Controlled versus assisted mechanical ventilation effects on respiratory motor output in sleeping humans

Central apneas occur after cessation of mechanical ventilation despite normocapnic conditions. We asked whether this was due to ventilator-induced increases in respiratory rate or VT. Accordingly, we compared the effects of increased VT (135 to 220% of eupneic VT) with and without increased respiratory rate, using controlled and assist control mechanical ventilation, respectively, upon transdiaphragmatic pressure in sleeping humans. Increasing ventilator frequency +1 per minute and VT to 165-200% of baseline eupnea eliminated transdiaphragmatic pressure during controlled mechanical ventilation and prolonged expiratory time (two to four times control) after mechanical ventilation. During and after assist control mechanical ventilation at 135-220% of eupneic VT, transdiaphragmatic pressure was reduced in proportion to the increase in ventilator volume. However, every ventilator cycle was triggered by an active inspiration, and immediately after mechanical ventilation, expiratory time during spontaneous breathing was prolonged less than 20% of that observed after controlled mechanical ventilation at similar VT. We conclude that both increased frequency and VT during mechanical ventilation significantly inhibited respiratory motor output via nonchemical mechanisms. Controlled mechanical ventilation at increased frequency plus moderate elevations in VT reset respiratory rhythm and inhibited respiratory motor output to a much greater extent than did increased VT alone.

On the function of groaning and hyperventilation during sexual intercourse: intensification of sexual experience by altering brain metabolism through hypocapnia

Sexual arousal is accompanied by some typical physiological reaction patterns. Another typical feature of sexual intercourse is involuntary sound production implying in its more intense forms acceleration of breathing (hyperventilation). Up to now no study examined spCO2 during intense sexual intercourse, but there is evidence that some degree of hyperventilation with its physiological consequences may often be induced during sexual intercourse. This article discusses implications of hyperventilation during sexual intercourse for alterations of consciousness and subjective experience in the light of recent studies of brain metabolic changes during states of hyperventilation. Groaning and hyperventilation are interpreted in this context as a psychophysiological mechanism to deepen states of sexual trance.

Hypoxic respiratory response during acute stable hypocapnia

The hypoxic ventilatory response during hypocapnia has been studied with divergent results. We used volume-cycled ventilation in spontaneously breathing normal subjects to study their hypoxic ventilatory response under conditions of stable hypocapnia. Subjects were studied at three different levels of end-tidal (partial) carbon dioxide pressure (PETCO2), eucapnia and 6 and 12 mm Hg below eucapnia (mild and moderate hypocapnia, respectively). The response to hypoxia was assessed by changes in muscle pressure output (Pmus) and respiratory rate. Compared with the Pmus response at eucapnia (0.53 +/- 0.59 cm H2O/percentage oxygen saturation [% O2sat]), the response at mild hypocapnia was attenuated (0.26 +/- 0.33 cm H2O/% O2sat), whereas the response at moderate hypocapnia was negligible (0.003 +/- 0.09 cm H2O/% O2sat). Similar reductions were seen with the respiratory rate (eucapnia, 0.17 +/- 0.2 breaths/minute/% O2sat; mild hypocapnia, 0.11 +/- 0.11 breaths/minute/% O2sat; moderate hypocapnia, 0.01 +/- 0.06 breaths/minute/% O2sat). The Pmus and respiratory rate responses at the three levels of PETCO2 were significantly different (p < 0.05, analysis of variance). The responses at moderate hypocapnia were not significantly different from zero. We conclude that when apnea occurs under conditions in which central PCO2 is well below the CO2 setpoint, subjects are at risk of developing dangerous hypoxemia due to absence of a hypoxic ventilatory response.

Cerebral hemodynamics during early neonatal period in preterm infants with periventricular leukomalacia

We prospectively investigated the relation among cerebral blood flow, periventricular leukomalacia (PVL) and hypocarbia using Doppler ultrasonography in 53 preterm infants with gestational age between 27 and 34 weeks who required mechanical ventilation during the first 72 h of life. Cerebral blood flow of pericallosal artery was assessed by Doppler ultrasonography at the first and the third day of life. Mean velocity (MV) and Resistance index (RI) of anterior cerebral artery were calculated from the data obtained by Doppler ultrasonography. The diagnosis of PVL was made in 12 infants on the basis of the results of ultrasonography and MRI. Hypocarbia was judged as positive when both arterial blood gas analyses before and after the ultrasonography revealed PaCO(2) values < 25 mmHg. On the first day of life, RI was 0.62 +/- 0.022 in infants with PVL and 0.71 +/- 0.014 in those without PVL. On the third day of life, RI was 0.60 +/- 0.032 in infants with PVL and 0.66 +/- 0.013 in those without PVL. There was a significant difference in RI between the two groups at either point. MV was not significantly different between the two groups at either point. There was no significant difference in RI or MV between infants with and without hypocarbia at either point. RI was significantly lower in infants with PVL during the first 72 h of life, which is suggestive of vasoparalysis in such infants at the level of major cerebral arteries. However, RI or MV was no different between infants with and without hypocarbia.

Caudoputamen is damaged by hypocapnia during mechanical ventilation in a rat model of chronic cerebral hypoperfusion

BACKGROUND AND PURPOSE

Postoperative brain dysfunction, such as delirium, is a common complication of anesthesia and is sometimes prolonged, especially in patients with cerebrovascular disease. In the present study we investigated the effect of hypocapnia during anesthesia on neuronal damage using a rat model of chronic cerebral hypoperfusion.

METHODS

Chronic cerebral hypoperfusion was induced by clipping the bilateral common carotid arteries in male Wistar rats. Fourteen days after the operation, these animals were mechanically ventilated for 2 hours and then kept in suitable conditions for an additional 14 days. Twenty-four rats were assigned to 4 groups: those with chronic cerebral hypoperfusion with either hypocapnia or normocapnia during anesthesia, and those given sham operation with either hypocapnia or normocapnia. White matter lesions in the brain sections were evaluated with Klüver-Barrera staining. Proliferation of glial cells was estimated with the use of immunohistochemistry of glial fibrillary acidic protein, a marker for astroglia, and CD11b, a marker for microglia. Computer-assisted morphometry was applied to the immunohistochemical results of microtubule-associated protein 2 to evaluate the loss of neurons.

RESULTS

The histological damage was localized almost exclusively in the white matter in the rats subjected to chronic cerebral hypoperfusion but without hypocapnia. Neuronal damage and astroglial proliferation occurred with aggravated white matter lesions in the caudoputamen in the rats with chronic cerebral hypoperfusion and hypocapnia. No lesions were observed in sham-operated rats with either hypocapnia or normocapnia.

CONCLUSIONS

These results indicate that hypocapnia during anesthesia causes tissue damage in the caudoputamen, which may be responsible for long-lasting postoperative delirium in patients with stroke and/or dementia.

Effect of moderate hypocapnic ventilation on nuclear DNA fragmentation and energy metabolism in the cerebral cortex of newborn piglets

Previous studies have shown that severe hypocapnic ventilation [arterial carbon dioxide partial pressure (PaCO(2)) 7-10 mm Hg] in newborn animals results in decreased cerebral blood flow and decreased tissue oxidative metabolism. The present study tests the hypothesis that moderate hypocapnic ventilation (PaCO(2) 20 mm Hg) will result in decreased cerebral oxidative metabolism and nuclear DNA fragmentation in the cerebral cortex of normoxemic newborn piglets. Studies were performed in 10 anesthetized newborn piglets. The animals were ventilated for 1 h to achieve a PaCO(2) of 20 mm Hg in the hypocapnic (H) group (n = 5) and a PaCO(2) of 40 mm Hg in the normocapnic, control (C) group (n = 5). Tissue oxidative metabolism, reflecting tissue oxygenation, was documented biochemically by measuring tissue ATP and phosphocreatine (PCr) levels. Cerebral cortical nuclei were purified, nuclear DNA was isolated, and DNA content was determined. DNA samples were separated, stained, and compared with a standard DNA ladder. Tissue PCr levels were significantly lower in the H group than the C group (2.32 +/- 0.66 versus 3.73 +/- 0.32 micromol/g brain, p < 0.05), but ATP levels were preserved. Unlike C samples, H samples displayed a smear pattern of small molecular weight fragments between 100 and 12,000 bp. The density of DNA fragments was eight times higher in the H group than the C group, and DNA fragmentation varied inversely with levels of PCr (r = 0.93). These data demonstrate that moderate hypocapnia of 1 h duration results in decreased oxidative metabolism that is associated with DNA fragmentation in the cerebral cortex of newborn piglets. We speculate that hypocapnia-induced hypoxia results in increased intranuclear Ca(2+) flux, which causes protease and endonuclease activation, DNA fragmentation, and periventricular leukomalacia in newborn infants.

Cerebral blood volume (CBV) in humans during normo- and hypocapnia: influence of nitrous oxide (N(2)O)

BACKGROUND

It is generally argued that variations in cerebral blood flow create concomitant changes in the cerebral blood volume (CBV). Because nitrous oxide (N(2)O) inhalation both increases cerebral blood flow and may increase intracranial pressure, it is reasonable to assume that N(2)O acts as a general vasodilatator in cerebral vessels both on the arterial and on the venous side. The aim of the current study was to evaluate the effect of N(2)O on three-dimensional regional and global CBV in humans during normocapnia and hypocapnia.

METHODS

Nine volunteers were studied under each of four conditions: normocapnia, hypocapnia, normocapnia + 40-50% N(2)O, and hypocapnia + 40-50% N(2)O. CBV was measured after (99m)Tc-labeling of blood with radioactive quantitative registration via single photon emission computer-aided tomography scanning.

RESULTS

Global CBV during normocapnia and inhalation of 50% O(2) was 4.25 +/- 0.57% of the brain volume (4.17 +/- 0.56 ml/100 g, mean +/- SD) with no change during inhalation of 40-50% N(2)O in O(2). Decreasing carbon dioxide (CO(2)) by 1.5 kPa (11 mmHg) without N(2)O inhalation and by 1.4 kPa (11 mmHg) with N(2)O inhalation reduced CBV significantly (F = 57, P < 0.0001), by 0.27 +/- 0.10% of the brain volume per kilopascal (0.26 +/- 0.10 ml x 100 g(-1) x kPa(-1)) without N(2)O inhalation and by 0.35 +/- 0.22% of the brain volume per kilopascal (0.34 +/- 0.22 ml x 100 g(-1) x kPa(-1)) during N(2)O inhalation (no significant difference). The amount of carbon dioxide significantly altered the regional distribution of CBV (F = 47, P < 0.0001), corresponding to a regional difference in Delta CBV when CO(2) is changed. N(2)O inhalation did not significantly change the distribution of regional CBV (F = 2.4, P = 0.051) or Delta CBV/Delta CO(2) in these nine subjects.

CONCLUSIONS

Nitrous oxide inhalation had no effect either on CBV or on the normal CBV-CO(2) response in humans.

Brain compared to heart tissue oxygen pressure during changes in arterial carbon dioxide in the dog

Myocardial tissue oxygen pressure (PmO2 ) and left anterior descending (LAD) artery blood flow were measured in dogs anesthetized with 1.5% isoflurane, and were then compared to brain tissue oxygen pressure (PbO2 ) and middle cerebral artery (MCA) blood flow during normocapnia, hypocapnia, and hypercapnia. A craniotomy was performed and a tissue probe (Codman, Inc.) that measures PO2, PCO2, and pH was inserted into the brain cortex in the MCA region (n = 8). Separately, after a thoracotomy, a probe was inserted into the middle myocardium of the left ventricle, within the distribution of the LAD, in eight dogs. Blood flow probes were placed on the LAD or MCA. Blood flow and tissue gases were measured during normocapnia (PaCO2 = 38 mm Hg), hypocapnia (PaCO2 = 26 mm Hg), and hypercapnia (PaCO2 = 53 mm Hg). Mean arterial pressure, heart rate, arterial gases, and pH were not different between brain and heart measurements. PbO2 was 21 +/- 9 mm Hg (mean +/- SD ), 40 +/- 16 mm Hg, and 47 +/- 11 mm Hg. PmO2 was 35 +/- 12 mm Hg, 40 +/- 14 mm Hg, and 48 +/- 15 mm Hg during hypocapnia, normocapnia, and hypercapnia respectively. During hypercapnia, LAD and MCA flow increased 50% and tissue oxygenation increased 20% ( P < .05). During hypocapnia, MCA flow and PbO2 decreased 50% ( P < .05), but LAD flow and PmO2 did not significantly change. These results indicated that LAD flow and myocardial PO2 were less responsive to hypocapnia than MCA flow and PbO2.

Effects of hypercapnia and hypocapnia on [Ca2+]i mobilization in human pulmonary artery endothelial cells

The hydrogen ion is an important factor in the alteration of vascular tone in pulmonary circulation. Endothelial cells modulate vascular tone by producing vasoactive substances such as prostacyclin (PGI2) through a process depending on intracellular Ca2+ concentration ([Ca2+]i). We studied the influence of CO2-related pH changes on [Ca2+]i and PGI2 production in human pulmonary artery endothelial cells (HPAECs). Hypercapnic acidosis appreciably increased [Ca2+]i from 112 +/- 24 to 157 +/- 38 nmol/l. Intracellular acidification at a normal extracellular pH increased [Ca2+]i comparable to that observed during hypercapnic acidosis. The hypercapnia-induced increase in [Ca2+]i was unchanged by the removal of Ca2+ from the extracellular medium or by the depletion of thapsigargin-sensitive intracellular Ca2+ stores. Hypercapnic acidosis may thus release Ca2+ from pH-sensitive but thapsigargin-insensitive intracellular Ca2+ stores. Hypocapnic alkalosis caused a fivefold increase in [Ca2+]i compared with hypercapnic acidosis. Intracellular alkalinization at a normal extracellular pH did not affect [Ca2+]i. The hypocapnia-evoked increase in [Ca2+]i was decreased from 242 +/- 56 to 50 +/- 32 nmol/l by the removal of extracellular Ca2+. The main mechanism affecting the hypocapnia-dependent [Ca2+]i increase was thought to be the augmented influx of extracellular Ca2+ mediated by extracellular alkalosis. Hypercapnic acidosis caused little change in PGI2 production, but hypocapnic alkalosis increased it markedly. In conclusion, both hypercapnic acidosis and hypocapnic alkalosis increase [Ca2+]i in HPAECs, but the mechanisms and pathophysiological significance of these increases may differ qualitatively.

Alteration of hemoglobin oxygenation in the frontal region in elderly depressed patients as measured by near-infrared spectroscopy

In an attempt to elucidate the neurobiological basis of hypofrontality in depression, alterations of oxyhemoglobin and deoxyhemoglobin were examined by using near-infrared spectroscopy of the left frontal region in 9 elderly patients with depressive disorders and 10 control subjects. Verbal repetition task, verbal fluency test, hyperventilation, and paper-bag breathing were performed. During the verbal fluency test, oxyhemoglobin significantly increased and deoxyhemoglobin significantly decreased in control subjects, whereas no significant change was observed in patients. During hyperventilation, oxyhemoglobin significantly decreased and deoxyhemoglobin significantly increased in both groups. These findings suggest that functional hypofrontality in elderly depression is not due to altered vasodilator response.

Reduced cerebrovascular CO(2) reactivity in CADASIL: A transcranial Doppler sonography study

BACKGROUND AND PURPOSE

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukencephalopathy (CADASIL) is a hereditary angiopathy caused by mutations in Notch3. Cerebral microvessels show an accumulation of granular osmiophilic material in the vicinity of degenerating vascular smooth muscle cells. To study cerebrovascular function in CADASIL, we performed measurements on cerebral hemodynamics by using transcranial Doppler sonography.

METHODS

Middle cerebral artery (MCA) mean blood flow velocity (MFV), cerebrovascular CO(2) reactivity, and the resistance index were measured by bilateral transcranial Doppler sonography in 29 CADASIL individuals (mean age, 49.0+/-2.4 years) and an equal number of age- and sex-matched control subjects.

RESULTS

Compared with control subjects, CO(2) reactivity was reduced in CADASIL (33.4+/-2.7% versus 45.3+/-3.0%; P:<0.01). This difference remained significant when only nondisabled CADASIL individuals (Rankin=0, n=21) were included in the analysis (P:<0.05). CO(2) reactivity was significantly lower in disabled than in nondisabled CADASIL individuals (24.5+/-2.7% versus 36.8+/-3.4%; P:<0.05). MCA MFV was reduced in CADASIL (45.6+/-2.2 cm/s versus 54.2+/-2.4 cm/s; P:<0.05) and correlated negatively with age both in affected individuals (r=-0.314; P:<0.05) and control subjects (r=-0.339; P:<0.05). Resistance index was not significantly altered (59.0+/-1.0% versus 57.7+/-1.2%; P:=0.42).

CONCLUSIONS

In CADASIL, there is a reduction of both CO(2) reactivity and basal MCA MFV. The reduced CO(2) reactivity suggests functional impairment of cerebral vasoreactivity probably related to vascular smooth muscle cell dysfunction. The reduction of CO(2) reactivity in nondisabled CADASIL individuals suggests an early role of impaired cerebral vasoreactivity in the evolution of the disease.

Hypocapnic constriction in rabbit basilar artery in vitro: triggering by serotonin and dependence on endothelin-1 and alkalosis

This study tested whether hypocapnic constriction of the rabbit basilar artery in vitro can be triggered by serotonin, and whether the resulting constriction is (1) due to the alkaline pH associated with hypocapnia, and (2) endothelin-1 mediated. Hypocapnic alkaline solution (25 mM NaHCO(3); pH 7.76; pCO(2) 14.2) or isocapnic alkaline solution (50 mM NaHCO(3); pH 7.73; pCO(2) 35.0) rarely altered basal tension. Serotonin (3 microM) challenge in hypocapnic or isocapnic alkaline solution resulted in near maximal tension. Washout of the serotonin did not decrease tension in 54% of the tissues, as plateau tension was maintained for 2-2.5 h. The plateau tension of washed tissues was relaxed by 1-3 microM PD145065 (Ac-D-Bhg-L-Leu-Asp-L-Ile-L-Ile-L-Trp), BQ610 (homopiperidinyl-CO-Leu-D-Trp(CHO)-D-Trp), and BQ788 (N-cis-2, 6-dimethyl-piperidinocarbonyl-L-gamma-MeLeu-D-Trp (COOCH(3))-Nle), endothelin ET(A)/ET(B), endothelin ET(A), and endothelin ET(B) receptor antagonists, respectively. In contrast, serotonin-induced tension in normal solution (25 mM NaHCO(3); pH 7.42; pCO(2) 36.9) was maintained for only 40 min (mean). These results demonstrate that (1) constriction due to hypocapnia in vitro can be triggered by serotonin and is endothelin-1 mediated and (2) alkaline pH in the absence of decreased pCO(2) is sufficient to elicit the constriction triggered by serotonin.

Concentrations of lidocaine and monoethylglycine xylidide in brain, cerebrospinal fluid, and plasma during lidocaine-induced epileptiform electroencephalogram activity in rabbits: the effects of epinephrine and hypocapnia

When injecting lidocaine into tissues, the mean toxic dose of lidocaine may be increased by adding epinephrine to lidocaine and by decreasing the PaCO(2). In contrast, when lidocaine is introduced directly into an artery or vein, adding epinephrine to lidocaine may decrease the mean toxic dose of lidocaine. Less is known about the effects of decreased PaCO(2) on intravascular lidocaine toxicity. We infused lidocaine in 24 rabbits at 4 mg. kg(-1). min(-1) with/without epinephrine and with/without hypocapnia. We measured the time to onset of lidocaine-induced seizures, total dose of lidocaine at the time of seizures, and concentrations of lidocaine and monoethylglycine xylidide (MEGX), a metabolite of lidocaine, in plasma, brain, and cerebrospinal fluid. Epinephrine decreased onset time by 11% with hypocapnia and by 21% with normocapnia, and it increased plasma MEGX by 1 microg/mL with hypocapnia and 2 microg/mL with normocapnia. Hypocapnia increased onset time by 18% without epinephrine and by 33% with epinephrine, and it increased whole-brain MEGX by 10 microg/mL without epinephrine and by 14 microg/mL with epinephrine. We conclude that, when lidocaine is given intravascularly, hypocapnia increases onset time and lidocaine dose required for seizures. These effects occur with no change in the concentration of lidocaine in plasma or the brain.

IMPLICATIONS

Hypocapnia increases the toxic dose of lidocaine given IV without altering lidocaine concentrations in blood, brain, or cerebrospinal fluid. Whole-brain monoethylglycine xylidide concentration is greater during hypocapnia than during normocapnia, and the addition of epinephrine to lidocaine increases the concentration of monoethylglycine xylidide in plasma.

Effect of hyperventilation on brain tissue oxygenation and cerebrovenous PO2 in rats

Previous studies have shown that cortical tissue oxygenation is impaired during hyperventilation. However, it is important to quantify the effect of hyperventilation on brain tissue PO(2) and cerebrovenous PO(2) simultaneously especially since cerebral venous oxygenation is often used to assess brain tissue oxygenation. The present study was designed to measure the sagittal sinus PO(2) (PvO(2)), brain tissue PO(2) in the thalamus (PtO(2)), and brain temperature (Bt) simultaneously during acute hyperventilation. Isoflurane-anesthetized rats were hyperventilated for 10 min during which time the arterial carbon dioxide tension (PaCO(2)) dropped from 40.3+4.9 mmHg to 23.5+2.8 mmHg. PtO(2) declined from 26.0+/-4.2 mmHg to 14.8+/-5.2 mmHg (P=0.004) while brain temperature decreased from 36.5+0.3 degrees C to 36.2+0.3 degrees C (P=0.02). However, PvO(2) and arterial blood pressure (BP) did not change during hyperventilation. The maintenance of PvO(2) when perfusion is thought to decline and PtO(2) decreases suggests that there may be a diffusion limitation, possibly due to selective perfusion. Therefore, cerebrovenous PO(2) may not give a good assessment of brain tissue oxygenation especially in conditions of acute hyperventilation, and deeper brain regions other than the cortex also show impaired tissue oxygenation following hyperventilation.

Effect of acute moderate changes in PaCO2 on global hemodynamics and gastric perfusion

OBJECTIVE

To describe global hemodynamics and splanchnic perfusion changes in response to acute modifications in Paco2 in hemodynamically stable patients.

DESIGN

Prospective, randomized crossover study.

SETTING

Medical-surgical intensive care unit at a community hospital (400,000 inhabitants).

PATIENTS

Ten critically ill patients who were sedated, paralyzed, and mechanically ventilated.

INTERVENTIONS

Hypercapnia and hypocapnia were obtained by increasing and reducing instrumental deadspace in random order. After each intervention, patients returned to the basal condition. Each period lasted 80 min: 20 min to achieve stable Paco2 and 60 min for tonometer equilibration. In each period, global hemodynamic variables and tonometric data were collected. The periods were compared using analysis of variance.

MEASUREMENTS AND MAIN RESULTS

Acute hypercapnia (Paco2 from 40+/-3 to 52+/-3 torr, p<.05) increased cardiac index (3.43+/-0.37 vs. 3.97+/-0.43 mL/min/m2, p<.05), heart rate (95+/-6 vs. 105+/-3 beats/min, p<.05), and mean pulmonary artery pressure (21+/-1 vs. 24+/-1 mm Hg, p<.05) and reduced systemic vascular resistance (992+/-98 vs. 813+/-93 dyne x sec/ cm5, p<.05) and oxygen extraction ratio (27+/-3% vs. 22+/-2%, p<.05). Standardized intramucosal Pco2 increased from 49+/-2 to 61+/-3 torr (p<.05) with an associated decrease in calculated intramucosal pH ([pHi] 7.35+/-0.03 vs. 7.25+/-0.02, p<.05), but the gastro-arterial Pco2 gradient (deltaPco2) did not change. Acute hypocapnia (Paco2 from 41+/-3 to 34+/-3 torr, p<.05; pH 7.41+/-0.01 to 7.47+/-0.02, p<.05) induced slight increments in systemic vascular resistance (995+/-117 vs. 1088 +/- 160 dyne x sec/cm5, p<.05) and oxygen extraction ratio (28+/-2% vs. 30+/-2%, p<.05). Standardized intramucosal Pco2 decreased (50+/-4 vs. 44+/-3 torr, p<.05), pHi increased (7.33+/-0.03 vs. 7.36+/-0.02; p<.05), but deltaPco2 did not change.

CONCLUSIONS

In this small group of stable patients, moderate acute variations in Paco2 had a significant effect on global hemodynamics, but splanchnic perfusion, assessed by deltaPco2, did not change. In these conditions, the use of pHi to evaluate gastric perfusion appears unreliable.

Gut mucosal-arterial Pco2 gradient as an indicator of splanchnic perfusion during systemic hypo- and hypercapnia

OBJECTIVES

By accounting for influences of systemic acid-base disturbances, gut mucosal-arterial Pco2 gradient (Pico2 - Paco2) has been increasingly advocated as a more specific marker of splanchnic perfusion than Pico2 alone. We examined the stability of the Pico2 - Paco2 gradient compared with raw Pico2 measurements during induced systemic hypo- and hypercapnia.

DESIGN

A prospective animal study.

SETTINGS

A university research laboratory.

SUBJECTS

Twenty anesthetized, paralyzed, and mechanically ventilated mongrel dogs.

INTERVENTIONS

After a baseline period during which Paco2 was maintained near 40 torr, the animals were divided into four groups. Minute ventilation was then altered by adjusting tidal volume, frequency, or both to achieve group Paco2 values of 15, 20, 60, and 80 torr for groups 1 through 4, respectively. Portal blood flow was monitored and maintained near baseline levels by infusion of intravenous fluids. Intestinal Pico2 was measured continuously by using capnometric recirculating gas tonometry.

MEASUREMENTS AND MAIN RESULTS

Mean (+/- SE) aggregate baseline Pico2 - Paco2 was 16.9+/-3.3 torr. After 60 mins of hypoventilation, Pico2 - Paco2 decreased to 14.2+/-1.1 and to 13.7+/-2.7 torr in groups 3 and 4, respectively (p = NS, compared with baseline for both). On the other hand, after 60 mins of hyperventilation, Pico2 - Paco2 increased to 37.9+/-3.6 and 28.0+/-6.3 torr in groups 1 and 2, respectively (p < .0001, compared with baseline for both).

CONCLUSIONS

In this model of maintained portal blood flow, Pico2 - Paco2 remained essentially stable after hypoventilation but increased significantly after inducing hyperventilation. Our findings warrant cautious interpretation of Pico2 - Paco2 as an indicator of splanchnic perfusion during systemic hypocapnia.

Impeding O(2) unloading in muscle delays oxygen uptake response to exercise onset in humans

We tested whether the leftward shift of the oxygen dissociation curve of hemoglobin with hyperpnea delays the oxygen uptake (VO(2)) response to the onset of exercise. Six male subjects performed cycle ergometer exercise at a work rate corresponding to 80% of the ventilatory threshold (VT) VO(2) of each individual after 3 min of 20-W cycling under eupnea [control (Con) trial]. A hyperpnea procedure (minute ventilation = 60 l/min) was undertaken for 2 min before and during 80% VT exercise in hypocapnia (Hypo) and normocapnia (Normo) trials. In the Normo trial, the inspired CO(2) fraction was 3% to prevent hypocapnia. The subjects completed two repetitions of each trial. To determine the kinetic variables of VO(2) and heart rate (HR) at the onset of exercise, a nonlinear least-squares fitting was applied to the data averaged from two repetitions by a monoexponential model. The end-tidal CO(2) partial pressure before the onset of exercise was significantly lower in the Hypo trial than in the Con and Normo trials (22 +/- 1 vs. 38 +/- 3 and 36 +/- 1 mmHg, respectively, P < 0.05). The time constant of VO(2) and HR was significantly longer in the Normo trial (28 +/- 7 and 39 +/- 18 s, respectively) than in the Con trial (21 +/- 7, 34 +/- 16 s, respectively, P < 0.05). The VO(2) time constant of the Hypo trial (37 +/- 12 s) was significantly longer than that of the Normo trial, although no significant difference in the HR time constant was seen (Hypo, 41 +/- 28 s). These findings suggested that respiratory alkalosis delayed the kinetics of oxygen diffusion in active muscle as a result of the leftward shift of the oxygen dissociation curve of hemoglobin. This supports an important role for hemoglobin-O(2) offloading in setting the VO(2) kinetics at exercise onset.

Re-evaluation of the hypoxia theory as the mechanism of hyperventilation-induced EEG slowing

To determine whether the well-accepted hypoxia theory accounts for hyperventilation-induced electroencephalogram (EEG) slowing, the authors monitored changes in cerebral oxygenation and end-tidal concentrations of carbon dioxide in 67 patients with epilepsy (age range = 5-12 years) during the hyperventilation activation test in a routine EEG examination. Relative concentration changes in cerebral oxygenated, deoxygenated, total hemoglobin, and oxidized cytochrome oxidase were measured by near-infrared spectroscopy in the frontal region. In all patients, except one who demonstrated EEG slowing, total and oxygenated hemoglobin decreased, and cytochrome oxidase was not reduced. EEG slowing occurred intermittently in 22 patients and was not synchronous with changes in either the cerebral oxygenation or end-tidal concentration of carbon dioxide. The degree of EEG slowing was diminished or the slow waves disappeared abruptly within 1 second after the cessation of hyperventilation in 22 patients when both the cerebral oxygenation and end-tidal concentration of carbon dioxide were still at low levels. The findings during the recovery periods do not confirm the hypoxia theory. It is thus supposed that more subtle mechanisms are the cause of EEG slowing.

Splanchnic hemodynamics and gut mucosal-arterial PCO(2) gradient during systemic hypocapnia

The effects of hypocapnia [arterial PCO(2) (Pa(CO(2))) 15 Torr] on splanchnic hemodynamics and gut mucosal-arterial P(CO(2)) were studied in seven anesthetized ventilated dogs. Ileal mucosal and serosal blood flow were estimated by using laser Doppler flowmetry, mucosal PCO(2) was measured continuously by using capnometric recirculating gas tonometry, and serosal surface PO(2) was assessed by using a polarographic electrode. Hypocapnia was induced by removal of dead space and was maintained for 45 min, followed by 45 min of eucapnia. Mean Pa(CO(2)) at baseline was 38.1 +/- 1.1 (SE) Torr and decreased to 13.8 +/- 1.3 Torr after removal of dead space. Cardiac output and portal blood flow decreased significantly with hypocapnia. Similarly, mucosal and serosal blood flow decreased by 15 +/- 4 and by 34 +/- 7%, respectively. Also, an increase in the mucosal-arterial PCO(2) gradient of 10.7 Torr and a reduction in serosal PO(2) of 30 Torr were observed with hypocapnia (P < 0.01 for both). Hypocapnia caused ileal mucosal and serosal hypoperfusion, with redistribution of flow favoring the mucosa, accompanied by increased PCO(2) gradient and diminished serosal PO(2).

In vivo microdialysis of 2-deoxyglucose 6-phosphate into brain: a novel method for the measurement of interstitial pH using 31P-NMR

A unique method for simultaneously measuring interstitial (pHe) as well as intracellular (pHi) pH in the brains of lightly anesthetized rats is described. A 4-mm microdialysis probe was inserted acutely into the right frontal lobe in the center of the area sampled by a surface coil tuned for the collection of 31P-NMR spectra. 2-Deoxyglucose 6-phosphate (2-DG-6-P) was microdialyzed into the rat until a single NMR peak was detected in the phosphomonoester region of the 31P spectrum. pHe and pHi values were calculated from the chemical shift of 2-DG-6-P and inorganic phosphate, respectively, relative to the phosphocreatine peak. The average in vivo pHe was 7.24+/-0.01, whereas the average pHi was 7.05+/-0.01 (n = 7). The average pHe value and the average CSF bicarbonate value (23.5+/-0.1 mEq/L) were used to calculate an interstitial Pco2 of 55 mm Hg. Rats were then subjected to a 15-min period of either hypercapnia, by addition of CO2 (2.5, 5, or 10%) to the ventilator gases, or hypocapnia (PCO2 < 30 mm Hg), by increasing the ventilation rate and volume. pHe responded inversely to arterial Pco2 and was well described (r2 = 0.91) by the Henderson-Hasselbalch equation, assuming a pKa for the bicarbonate buffer system of 6.1 and a solubility coefficient for CO2 of 0.031. This confirms the view that the bicarbonate buffer system is dominant in the interstitial space. pHi responded inversely and linearly to arterial PCO2. The intracellular effect was muted as compared with pHe (slope = -0.0025, r2 = 0.60). pHe and pHi values were also monitored during the first 12 min of ischemia produced by cardiac arrest. pHe decreases more rapidly than pHi during the first 5 min of ischemia. After 12 min of ischemia, pHe and pHi values were not significantly different (6.44+/-0.02 and 6.44+/-0.03, respectively). The limitations, advantages, and future uses of the combined microdialysis/31P-NMR method for measurement of pHe and pHi are discussed.

Carbon dioxide dynamics during apneic oxygenation: the effects of preceding hypocapnia

STUDY OBJECTIVE

To compare the rate of increase of arterial carbon dioxide tension (PaCO2) during apneic oxygenation preceded by acute respiratory hypocapnia with that during apnea preceded by respiratory eucapnia.

DESIGN

Randomized, prospective, single crossover study.

SETTING

Operating room at a teaching hospital.

PATIENTS

19 ASA physical status I, II, and III patients requiring general endotracheal anesthesia and invasive monitoring for elective surgery.

INTERVENTIONS

Two ventilatory states preceding apneic oxygenation were studied in each patient. The first respiratory state was established using controlled mechanical ventilation in the stable, anesthetized patient, followed by a 5-minute period of apneic oxygenation. Arterial and mixed central venous blood gas samples were obtained simultaneously prior to and at 1-minute intervals during apnea. Ventilatory parameters were then changed to establish the second respiratory state. During the subsequent period of apnea, the study was repeated as above.

MEASUREMENTS AND MAIN RESULTS

Arterial and mixed central venous partial pressure of carbon dioxide (pCO2) levels were measured. A greater increase in PaCO2 was found during the first minute of apneic oxygenation in the hypocapneic group compared with the eucapneic group. The venoarterial gradient of pCO2 was also greater in the hypocapneic group prior to apnea.

CONCLUSIONS

Acute hypocapnia compared with eucapnia prior to apneic oxygenation is associated with a greater rise in PaCO2 in the anesthetized patient. This finding may be due in part to widening of the venoarterial gradient of pCO2.

Near-infrared monitoring of cerebral tissue oxygen saturation and blood volume in newborn piglets

Near-infrared spectrophotometry (NIRS) potentially provides a tool for noninvasive tissue oxygenation and blood volume monitoring. Cerebral monitoring could be useful in the prevention of hypoxic ischemic brain injury in newborns. This study sought to validate such NIRS measurements in normoventilated, hypocapnic, and hypoxemic states in the brain of newborn piglets vs. arterial (SaO2) and sagittal sinus blood hemoglobin saturation (SssO2) and blood volume measurements with 99mTc-labeled erythrocytes. NIRS measurements of cerebral blood volume (CBV) were performed with both oxyhemoglobin and indocyanine green as tracers, and changes in CBV were monitored by following the change in the concentration of total hemoglobin (i.e., oxyhemoglobin + deoxyhemoglobin). NIRS CBV measurements did not correlate well with the radioactive measurements. NIRS measurements of oxygenation, however, correlated well with a weighted mean value of SaO2 and SssO2 (r = 0.90; P < 0.0001). Multiple linear regression of the oxygenation index (i.e., oxyhemoglobin - deoxyhemoglobin) on SaO2 and SssO2 suggested that NIRS sees hemoglobin in tissue in a venous-to-arterial ratio of 2:1. Therefore, in this study, NIRS reliably monitored changes in cerebral tissue oxygenation but not in CBV.

Effect of carbon dioxide on cerebral metabolism during hypoxia-ischemia in the immature rat

We previously have demonstrated that hypocapnia aggravates and hypercapnia protects the immature rat from hypoxic-ischemic brain damage. To ascertain cerebral blood flow (CBF) and metabolic correlates, 7-d postnatal rats were subjected to hypoxia-ischemia during which they were rendered either hypo-(3.5 kPa), normo- (5.1 kPa), or hypercapnic (7.3 kPa) by the inhalation of either 0, 3, or 6% CO2, 8% O2, balance N2. CBF during hypoxia-ischemia was better preserved in the normo- and hypercapnic rat pups; these animals also exhibited a stimulation of cerebral glucose utilization. Brain glucose concentrations were higher and lactate lower in the normo- and hypercapnic animals, indicating that glucose was consumed oxidatively in these groups rather than by anaerobic glycolysis, as apparently occurred in the hypocapnic animals. ATP and phosphocreatine were better preserved in the normo- and hypercapnic rats compared with the hypocapnic animals. Cerebrospinal fluid glutamate, as a reflection of the brain extracellular fluid concentration, was lowest in the hypercapnic rats at 2 h of hypoxia-ischemia. The data indicate that during hypoxia-ischemia in the immature rat, CBF is better preserved during normo- and hypercapnia; the greater oxygen delivery promotes cerebral glucose utilization and oxidative metabolism for optimal maintenance of tissue high energy phosphate reserves. An inhibition of glutamate secretion into the synaptic cleft and its attenuation of N-methyl-D-aspartate receptor activation would further protect the hypercapnic animal from hypoxic-ischemic brain damage.

Activation of tyrosine hydroxylase in striatum of newborn piglets in response to hypocapnic ischemia and recovery

The present study describes the effect of hypocapnic ischemia caused by hyperventilation on striatal levels of dopamine, DOPAC, HVA and activity of tyrosine hydroxylase in striatal synaptosomes isolated from the brain of newborn piglets. Hyperventilation did not result in statistically significant changes in the striatal level of dopamine and its major metabolites; however, it was observed that after 20 min of recovery the levels of striatal tissue dopamine, DOPAC and HVA increase by 195%, 110% and 205%, respectively. The level of DOPA (3,4-dihydroxyphenylalanine), which was used as an index of tyrosine hydroxylase activity, also increased after recovery. The rate of dopamine synthesis was 32 pmoles/mg protein/10 min in control piglets and after recovery this increased to 132 pmoles/mg protein/10 min. Measurement of the tyrosine hydroxylase activity in Triton X-100 treated synaptosomes showed that, after 20 min of recovery, there was an increase in Vmax with no change in K(m) for pteridine cofactor, compared to control. This is consistent with the enzyme having been covalently modified (activated) during tissue ischemia caused by hyperventilation and remaining activated well into the recovery period. We postulate that ischemia can induce long lasting alterations in dopamine synthesis, which may play some role in mediation of hypoxic cell injury in immature brain.