pH, PCO2 and PO2 of cisternal cerebrospinal fluid in high altitude natives

Soman toxication in hypoxia acclimated rats: alterations in brain neuronal RNA and survival

Effects of prior hypoxia acclimation (14-day at 380 mm Hg) on soman (pinacolyl methylphosphonofluoridate) induced brain neuronal RNA and acetylcholinesterase (AChE) depletion and lethality were monitored in rats following their return to ambient oxygenation. Quantitative cytochemical techniques were used to measure RNA and AChE changes in individual cerebrocortical (Layer III) and striatal (caudate plus putamen) neurons. In ambient Po2 controls, soman eventuated in a moderate diminution of neuronal RNA in both brain regions and severe, dose-dependent suppression of AChE activity. Hypoxia acclimation per se induced RNA alterations as manifested in cortical RNA depletion and increased variability of striatal neuron RNA contents. In hypoxia acclimated rats, the extent of neuronal RNA depletion following soman injection was attenuated in both brain regions, yet there were no discernible differences in saline control AChE levels or in the extent of soman-induced AChE inhibition in ambient control versus hypoxia acclimated treatment groups. Hypoxia acclimated rats, however, were found to be even more susceptible to lethal actions of soman as assessed using 24- and 48-hour survival following a three-point treatment regimen. These data indicate that while compensatory systemic and central metabolic adjustments associated with 14d acclimation to reduced oxygen availability may retard soman-induced neuronal RNA depletion, resistance to lethal or near-lethal soman exposure is not enhanced. It is postulated that hypoxia acclimation is associated with complex adaptive and maladaptive neurophysiological alterations influencing CNS responsiveness to soman toxication, and that detrimental consequences exceed protection afforded by metabolic adaptation.

Dependence of high altitude sleep apnea on ventilatory sensitivity to hypoxia

Respiration in man exposed to 5400 m was studied during sleep over a period of 6-8 h at night. Subjects were adult males, and belonged to distinct groups: one was Sherpa high altitude residents of the Himalayas and the other consisted of causasian sojourners from near sea level. All the volunteers had spent at least 32 days at or above 5400 m before the study. The subjects were instrumented for the measurements of breath-by-breath ventilation, SaO2%, eye-movement and heart rate. Nasal prongs were secured for the administration of mixtures of O2, N2 or CO2. Also, ventilatory sensitivity to hypoxia was studied in the awake state by the transient N2 and O2 tests. The lowlanders who showed high respiratory sensitivity to hypoxia also manifested periodic breathing with apnea during sleep. A raised PIO2 and SaO2% decreased ventilation, raised PACO2, attenuated respiratory oscillations and eliminated apnea in the sojourners. CO2 inhalation in air also eliminated apnea but not the periodicity, indicating that respiratory alkalosis caused apnea but periodic breathing was independent of central stimulation by CO2-H+. None of the Sherpa highlanders with low ventilatory sensitivity to hypoxia showed any sustained periodic breathing with apnea. The large breathing oscillations and periodic apnea correlated well with the ventilatory sensitivity to hypoxia (r = 0.85), supporting the hypothesis that a high gain of the peripheral chemoreflex is conductive to periodic breathing. Sherpas by attenuating chemoreflexes have reduced instability as well as cost of breathing at high altitude.

Arterial blood gases and acid-base status in awake rats

Arterial blood gases and acid-base balance were measured in adult rats using a cannula implanted in the aortic arch. These measurements were performed both in awake, unrestrained animals and in animals submitted to various circumstances i.e. (a) different diet: high and low sodium chloride intake, (b) anesthesia by pentobarbital or inactine and, (c) repeated blood sampling with concomitant replacement with the same volume of blood. For each group investigated the [HCO3 -]a vs. PaCO2, [H+] vs. PaCO2, PaCO2 vs. PaO2 relationships were determined. The values obtained (m +/- SD) from awake, unrestrained adult rats were respectively 7.47 +/- 0.02 for arterial pH, 34.5 +/- 3.0 Torr for PaCO2 and 90 +/- 5.5 Torr for PaO2; the calculated [HCO3 -]a concentration was 25.5 +/- 1.5 mmol . 1-1. The present results indicate that plasma bicarbonate concentration, within normal range, highly depends on the prevailing resting level of PaCO2 (n = 202; r = 0.82; P less than 10(-3)). In addition, the PaCO2 versus PaO2 relationship was highly statistically significant (n = 202; r = -0.43; P less than 10(-3). In the other experimental groups of rats, these relationships were virtually the same as above although mean values (+/- SD) for PaCO2, PaO2, pHa and [HCO3 -]a might vary with the group investigated. The mean value for whole pHi, obtained by the DMO method, reached 6.81 for pHa = 7.47 and was not correlated to PaCO2 level in normal conditions. The present data argue for the existence of a respiratory component mediating individual acid-base variations in a normal population of rats. Arterial carbon dioxide partial pressure, by determining bicarbonate ions reabsorption rate, would ensure pH regulation under normal circumstances.

Control of acid-base status during hypothermia in man

Rahn's concepts of acid-base balance during hypothermia were tested in humans by studying eleven men who required extra-corporeal cooling for surgery. Hypothermia was moderate (27-28 degrees C) and maintained for 60-70 min. Extracorporeal blood perfusion (ECBP) was performed with a bubble-oxygenator which allowed changes in blood flow and gas concentrations. Arterial pH (pHa) at the person's body temperature was controlled by varying CO2 flow to the oxygenator in order to maintain in vitro pH measured at 37 degrees C in the normal range. During hypothermia and after rewarming to 37 degrees C, bicarbonate concentration and total CO2 content of arterial and mixed venous blood remained constant. A physiologic solution was introduced into the peritoneal cavity which was used as a tonometer; the values of equilibrated CO2 content in peritoneal fluid were constant. Neither metabolic acidosis nor hypercapnia developed. Blood acid-base balance in vivo during hypothermia was therefore identical to the behavior of blood in vitro. In addition, the interpretation of the results of acid-base studies, in humans with abnormal central temperature is facilitated when measurements are performed at 37 degrees C.

Cerebral hypoxia from bicarbonate infusion in diabetic acidosis

To study the delivery of oxygen to the cerebral tissue during metabolic acidosis and its therapy with bicarbonate (NaHCO3), oxygen partial pressure of the cisternal fluid was measured in 12 experiments of HCl-induced acidemia and eight experiments of diabetic ketoacidosis in 16 unanesthetized dogs. Full correction of acidosis with bicarbonate caused a significant (P less than 0.05) decrease in Pcsf 2: in the HCl acidotic dogs, Pcsfo2 decreased from 53.9 +/- 2.2 torr to 45.9 +/- 2.3 torr within one hour; in the ketoacidotic dogs, Pcsfo2 decreased for 48.5 +/- 5.4 torr to 26.7 +/- 6.6 torr within six hours. In the ketoacidotic dogs not treated with bicarbonate, there was no significant change in Pcsfo2. An inverse relationship (P less than 0.01) between the cisternal lactic acid level and the cisternal PO2 was also observed. It is concluded that NaHCO3 therapy in diabetic ketoacidosis decreases the cerebral O2 availability and that cerebral hypoxia contributes to the brain dysfunction encountered after the initiation of such therapy in acidemia.

Dependence of CSF on plasma bicarbonate during hypocapnia and hypoxemic hypocapnia

We have previoulsy shown pH compensation to be similar in CSF and arterial blood during chronic hypoxemic hypocapnia in man and pony, and postulated that the compensatory reduction in CSF [HCO3] was dependent upon corresponding changes in [HCO3]a. We tested this hypothesis in anesthetized, paralyzed dogs by determining the effects of 7 or 14 hours of hypocapnia (PaCO2 20 and 30 mm Hg), hypoxemia (PaO2 30, 38 and 48 mm Hg) and hypocapnic hypoxemia on CSF acid-base status. [hco3]a was either permitted to fall normally or was held near control levels by NaHCO3 infusion. In hypocapnia and hypoxemic hypocapnia, the decrease in [HCO3] and % pH compensation in CSF were less than or equal to that in arterial blood. Most (51-89%) of the compensatory decrease in CSF [HCO3] was prevented by preventing the corresponding reduction in [HCO3]a. This dependence of changes in CSF on plasma [HCO3] required a concurrent decrease in CSF PCO2, but was largely independent of variations in plasma pH. A minor but significant portion of the decrease in CSF [HCO3] was achieved independently of corresponding changes in [HCO3]a. The contribution of this local mechanism to CSF [HCO3] regulation increased with increasing severity of hypocapnia or hypoxemia and was usually associated with a selective increase in CSF lactate. It was concluded that [HCO3] regulation in the CSF during hypoxemic hypocapnia was primarily dependent upon, and therefore limited by, the concomitant decrease in plasma [HCO3].

Ventilatory acclimatization and csf acid-base balance in carotid chemodenervated dogs at 3550 m

In three awake dogs in a hypobaric chamber at 140 m and at 3550 m, resting ventilation, pulmonary gas exchanges, respiratory gases and pH of the arterial blood, acid-base status in the cerebrospinal fluid (csf), and ventilatory responses to transient O2-inhalation were studied before (intact) and after chronic bilateral carotid body denervation (cbd). 1. The hypoxic chemoreflex drive of ventilation was reduced by about half in cbd dogs. 2. At low altitude, sino-carotid body denervation resulted in hypoventilation and respiratory acidosis in the arterial blood and csf. 3. At high altitude, initial hypoxic hyperventilation, and the related alkalosis in blood and csf, occurred within 30 min in intact dogs, but was not observed in cbd ones. 4. Further increase in ventilation was achieved upon 3 hrs of altitude exposure in intact animals, while a delayed hyperventilation occurred after 24 hrs in cbd ones. 5. Neither in intact nor in cbd dogs, the ventilatory changes at altitude were related to the changes in csf pH. It is concluded that the rate of ventilatory acclimatization to altitude is dependent upon the strength of the arterial chemoreceptor drive. Integrity of this chemoreflex drive of breathing is essential in determining the eupneic level of ventilation and normal acid-base status of the blood and csf at low altitude and at high altitude.

Renal response to short-term hypocapnia in man

This study examines the renal response to moderate hyperventilation in healthy man. Eight men hyperventilated for 26 hr (PaCO2 approximately 30 to 32 mm Hg) in normoxia (barometric pressure, PB approximately 740 mm Hg) and hypobaric hypoxia (PB approximately530 mm Hg). Anaerobic samples of arterial blood and urine were studied at two-hour intervals. Plasma [HCO3-] fell with time during sustained hypocapnia and after 26 hr was reduced 2.5 mEq/liter, with plasma pH compensated approximately 60%. Statistically significant changes in renal H+ handling were observed within the initial 2 hr of hyperventilation and were evident over the first 12 hr. Over 26 hr, mean total HCO3-excretion in hypocapnia was 10.2 mEq above control and mean total acid excretion (UVTA + UVNH4+) was 17.5 mEq below control. An increased urinary excretion of cations, especially sodium, accompanied the decrease in acid excretion. Plasma lactic acid accumulation was negligible. We conclude that renal mechanisms contribute significantly and relatively quickly to plasma pH compensation during the early phase of adaptation to hypocapnia in man.

Ccomparison of cisternal and lumbar cerebrospinal fluid pH in high altitude natives

Samples of cisternal or lumbar cerebrospinal fluid were obtained from 20 young male volunteers born and living at high altitude (3500 to 4800 m). The pH, carbon dioxide and oxygen tensions, and bicarbonate concentration were measured and compared with those in the arterial and jugular venous blood. A consistent difference between the two CSF compartments was noted, particularly a lower pH (0.05), a higher PCO2 (7 Torr), and a lower PO2 (7 Torr) at the lumbar site. Mean bicarbonate concentration was not significantly different at the two sites. The main factor is PCO2, which controls the pH variation. These differences were more marked in high-altitude natives than in man at sea level. The existence of a consistent inhomogeneity of CSF acid-base content emphasizes the inaccuracy of using lumbar CSF pH to estimate the ECF pH as regulator of pulmonary ventilation and determinant of cerebral blood flow.