia (7% O₂) V?_O2did not respond to T, while the HR response was preserved. Stroke volume changed little with changes in T or O₂, except at 6 and 4% O₂, when it decreased by ～ 20 and 30%. In embryos growth-retarded because of incubation in chronic hypoxia, V?_O2and HR responses to T and hypoxia were similar to those of normal embryos. We conclude that in the early embryo during hypoxia cardiovascular O₂ convection is not responsible for the drop in V?_O2. The generalised hypometabolic response, in combination with the extremely small cardiac V?_O2, probably explains the minor effects of hypoxia on cardiac activity.     

Phase locking of the respiratory rhythm in cats to a mechanical ventilator

Mechanical ventilation of paralyzed, pentobarbital-anesthetized adult cats was performed while recording phrenic nerve activity. The periodic changes in lung volume owing to mechanical ventilation affected the rhythm of central respiratory activity, resulting in a variety of regular and irregular patterns of coupling between respiratory system output, monitored by phrenic activity, and the mechanical ventilator. Phase-locked patterns, in which phrenic burst onset occurred at specific and repetitive phase(s) of the mechanical ventilator, with ratios of ventilator frequency: phrenic burst frequency of 1:2, 1:1, 3:2, 2:1, and 3:1 were observed. Regular and irregular patterns occurred over specific ranges of frequency and volume of the mechanical ventilator. A careful study was made of the 1:1 phase locking as the frequency and inflation volume of the mechanical ventilator were changed. The inspiratory time (TI) was defined as the interval between the time when phrenic activity began to rise and the onset of its rapid decline, and the expiratory time (TE) as the time between inspirations. In the 1:1 phase-locking region, as the frequency of the ventilator was increased both TI and TE decreased, and the phase of phrenic onset in the ventilator cycle changed. During ventilation with frequencies higher than the intrinsic phrenic frequency (initial burst frequency of phrenic activity with the ventilator turned off) inspiratory activity was prematurely terminated by lung inflation (Hering-Breuer inspiratory inhibitory reflex). During ventilation with frequencies lower than the intrinsic phrenic frequency, the onset of phrenic activity was delayed (TE was prolonged) by lung inflation (Hering-Breuer expiratory promoting reflex).     

Effects of early hypoxia on breathing pattern in rabbit pups before and after vagotomy

We examined the influence of vagal pulmonary receptors exerted on the breathing pattern and inspiratory activities of phrenic nerve and intercostal electromyograms (EMG) during hypoxia in rabbit pups. Animals in their second week of life were anaesthetized with ketamine (50 mg/kg) and acepromazine (3 mg/kg) and tracheostomized. While they breathed spontaneously, we recorded tidal volume (VT), integrated phrenic activity (PHR), integrated external intercostal EMG (INT), and blood pressure (BP). To prevent secondary ventilatory depression, animals were exposed to 12% O2 (balanced with N2) for no longer than 5 min before and after vagotomy. All measurements were taken from 1 min following the onset of hypoxic exposure until the end of the run. During hypoxia, VT, PHR, and INT increased in intact rabbit pups. There was an almost immediate decrease in BP that was maintained during the total period of hypoxia exposure. Hypoxia resulted in inconsistent changes in inspiratory (TI) and expiratory (TE) time in intact animals. Following vagotomy, PHR, INT, VT, BP, and TE responses were the same as in intact animals. However, TI significantly decreased in all animals. In response to hypoxia with and without vagal feedback, INT increased less than PHR in most cases. Qualitatively similar effects of hypoxia were observed in an adult rabbit. The results reveal that the increase in VT and the shortening of TI in response to hypoxia do not depend on vagal feedback in rabbits during the early postnatal period. In fact TI shortening was significant only without vagal feedback.     

Expiratory effects of vagal stimulation in newborn kittens

Expiratory effects of electrical stimulation of vagal afferents were studied in 12 kittens during the first week of life. Animals anesthetized with ketamine (30 mg/kg, im) and acepromazine (1.1 mg/kg, im), tracheostomized, and paralyzed were artificially ventilated after bilateral vagotomy. Rectified and "integrated" activity of the C5 root of phrenic nerve, systemic blood pressure, and the stimulus train were recorded. The optimal stimulus parameters for expiratory prolongation were chosen. The results varied between animals. We found three types of response: A, expiratory prolongation when stimulus was applied within the initial 80% of control expiratory time (TEc); beyond this delay, a decreased response or no effect was observed in four kittens; B, graded expiratory prolongation was recorded to the end of this phase in three kittens; and C, expiratory prolongation when stimulus delay was less than 40% of TEc and expiratory shortening when the stimulus given with greater delays was observed in one kitten. Nonsignificant effects were observed in the remaining four animals. Types A and B of response suggest activation of the slowly adapting pulmonary stretch receptors. However, amplitude of stimulus and frequency of pulses were higher than those used in adult animals. Type C response indicates that fibers from both rapidly and slowly adapting stretch receptors could be activated. Our results imply that if the expiratory insensitive phase is present in kittens, it can be affected by experimental conditions. This is in contradiction to characteristics of expiratory response to vagal stretch receptor input in adult cats.