Control of expiratory duration by arterial CO2 oscillations in vagotomized dogs

Intiation of lung ventilation at birth

The newborn baby draws its first postnatal breath either during or within seconds of delivery. Within minutes, a regular breathing rhythm is established and this remains virtually continuous for the remainder of postnatal life. The mechanisms responsible for these sudden, dramatic and vital changes in the respiratory system at birth are only partially understood. Since fetuses make intermittent breathing movements long before birth, understanding the control of the fetal respiratory system may be essential to understanding the rapid onset of respiratory efforts at delivery. In this review the stimuli present at birth will be considered and, based on our current understanding of the fetal and neonatal respiratory control systems, those factors which are likely to play an important role in the initiation of lung ventilation at this time will be examined. Normal respiratory events in the early postnatal period will be concentrated on, but it is important to recognize that in some cases problems occur: a neonate may fail to initiate breathing efforts rapidly, or an apparently healthy premature neonate may suddenly stop breathing a few days after birth. In both cases, clinical intervention may be required to maintain adequate gas exchange and to prevent brain damage or death. Clearly, greater knowledge of respiratory control during this critical time of life would assist in the development of more appropriate and successful treatments for these life-threatening disorders.

Rhythmicity in single fiber postganglionic activity supplying the rat tail

Rhythmicity in single fiber postganglionic activity supplying the rat tail. The temporal pattern of ongoing sympathetic vasoconstrictor activity may play an important role for neurovascular transmission. Here we analyzed the activity of postganglionic fibers projecting into the ventral collector nerve of anesthetized and artificially ventilated vagotomized Wistar rats with respect to the presence of rhythmic firing under normocapnic conditions. Most of the fibers studied were likely vasoconstrictor and involved in thermoregulation. Accumulated histograms of sympathetic activity were produced synchronized with the electrocardiogram to detect cardiac rhythmicity, with phrenic nerve activity to detect modulation with the central respiratory cycle, and with tracheal pressure to uncover a reflex modulation associated with artificial ventilation. Sympathetic activity, phrenic activity, and tracheal pressure also were examined by spectral analysis and autocorrelation to detect rhythmicities distinct from respiration. Twenty-seven filaments containing two to seven fibers with spontaneous activity and 51 single fibers were analyzed. Ongoing activity was 1.12 +/- 0.65 imp/s (mean +/- SD, n = 51); conduction velocity was 0.62 +/- 0.06 m/s (n = 30). Cardiac rhythmicity in sympathetic activity was weak (46.2 +/- 16.4%). The dominant rhythm in the activity of 19/27 few-fiber preparations and 37/51 single fibers corresponded to the central respiratory cycle. The pattern consisted of an inhibition during inspiration and an activation in expiration. In 10/19 few-fiber preparations and 21/37 single fibers of this group, there was also a concomitant, less prominent rhythm related to artificial ventilation. By contrast, 8/27 few-fiber preparations and 11/51 single fibers exhibited a dominant pump-related modulation, whereas phrenic-related rhythmicity was subordinate. The dominant rhythm in the activity of two single fibers was related to neither central respiration nor artificial ventilation. We conclude that the ongoing activity of most postganglionic neurons supplying the rat tail is modulated by the central respiratory rhythm generator, suggesting that changes in respiratory drive may alter perfusion of the tail and therefore heat dissipation. Reflex modulation in parallel with artificial ventilation, independent of vagal afferents and possibly due to ventilatory changes of baroreceptor activity, is also an important source of rhythmicity in these neurons.

Aortic pH oscillations in conscious humans and anaesthetised cats and rabbits

Respiratory oscillations in arterial blood gas composition influence breathing in cats and dogs. Their role in the control of breathing in humans in less certain. To determine whether oscillations are very small or absent in mammals who are large or breathe fast, aortic pH oscillations, recorded with a tridodecylamine based hydrogen-ion selective electrode, were compared in humans (n = 13), cats (n = 7) and rabbits (n = 4) over a wide range of ventilation. For comparison, data were analysed in terms of the ratio of tidal volume to functional residual capacity (VT/FRC). During spontaneous breathing in rabbits, cats and humans (mean respiratory frequency fR = 61, 20.4 and and 17.5 min-1), mean VT/FRC were 1.35, 0.63 and 0.36 respectively. Corresponding pH amplitudes (pHamp) of 0.009 (0.004), 0.016 and 0.013 (0.005) pH units (mean +/- 1SD) were not significantly different. The pHamp decreased exponentially with increasing FR in each species and pHamp increased linearly with increasing VT in the 3 cats in which this was studied. The study confirms the dependence of pHamp on FR and VT and its comparability among species despite differences in body size. It also demonstrates that oscillations can be recorded in humans at FR in excess of 20 min-1.