Carbon dioxide sensitivity of the central chemosensitive mechanisms. An exploration by direct stimulation in rats

Carbon dioxide therapy in hypocapnic respiratory failure

Oxygen therapy, usually administered by a facemask or nasal cannulae, is the current default treatment of respiratory failure. Since respiration entails intake of oxygen and release of carbon dioxide from tissues as waste product, the notion of administering carbon dioxide in respiratory failure appears counter-intuitive. However, carbon dioxide stimulates the chemosensitive area of the medulla, known as the central respiratory chemoreceptor, which activates the respiratory groups of neurones in the brainstem and stimulates inspiration thereby initiating oxygen intake during normal breathing. This vital initiation of normal breathing is via a reduction in the pH of the cerebrospinal fluid and the medullary interstitial fluid. We hypothesise that in cases of type I respiratory failure in which the P_aCO₂ is low, administration of carbon dioxide by inhalation would stimulate the respiratory groups of brainstem neurones and facilitate breathing, which would be of therapeutic value. Preliminary clinical evidence in favour of this hypothesis is presented and we recommend that a formal randomised study be carried out.

Retrotrapezoid nucleus lesions decrease phrenic activity and CO2 sensitivity in rats

In chloralose-urethane anesthetized, paralyzed, and ventilated rats, we measured the effects of unilateral lesions in the region ventral and ventromedial to the facial nucleus, the retrotrapezoid nucleus (RTN), on eucapnic phrenic activity and the response to increased end-tidal CO2. Chemical (kainic acid injections; 4.7 mM; 10-100 nl) and electrolytic (5-15 mA; 5-15 sec) lesions, anatomically demonstrated to be in the RTN, resulted in a progressive decrease in the amplitude of the integrated phrenic nerve activity from baseline levels of 49-59% of maximum to values of 21-32% of maximum over 30 to 120 min. There were no consistent effects on frequency or on blood pressure. The initial slope of the response to hypercapnia was decreased by 86-92%. Bilateral carotid body ablation did not alter the general pattern of the responses. As in the cat, unilateral RTN lesions decrease baseline phrenic amplitude and virtually abolish the response to hypercapnia. We hypothesize that the RTN region provides; (1) a source of tonic activity which maintains eucapnic ventilatory output, and (2) allows expression of the response to hypercapnia.

Respiratory and vasomotor responses to focal cooling of the ventral medullary surface (VMS) of the rat

Experiments were performed on anesthetized, paralyzed and artificially ventilated rats after denervation of the vagus and carotid sinus nerves. The electrical activity of the phrenic and cervical sympathetic nerves (CS) along with the arterial blood pressure (BP) were monitored. Graded unilateral cooling of the ventral lateral surface (VMS) from 37 degrees C to 10 degrees C between 6th and 12th nerve rootlets did not affect the phrenic activity. Whereas, a significant depression or apnea was seen with cooling of an area between 1st cervical and 12th nerve rootlets. Bilateral cooling also produced similar respiratory responses. Respiratory depression could also be obtained during higher respiratory drive (7% CO2 in O2). On the other hand, a significant fall in BP and reduction in CS activity were observed with unilateral cooling in any of these VMS areas. However, the magnitude of BP decrease was less with 7% CO2 in O2 compared to 100% O2 breathing.

2-Deoxyglucose uptake in the central nervous system during systemic hypercapnia in the peripherally chemodenervated rat

Changes in 2-deoxyglucose (2-DG) uptake in the central nervous system during systemic hypercapnia were determined by the [3H]2-DG autoradiographic method in peripherally chemodenervated rats. Autoradiographs were made from serial transverse sections of the brain and analyzed by a computer-based interactive image processing system for areas having increases or decreases in metabolic activity compared with control animals. The most pronounced change shown by autoradiographs of the hypercapnic animals was a generalized decrease in the metabolism of the gray matter throughout the central nervous system with respect to the normocapnic controls. However, several central structures showed evidence of either no change or an increased metabolism in the hypercapnic animals. In the brain stem these areas were localized to the ventrolateral region of the nucleus of the solitary tract rostral to the obex, around the region of the nucleus retroambiguus, in a region of the ventrolateral medullary reticular formation extending rostrally from the obex to the level of the intramedullary rootlets of the facial nerve, in the region of the ventral nucleus raphe pallidus, and in the region of the lateral parabrachial nucleus. In the diencephalon these regions included the supraoptic nucleus and the dorsal hypothalamic area, extending into the caudal portion of the paraventricular nucleus. The thoracolumbar cord showed activation of the lateral aspects of the dorsal horns, the region of lamina X and the region of the intermediolateral nucleus. These data may be interpreted as a functional map of the central structures activated in hypercapnia in the peripheral chemodenervated rat. It appears likely that these structures are involved in mediating the cardiorespiratory responses associated with the activation of central chemoreceptors by the increased carbon dioxide concentrations.

Effects of urethane-chloralose anaesthesia on respiration in the rat

1. Respiratory effects were measured in rats during six hours' anaesthesia with urethane and chloralose. 2. One-hundred min from urethane administration, minute ventilation (VE) was minimal, arterial PO2 was low, arterial PCO2 was high; tidal volume (VT) and respiratory frequency (f) were relatively constant; hypercarbic and hypoxic responses were substantial. 3. Between 100 and 400 min from urethane administration, minute ventilation and frequency increased and became more variable, tidal volume remained relatively constant, arterial PO2 rose to 100 mmHg, PCO2 fell to 37 mmHg; hypercarbic sensitivity increased and hypoxic sensitivity decreased. 4. We conclude that the anaesthetic regime produced initial depression of respiration relative to metabolism but without great loss of respiratory chemosensitivity. The respiratory depression was prolonged by increased dosage with urethane and chloralose. 5. The variations between hypercarbic and hypoxic responses confirm that they operate through separate mechanisms.