Academic literature on the topic 'Respiration; Hypoxia; Hypocapnia'

1. In systemic hypertension the pulmonary vessels show an excessive tone at rest and hyper-react to adrenoceptor stimulation. Alterations in Ca2+ handling by the vascular smooth muscle cells seem to underlie these disorders. Alveolar hypoxia also constricts pulmonary arteries, increasing the intracellular Ca2+ availability for smooth muscle contraction. This suggests the hypothesis that hypoxic pulmonary vasoconstriction depends on similar biochemical disorders, and that the response to the hypoxic stimulus may be emphasized in high blood pressure. 2. In 21 hypertensive and 10 normotensive men, pulmonary arterial pressure and arteriolar resistance have been evaluated during air respiration and after 15 min of breathing 17, 15 and 12% oxygen in nitrogen. Curves relating changes in pulmonary arterial pressure and arteriolar resistance to the oxygen content of inspired gas had a similar configuration in the two populations, but in hypertension were steeper and significantly shifted to the left of those in normotension, reflecting a lower threshold and an enhanced vasoconstrictor reactivity. 3. This pattern was not related to differences in severity of the hypoxic stimulus, degree of hypocapnia and respiratory alkalosis induced by hypoxia, and plasma catecholamines. 4. The association of high blood pressure with enhanced pulmonary vasoreactivity to alveolar hypoxia could have clinical implications in patients who are chronically hypoxic and have systemic hypertension.

The ventilatory responses to immersion and changes in temperature are reviewed. A fall in skin temperature elicits a powerful cardiorespiratory response, termed “cold shock,” comprising an initial gasp, hypertension, and hyperventilation despite a profound hypocapnia. The physiology and neural pathways of this are examined with data from original studies. The respiratory responses to skin cooling override both conscious and other autonomic respiratory controls and may act as a precursor to drowning. There is emerging evidence that the combination of the reestablishment of respiratory rhythm following apnea, hypoxemia, and coincident sympathetic nervous and cyclic vagal stimulation appears to be an arrhythmogenic trigger. The potential clinical implications of this during wakefulness and sleep are discussed in relation to sudden death during immersion, underwater birth, and sleep apnea. A drop in deep body temperature leads to a slowing of respiration, which is more profound than the reduced metabolic demand seen with hypothermia, leading to hypercapnia and hypoxia. The control of respiration is abnormal during hypothermia, and correction of the hypoxia by inhalation of oxygen may lead to a further depression of ventilation and even respiratory arrest. The immediate care of patients with hypothermia needs to take these factors into account to maximize the chances of a favorable outcome for the rescued casualty.

Kuna, Samuel T., and Christi R. Vanoye.Respiratory-related pharyngeal constrictor muscle activity in decerebrate cats. J. Appl. Physiol.83(5): 1588–1594, 1997.—Respiratory-related activity of the hyopharyngeus (middle pharyngeal constrictor) and thyropharyngeus (inferior pharyngeal constrictor) muscles was determined in decerebrate, tracheotomized adult cats and compared with the electromyographic activity of the thyroarytenoid, a vocal cord adductor. During quiet breathing, the hyopharyngeus and usually the thyroarytenoid exhibited phasic activity during expiration and tonic activity throughout the respiratory cycle. Respiratory-related thyropharyngeus activity was absent under these conditions. Progressive hyperoxic hypercapnia and progressive isocapnic hypoxia increased phasic expiratory activity in both pharyngeal constrictor (PC) muscles but tended to suppress thyroarytenoid activity. Passively induced hypocapnia and the central apnea that followed the cessation of the mechanical hyperventilation were associated with tonic activation of the hyopharyngeus and thyroarytenoid but no recruitment in thyropharyngeus activity. The expiratory phase of a sigh and progressive pneumothorax were associated with an increase in phasic thyroarytenoid activity but no change in phasic PC activity. The results indicate that a variety of stimuli modulate respiratory-related PC activity, suggesting that the PC muscles may have a role in the regulation of upper airway patency during respiration.

In normoxic conscious dogs, increased angiotensin II (ANG II), or activation (disinhibition) of the renin-angiotensin system by vasopressin (AVP) V1-receptor block, increases ventilation and decreases arterial PCO2. Both hormones can be increased during hypoxia and might modulate ventilatory drive. Six conscious dogs were studied before and during hypocapnic, isocapnic, and hypercapnic hypoxia. To study potential hormonal effects during hypocapnic hypoxia, experiment 1 included three protocols in which 12.8% O2 was breathed for 60 min: protocol 1, control studies without block; protocol 2, AVP V1 receptors were blocked at the onset of hypoxia; and protocol 3, ANG II receptors were blocked 20 min before hypoxia. To study potential effects of acid-base changes during acute hypoxia, experiment 2 included two protocols (with and without AVP V1-receptor block). A 40-min period of hypocapnic hypoxia was followed by two successive 20-min periods with hypoxia maintained but inspired CO2 progressively increased. Neither hormonal block affected respiration during the hypoxic conditions. Unlike normoxia in conscious dogs, during acute hypoxia, respiratory control by ANG II is not modulated by AVP and acid-base effects on receptors do not account for this difference.

The effects of systemic hypoxia with different levels of CO2 on R-R interval (RRI) and systolic blood pressure (SBP) variabilities were investigated in conscious rats. Wistar rats chronically instrumented for the measurement of blood pressure, electrocardiogram, and renal sympathetic nerve activity (RSNA) were exposed to hypocapnic (Hypo), isocapnic (Iso), and hypercapnic (Hyper) hypoxia. On another day, the rats were treated with atropine and exposed to the same type of hypoxia. Sinoaortic denervation (SAD)-treated rats were exposed to Iso and Hyper, and RRI and SBP variabilities before and during hypoxia were analyzed using the maximum-entropy method with high resolution. With regard to RRI variability, very low frequency (VLF), low frequency (LF), and high frequency (HF) powers all decreased during Hypo, increased during Hyper, and did not change during Iso in intact rats. Changes during Hypo were attenuated by atropine, and those during Hyper were abolished by either atropine or SAD. The ratio of LF power to HF power decreased independently of increases in RSNA during each type of hypoxia. On the other hand, there were no changes in VLF, LF, or HF power in SBP variability during each type of hypoxia in intact rats. In atropine-treated rats, LF power increased during Iso and Hyper and HF power increased during each type of hypoxia. There was no difference in respiratory frequency among the three kinds of hypoxia in both intact and atropine-treated rats. The results suggest that arterial[Formula: see text] level rather than respiration frequency produces changes in powers of RRI variability through changes in parasympathetic nerve activity and that with regard to SBP variability, parasympathetic nerve activity masks changes in LF power that reflect an increase in RSNA and those in HF power that reflect a mechanical consequence of respiration.