Hypothetical roles of angiogenesis, osmotic swelling, and ischemia in high-altitude cerebral edema

High-altitude cerebral edema (HACE) has been tentatively attributed to either cellular ion pump failure from ATP depletion or high cerebral blood flow inducing high capillary pressure. These hypotheses are inadequate because 1) ATP decrease occurs only after anoxia has silenced neuronal activity and 2) prolonged hypercapnic hyperemia generates only minor transcapillary protein leakage localized to the less hyperemic brain regions. In connection with this review of HACE and its causes, three other hypothetical mechanisms that might contribute are presented. 1) Osmotic cell swelling: cellular and mitochondrial osmotic pressure may rise 30 mosmol in ischemia or anoxia (potentially a 7-10% expansion). Smaller rises caused by hypoxia may be significant in the closed calvarium. 2) Focal ischemia: this may result from intracranial hypertension from hyperemia and osmotic swelling. 3) Angiogenesis: cellular hypoxia initially attracts and activates macrophages that express vascular endothelial growth factor and other cytokines, dissolving capillary basement membranes and degrading extracellular matrix, resulting in capillary leakage. In HACE, petechial hemorrhages are seen in the nerve cell layers of the retina, and similar changes have been described throughout the brain. Evidence linking HACE to angiogenesis is that dexamethasone, an effective inhibitor of angiogenesis, has demonstrated unique success in preventing and treating HACE.

Hypoxic ventilatory response predicts the extent of maximal breath-holds in man

To understand the factors influencing breath-holding performance, we tested whether the hypoxic (HVR) and hypercapnic ventilatory responses (HCVR) were predictors of the extent of maximal breath-holds as measured by breath-hold duration, the lowest oxyhemoglobin saturation (SpO2min), lowest calculated PaO2 (PaO2min) and highest end-tidal PCO2 (PETCO2max) reached. Steady state isocapnic HVR and hyperoxic HCVR were measured in 17 human volunteers. Breath-holds were made at total lung capacity (TLC), at TLC following hyperventilation, at functional residual capacity, and at TLC with FIO2 = 0.15. SpO2 was measured continuously by pulse oximetry, and alveolar gas was measured at the end of breath-holds by mass spectrometry. PaO2min was calculated from SpO2min and PETCO2max. HVR was a significant predictor of both SpO2min and PaO2min. HVR and forced vital capacity were predictors of breath-hold duration by multiple linear regression. HCVR had no significant predictive value. We conclude that HVR, but not HCVR, is a significant predictor of breath-holding performance.

Hypoxic ventilatory depression may be due to central chemoreceptor cell hyperpolarization

By re-examining the results of various studies of HVD, of the localization of medullary CO2 chemosensory cells, and of their acid secretion, an hypothesis has been developed suggesting that the neurones which detect increased CO2 or CSF acid respond to decreased transmembrane H+ gradient, i.e. a greater fall in ECF than in ICF pH. Hypoxic lactic acid generated within these cells depresses activity, which can be restored by an appropriate rise of Paco2, disclosing both normal peripheral chemoreceptor hypoxic sensitivity and normal medullary integrative response.

Exercise O2 transport model assuming zero cytochrome PO2 at VO2 max

An analogy is drawn between cytochrome aa3 function and a polarographic cathode at which the potential of -0.6 V captures all O2 diffusing to the surface, achieving maximal O2 consumption (VO2max) by eliminating O2 backpressure and outward diffusion from the surface, defined herein as zero surface PO2. The relationship of O2 consumption (as %VO2max) to muscle venous, myoglobin, and cytochrome PO2 is modeled assuming that cytochrome aa3 PO2 reaches zero at VO2max, incorporating published data on the profile of leg venous PO2, pH, and blood lactate vs. work. Equations describe hemoglobin and myoglobin O2 dissociation and the Bohr effect of acid on O2 unloading. The O2 gradient from capillary blood to cytochrome aa3 is assumed to be proportional to O2 consumption. The model suggests that 1) to extract 75% of the O2 from myoglobin at VO2max, myoglobin must lie 90% down the O2 gradient from capillary to cytochrome; 2) the Bohr effect adds 15-30% to VO2max and keeps venous PO2 almost constant as work rises from 60 to 100% of VO2max; and 3) in steady heavy work, the rising arterial lactate may impede lactate excretion from muscle, reduce anaerobic ATP generation, and shift the energy balance toward aerobic metabolism. The zero PO2 hypothesis facilitates modeling and may be the key to understanding the physiological limitation of work.

Time course of augmentation and depression of hypoxic ventilatory responses at altitude

Hypoxic ventilatory response (HVR) and hypoxic ventilatory depression (HVD) were measured in six subjects before, during, and after 12 days at 3,810-m altitude (barometric pressure approximately 488 Torr) with and without 15 min of preoxygenation. HVR was tested by 5-min isocapnic steps to 75% arterial O2 saturation measured by pulse oximetry (Spo2) at an isocapnic PCO2 (P*CO2) chosen to set hyperoxic resting ventilation to 140 ml.kg-1.min-1. Hypercapnic ventilatory response (HCVR, 1.min-1.Torr-1) was tested at ambient and high SPO2 6-8 min after a 6- to 10-Torr step increase of end-tidal PCO2 (PETCO2) above P*CO2. HCVR was independent of preoxygenation and was not significantly increased at altitude (when corrected to delta logPCO2). Preoxygenated HVR rose from -1.13 +/- 0.23 (SE) l.min-1.%SPO2(-1) at sea level to -2.17 +/- 0.13 by altitude day 12, without reaching a plateau, and returned to control after return to sea level for 4 days. Ambient HVR was measured at P*CO2 by step reduction of SPO2 from its ambient value (86-91%) to approximately 75%. Ambient HVR slope was not significantly less, but ventilation at equal levels of SPO2 and PCO2 was lower by 13.3 +/- 2.4 l/min on day 2 (SPO2 = 86.2 +/- 2.3) and by 5.9 +/- 3.5 l/min on day 12 (SPO2 = 91.0 +/- 1.5; P < 0.05). This lower ventilation was estimated (from HCVR) to be equivalent to an elevation of the central chemoreceptor PCO2 set point of 9.2 +/- 2.1 Torr on day 2 and 4.5 +/- 1.3 on day 12.(ABSTRACT TRUNCATED AT 250 WORDS)

Siggaard-Andersen and the "Great Trans-Atlantic Acid-Base Debate"

In the late 1950's, while working with Poul Astrup's equilibration method of blood gas analysis, Siggaard-Andersen introduced a new parameter called base excess (BE) to quantify the non-respiratory acid-base imbalance. "The Great-Transatlantic Acid-Base Debate" arose when the "Boston" school, whose bicarbonate based analysis had been developed during pre-1950 Van Slyke days, (initially) argued that BE was not independent of PCO2 in vivo. Although Siggaard-Andersen and others then introduced a standard BE independent of PCO2, the Boston and Copenhagen schools are "unreconciled". While SBE is now used by most physicians, teaching and interpretation of acid-base chemistry remains confusing, "Boston" school laboratories refusing to report SBE, their students being asked to learn the 6 bicarbonate equations and rules, an old concept being reintroduced as "strong ion difference", or SID, and some wanting to discard pH in favor of nanomoles of H+, and end the era of "Arrhenius, Severinghaus and Henderson-Hasselbalch".

Effect of a vecuronium-induced partial neuromuscular block on hypoxic ventilatory response

BACKGROUND

A previous study has demonstrated a decrease in the hypoxic ventilatory response in volunteers partially paralyzed with vecuronium. However, in this study, hypocapnia was allowed to occur. Because hypocapnia counteracts the ventilatory response to hypoxia during partial vecuronium-induced neuromuscular block and isocapnia, the hypoxic ventilatory response (HVR) was tested in 10 awake volunteers.

METHODS

To avoid hypocapnia, the resting hyperoxic control end-tidal PCO2 was increased to 43.3 +/- 2.4 mmHg, raising inspiratory minute ventilation (VI) to 140 ml.kg-1.min-1. Hypoxic ventilatory response (delta VI/delta SpO2, L.min-1.%-1) was measured during a 5-min isocapnic step reduction to a mean arterial hemoglobin oxygen saturation (SpO2) of 84.8 +/- 1.4%. Immediately thereafter, hypercapnic ventilatory response (HCVR; delta VI/delta PETCO2, L.min-1.mmHg-1) was determined at the end of a 6-min step increase of PETCO2 to 50.5 +/- 2.7 mmHg. During a subsequent 30-40-min pause, an intravenous infusion of vecuronium was adjusted to reduce the adductor pollicis train-of-four ratio to 0.70, as monitored using mechanomyography. Ventilatory parameters, HVR and HCVR, were then redetermined.

RESULTS

Resting VI, PETCO2, and SpO2 were unchanged by drug infusion. Hypoxic ventilatory response decreased from control (a) of 0.97 +/- 0.43 to 0.74 +/- 0.41 L.min-1.%-1 (P < 0.02) during drug infusion (b), while HCVR was unchanged (a = 1.91 +/- 0.82, b = 1.62 +/- 0.46 L.min-1.mmHg-1; NS). To correct HVR for possible vecuronium-induced respiratory muscle weakness or otherwise altered central nervous system reactivity, the drug/control ratio (HVRb/a) was divided by the associated HCVRb/a ratio. This HVR index, FHVR, was 0.84 +/- 0.12 (P < 0.01).

CONCLUSIONS

We conclude that a vecuronium-induced partial neuromuscular block impairs HVR more than it does HCVR in humans, suggesting an effect of vecuronium on carotid body hypoxic chemosensitivity.

Topography of cat medullary ventral surface hypoxic acidification

The topographic relationship between previously identified medullary ventral surface respiratory chemosensitive regions and brain surface extracellular fluid (ECF) acid production during acute hypoxia was explored in anesthetized, paralyzed, and artificially ventilated cats. Glass pH electrodes (0.8-mm diam, sheathed in stainless steel tubing) were mounted in mechanical contact with surfaces of medullary surface or adjacent pyramids, pons, spinal cord, or parietal cortex. Isocapnic hypoxia of 5 min [at arterial O2 saturation (SaO2) = 48 +/- 10%] reduced pH over rostral (Mitchell) and caudal (Loeschcke) areas by 0.12 +/- 0.09 and 0.07 +/- 0.04, respectively (n = 10, P < 0.05). Change in pH (delta pH) was proportional to desaturation with slopes 100 delta pH/delta SaO2 of 0.45 (rostral) and 0.20 (caudal) (R = 0.91 and 0.88, respectively). pH drop usually began within 3 min of hypoxia, became stable between 5 and 15 min, began to rise within 2 min of reoxygenation, and returned to control within 10 min. During equally hypoxic tests, intermediate area (Schläfke), pons, and spinal cord surfaces showed no significant acid shift. Parietal cortex ECF pH dropped more slowly but steadily by 0.079 +/- 0.034 during 20 min at SaO2 = 50% after a small but significant initial alkaline shift, and acidification of cortical surface continued for > 5 min after reoxygenation. We conclude that medullary ventral chemosensitive regions produce more lactic acid during hypoxia than neighboring brain surfaces.(ABSTRACT TRUNCATED AT 250 WORDS)

Augmented hypoxic ventilatory response in men at altitude

To test the hypothesis that the hypoxic ventilatory response (HVR) of an individual is a constant unaffected by acclimatization, isocapnic 5-min step HVR, as delta VI/delta SaO2 (l.min-1.%-1, where VI is inspired ventilation and SaO2 is arterial O2 saturation), was tested in six normal males at sea level (SL), after 1-5 days at 3,810-m altitude (AL1-3), and three times over 1 wk after altitude exposure (PAL1-3). Equal medullary central ventilatory drive was sought at both altitudes by testing HVR after greater than 15 min of hyperoxia to eliminate possible ambient hypoxic ventilatory depression (HVD), choosing for isocapnia a P'CO2 (end tidal) elevated sufficiently to drive hyperoxic VI to 140 ml.kg-1.min-1. Mean P'CO2 was 45.4 +/- 1.7 Torr at SL and 33.3 +/- 1.8 Torr on AL3, compared with the respective resting control end-tidal PCO2 of 42.3 +/- 2.0 and 30.8 +/- 2.6 Torr. SL HVR of 0.91 +/- 0.38 was unchanged on AL1 (30 +/- 18 h) at 1.04 +/- 0.37 but rose (P less than 0.05) to 1.27 +/- 0.57 on AL2 (3.2 +/- 0.8 days) and 1.46 +/- 0.59 on AL3 (4.8 +/- 0.4 days) and remained high on PAL1 at 1.44 +/- 0.54 and PAL2 at 1.37 +/- 0.78 but not on PAL3 (days 4-7). HVR was independent of test SaO2 (range 60-90%). Hyperoxic HCVR (CO2 response) was increased on AL3 and PAL1. Arterial pH at congruent to 65% SaO2 was 7.378 +/- 0.019 at SL, 7.44 +/- 0.018 on AL2, and 7.412 +/- 0.023 on AL3.(ABSTRACT TRUNCATED AT 250 WORDS)

Medullary CO2 chemoreceptor neuron identification by c-fos immunocytochemistry

In a search for CO2 chemoreceptor neurons in the brain stem, we used immunocytochemistry to monitor the expression of neuronal c-fos, a marker of increased activity, after 1 h of exposure to CO2 in five groups of Sprague-Dawley rats (294 +/- 20 g): five air breathing controls, three breathing 10% CO2, three breathing 13% CO2, three breathing 15% CO2, and three breathing 15% CO2 and treated with morphine (10 mg/kg sc). After exposure the rats were anesthetized with pentobarbital sodium and perfused intracardially with 4% paraformaldehyde. The brain stem was removed and cryoprotected, and then 50-microns frozen sections were cut and immunostained for the fos protein. Brain stem fos-immunoreactive neurons were plotted and counted in the superficial 0.5 mm of the ventral medullary surface. Thirteen to 15% CO2 evoked fos-like immunoreactivity (FLI) in 321 +/- 146 neurons/rat. Significant CO2-induced labeling was confined within the superficial 150 microns: 67% of identified cells were less than 50 microns below the surface, greater than 90% between 1.0 and 3.0 mm from the midline, and approximately 60% in the rostral half of the medulla. Thirteen to 15% CO2 also evoked FLI in the area of the nucleus tractus solitarius but not in other medullary regions. Morphine (10 mg/kg sc) did not suppress high CO2-evoked FLI in either the ventral medullary surface or the nucleus tractus solitarius, although it eliminated excitement and hyperventilation. We suggest that respiratory CO2 chemoreceptor neurons can be identified in rats by their expression of c-fos after 1 h of hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)

Anomalous hypoxic acidification of medullary ventral surface

In castrated male goats, two flexible catheters, one open ended for reference and the other ending in a 1-mm-diam glass bulb pH electrode, were advanced ventrally through a left posterior fossa craniotomy into the subarachnoid space between the 9th and 10th cranial nerve roots, passing medially into cerebrospinal fluid (CSF) over the medullary ventral surface (MVS). They were anchored to dura and fascia, tunneled under the scalp, and terminated in connectors on the left horn. After several days for recovery, while the animals were awake, the effects of CO2 and hypoxia on pH of the film of CSF between the pia and arachnoid (pHMVS) were recorded along with end-tidal PCO2 and PO2 (mass spectrometer), ventilation (pneumotachometer) through a permanent tracheostomy, and, when possible, ear arterial O2 saturation (SaO2). High PCO2 acidified MVS as expected: delta pH MVS/delta log PCO2. = -0.64 +/- 0.14, producing a ventilatory response slope delta VI/delta pHMVS = 372 l/min. Hypoxia resulted in acid shifts even when PCO2 was allowed to fall. The development of hypoxic acidosis was related to the location of pH electrodes determined at necropsy. In isocapnic hypoxia, pH over putative chemoreceptor surfaces fell in proportion to desaturation: delta pHMVS = 0.0033(SaO2)-0.34, r = 0.80, Sy.x = 0.025. With uncontrolled arterial PCO2, similar acidosis occurred when SaO2 fell below 85-90%: delta pHMVS = 0.0039(SaO2)-0.34, r = 0.88, Sy.x = 0.032. With constant hypoxia, pH fell (tau = 3.7 +/- 2.2 min) to a plateau after 10-20 min and showed rapid recovery (tau = 2.0 +/- 1.3 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Intracranial pressure and brain redox balance in rabbits

The effects of elevated intracranial pressure (ICP) on intracellular oxygenation and cerebrocortical blood volume (CBV) were studied in rabbits. Intracellular oxygen (O2) concentration was assessed as the level of pyridine nucleotide concentration ([NADH]) oxidation/reduction balance and relative cerebrocortical blood volume (CBV) were measured with a fibreoptic fluororeflectometer probe placed on the cerebrocortical surface. Experiments were conducted in six urethane anaesthetized, normocarbic animals at different fractions of inspired O2 (FIO2). During gradual increases in ICP, [NADH] began to increase (representing decreased intracellular mitochondrial PO2) for all values of FIO2 as ICP exceeded a threshold of 18 +/- 2.2 cmH2O (P less than 0.05). The decline in intracellular oxygenation with elevated ICP was inversely related to FIO2 (P less than 0.05). With ICP greater than 18 +/- 2.2 cmH2O, intracellular mitochondrial oxygenation showed an improvement between an FIO2 of 0.21 and 0.5 (P less than 0.05) but increasing FIO2 from 0.5 to 1.0 resulted in no statistically significant improvement in tissue redox balance. The CBV, largely representing tissue capillary blood, increased when ICP reached greater 18 +/- 1.2 cmH2O probably reflecting local autoregulation or venous distension (P less than 0.05). However, above 30 +/- 1.1 cmH2O, CBV decreased (P less than 0.05). The results demonstrate the interdependence of inspired oxygen concentration, elevated ICP, and brain intracellular oxygenation, and suggest that brain oxygen utilization deteriorates above an ICP of about 18 cmH2O.

Effect of halothane on hypoxic and hypercapnic ventilatory responses of goats

We have measured the ventilatory responses to increased inspired carbon dioxide and to hypoxia in four goats awake and at 0.5%, 1.0% and 1.25% end-tidal halothane concentration. While maintaining PE'CO2 constant at each of three values (means 5.86, 6.45 and 7.2 kPa), PE'O2 was reduced rapidly from more than 25 kPa to 5.3-6 kPa for 3 min to record the increase in ventilation. Eleven sets of these 24 steady state points were obtained (2 PO2 x 3 PCO2 x 4 anaes. = 24). The mean isocapnic hypoxic ventilatory response (HVR) was 6.52 (SD 2.58) litre min-1 (n = 33) when awake, 5.62 (3.48) litre min-1 at 0.5% end-tidal halothane (ns), 3.05 (2.02) litre min-1 at 1% and 2.91 (2.12) litre min-1 at 1.25%, the last two being reduced significantly from awake and 0.5% halothane (P less than 0.05). With 1.25% halothane, HVR was reduced to 44.5 (18.6)% of the awake HVR. However, when HVR was expressed as % increase in ventilation produced by isocapnic hypoxia, it was 71 (19)% awake but 124 (65)% with 1.25% halothane, a significant increase with halothane (P less than 0.05). With 1.25% halothane, the carbon dioxide response slope decreased to 36.4 (26.4)% of control; hypoxia did not increase the slope significantly. Whereas previous studies in man have shown that halothane preferentially depresses hypoxic chemosensitivity and has a significant effect at 0.1 MAC, in the goat the hypoxic and carbon dioxide chemosensitivities were depressed equally. At 0.5% end-tidal concentration (about 0.5 MAC), halothane did not significantly depress hypoxic response.

Pulse oximeter failure thresholds in hypotension and vasoconstriction

The degree of systolic hypotension causing failure and recovery were tested simultaneously with three oximeters (CSI 504US, Nellcor N-200, and Ohmeda 3740) in nine normal male volunteers. Perfusion of the right hand was slowly reduced and restored by 1) elevation of the hand plus systemic hypotension with nitroprusside if needed (EL); 2) clamp compression of the brachial artery (CL); 3) brachial cuff inflation (CU); and 4) intraarterial norepinephrine (NE). With EL, pulse pressure was normal whereas right radial arterial systolic pressure (SP) was 25.3 +/- 12.4 mmHg at failure and 34.1 +/- 13.3 at recovery (mean of three oximeters, n = 189). With CL, pulse pressure fell more than did mean pressure, and failure occurred at 37.3 +/- 9.8 and recovery at 46.8 +/- 17.6 mmHg, n = 84. With CL, threshold of function, defined as the average of failure SP and recovery SP, was 47.1 +/- 13.5, n = 41 for Nellcor, higher than for either CSI (38.7 +/- 14.5, n = 17) or Ohmeda (36.0 +/- 3.4, n = 26) (P less than 0.05). With EL, no difference among instruments was found (mean 29.7 +/- 12.8, n = 189). Threshold was 58.2 +/- 8.4, n = 17 with CU if cuff inflation was slow (filling veins), but recovery was similar to EL after rapid cuff occlusion. With NE, SP threshold was increased to 58.3 +/- 21.0 with CL but only to 41.0 +/- 13.8 with EL.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcutaneous PCO2 and PO2: a multicenter study of accuracy

A multicenter study used 756 samples from 251 patients in 12 institutions to compare arterial (PaO2, PaCO2) with transcutaneous (PsO2, PsCO2) oxygen and carbon dioxide tensions, measured usually at 44 degrees C. Of these samples, 336 were obtained from 116 neonates, 27 from 25 children with cystic fibrosis, and 140 from 40 patients under general anesthesia. Ninety-one patients were between 4 weeks and 18 years of age, 32 were between 18 and 60 years, and 12 were over 60. The ratio of transcutaneous to arterial P(s/a)CO2 was 1.01 +/- 0.11 with PaCO2 less than 30 mm Hg, increasing to 1.04 +/- 0.08 at PaCO2 greater than 40 mm Hg. Mean bias and its standard deviation (PsCO2 - PaCO2) were + 1.3 +/- 3.9 mm Hg in the entire group, + 1.8 +/- 4.2 mm Hg in neonates (NS). Bias was + 0.2 +/- 2.7 mm Hg when PaCO2 was less than 30 mm Hg (N = 175, NS), 1.0 +/- 3.4 with 30 less than PaCO2 less than 40 (n = 329, p less than 0.001), and + 2.04 +/- 4.00 mm Hg with 40 less than PaCO2 less than 70 (n = 229, p less than 0.001). These data suggest that, using transcutaneous PCO2 monitors with inbuilt temperature correction of 4.5%/degrees C, the skin metabolic offset should be set to 6 mm Hg. The linear regression was PsCO2 = 1.052(PaCO2) - 0.56, Sy.x = 3.92, R = 0.929 (n = 756); and PsCO2 = 1.09(PaCO2) - 1.57, Sy.x = 4.17, R = 0.928 in neonates (n = 336).(ABSTRACT TRUNCATED AT 250 WORDS)