Comparison of the Effect of Etomidate and Desflurane on Brain Tissue Gases and pH during Prolonged Middle Cerebral Artery Occlusion

Background: The authors compared the effects of etomidate and desflurane on brain tissue oxygen pressure (PO2), carbon dioxide pressure (PCO2), and pH in patients who had middle cerebral artery occlusion for > 15 min.

Methods: After a craniotomy, a probe that measures PO2, P (CO)2, and pH was inserted into cortical tissue at risk for ischemia during middle cerebral artery occlusion. A burst suppression pattern of the electroencephalogram was induced with etomidate (n = 6) or 9% end-tidal desflurane (n = 6) started before middle cerebral artery occlusion. Mean blood pressure was supported with phenylephrine to 90-95 mmHg.

Results: During baseline conditions, tissue PO2, PCO (2), and pH were similar between the two groups (PO2 = 15 mmHg, PCO2 = 60 mmHg, pH = 7.1). During administration of etomidate before middle cerebral artery occlusion, tissue PO2 decreased in five of six patients without a change in PCO2 or pH. During administration of 9% desflurane, tissue PO2 and pH increased before middle cerebral artery clipping. Middle cerebral artery occlusion for an average of 33 min with etomidate and 37 min with desflurane produced a decrease in pH with etomidate (7.09 to 6.63, P <0.05) but not with desflurane (7.12 to 7.15).     

Subcutaneous tissue oxygen and carbon dioxide tensions during hyperbaric oxygenation: an experimental study in rats.

OBJECTIVE: To investigate the response of subcutaneous tissue oxygen (O2) and carbon dioxide (CO2) tensions to hyperbaric oxygenation. DESIGN: Experimental study. SETTING: University hospital, Finland. SUBJECTS: 10 Wistar rats. INTERVENTION: Subcutaneous tissue PO2 and PCO2 were directly measured with an implanted Silastic tube tonometer and capillary sampling technique while breathing air and exposed to hyperbaric oxygen (HBO) at 2.5 or 2.8 ATA pressure. Hyperbaric exposures were carried out in a large multiplace chamber pressurised with air. MAIN OUTCOME MEASURES: Subcutaneous tissue PO2 and PCO2. RESULTS: The mean subcutaneous PO2 rose from the baseline of 8 kPa (60 mmHg) to 16 kPa (112 mmHg) when rats breathed room air during pressurisation to 2.8 atm. When the rats breathed oxygen at 2.5 ATA the maximal mean tissue PO2 was four times higher than the mean starting value. During the HBO treatment at 2.8 ATA the tissue PO2 rose to a value about five times above baseline. The tissue PCO2 values almost doubled during the exposure to HBO at 2.5 ATA, probably because elimination of carbon dioxide was impaired. CONCLUSION: Measurements of tissue PO2 and PCO2 with an implanted Silastic tonometer and a capillary sampling technique can successfully be adapted to hyperbaric conditions. The method yielded reproducible results and is applicable to clinical use in hyperbaric medicine.     

Intramyocardial gas tensions in the canine heart

Intramyocardial gas tensions were determined in the left ventricular wall of the canine heart by means of chronically implanted silastic tonometers. In the central zone of the myocardial wall the mean baseline PO2 was 22 mmHg and the mean baseline PCO2 28 mmHg. Breathing of pure oxygen elevated the myocardial PO2 to a level of 60 to 90 mmHg. Hypovolaemic shock induced by shedding 30% of the blood volume resulted in a sharp decrease of the myocardial PO2 and a marked increase of the myocardial PCO2. During hypovolaemia, ventilation with pure oxygen elevated the oxygen tension to the control level and, after re-infusion of the shed blood, the response of the myocardial PO2 to oxygen breathing became higher than normal. Correction of hypovolaemic with a plasma expander Haemaccel restored the myocardial oxygen tension to the baseline level. Ligation of the left anteior descending coronary artery produced a rapid fall of PO2 and a profound increase of PCO2 in the corresponding myocardial area. After coronary ligation, developing tissue ischaemia usually resulted in ventricular fibrillation within a few minutes. Cardiac massage was started immediately, but produced no correction of tissue gas tensions, although the arterial blood gases remained normal. Ligation of the right coronary artery or the left circumflex coronary artery for short periods increased the myocardial PO2 in the vicinity of the left anterior descending branch, probably due to reactive hyperaemia around the ischaemic tissue.     

Effect of vascular reconstructions on tissue gas tensions in calf muscles of patients with occlusive arterial disease

Oxygen and carbon dioxide tensions were measured pre- and postoperatively in the gastrocnemius muscles of 6 patients undergoing an aorto- femoral reconstruction due to occlusive arterial disease. Four volunteer patients showing no signs of arterial ischaemia formed the control group. Continuous recordings of tissue gas tensions were carried out by means of an implanted Silastic tonometer during rest and heel lifting exercise. The preoperative measurements were performed a few days to a few weeks prior to surgery and the postoperative determinations 3-6 months after operation. In preoperative measurements of patients with limb ischaemia calf muscle PO2 levels during rest were only slightly lower than in controls, while no differences were detected in the corresponding PCO2 values between the two groups. Arterial surgery elevated the levels of basal tissue PO2 measured at rest. During and immediately after a preoperative exercise test of patients with arterial occlusive disease the muscle PO2 decreased sharply and the PCO2 increased. When the heel lifting exercise test was repeated postoperatively in these patients the calf muscle PO2 and PCO2 underwent no essential changes from the pre-exercise levels. In control patients calf muscle gas tensions showed a totally different behaviour during and immediately after exercise: both PO2 and PCO2 increased moderately. During rest following the exercise test tissue gas tensions resumed their original levels within a few minutes in both patient groups. When the heel lifting exercise was repeated after 20-30 minutes, analogous changes of tissue gas tensions were observed. To summarize, the present data show that pre- and postoperative recordings of calf muscle PO2 and PCO2 combined with an exercise test provide an accurate means of evaluating the change of tissue perfusion and nurtrition after an arterial reconstruction.     

Tissue gas tensions in the calf muscles of patients with lower limb arterial ischaemia.

O2 and CO2 tensions were measured in the gastrocnemius muscles of patients submitted for reconstructive arterial surgery due to obstructive arteriosclerosis (37) or abdominal aortic aneurysm (5). Four patients without signs of arterial ischaemia served as controls. Measurements were carried out by means of implanted silastic tonometers during breathing of air and 100% O2 and immediately after walking on a treadmill. Peripheral blood pressures in the ankles were recorded with a Doppler apparatus. Baseline tissue gas tensions showed no essential differences between the various groups of patients: intermittent claudication, pain at rest, praegangrene, abdominal aortic aneurysm and controls. In contrast, baseline ankle pressures correlated well with the severity of the disease. During breathing of oxygen, the smallest increases of muscle PO2 were observed in extremities with pain at rest or praegangrene and the highest responses were recorded in controls and aneurysm patients. Muscle PCO2 values showed no alterations during oxygen breathing. In physical exercise, muscle PO2 and PCO2 levels as well as ankle blood pressures remained unchanged in controls and patients with aneurysm but no claudication. However, in all groups with arterial ischaemia, the exercise test resulted in a profound fall of muscle PO2 and ankle blood pressure and an increase of muscle PCO2.     

Effects of profound anemia on brain tissue oxygen tension, carbon dioxide tension, and pH in rabbits

This study sought to determine the maximum tolerable limit of anemia for the brain during halothane anesthesia. Using a multiparameter sensor, we continuously monitored brain tissue oxygen tension (PO2), carbon dioxide tension (PCO2), and pH during profound hemodilution and subsequent transfusion. Twelve New Zealand White rabbits were anesthetized, intubated, and mechanically ventilated at a fraction of inspired oxygen (FiO2) of 21% to produce an arterial carbon dioxide tension (PaCO2) of 35 to 40 mm Hg. The femoral artery was cannulated to continuously monitor arterial blood pressure and to intermittently measure arterial blood gases. The electroencephalogram (EEG) was recorded throughout the course of the study. A fiberoptic sensor was inserted into the brain for the continuous measurement of brain PO2, PCO2, pH, and temperature. Cerebral blood flow (CBF) was measured by the hydrogen clearance method. Severe anemia was induced by repeatedly withdrawing 50-mL aliquots of blood and infusing an equal volume of 6% hetastarch. This procedure was performed four times for each rabbit. After the forth blood draw and fluid infusion, a total of 60 mL of packed red blood cells were transfused. Upon completion of the hemodilution, the hemoglobin concentration was 2.4 +/- 0.3 g/dL (mean +/- SEM). Brain tissue PO2 decreased from 27 +/- 3 mm Hg to a minimum of 12 +/- 2 mm Hg. Brain tissue pH also decreased from 7.22 +/- 0.03 to 7.12 +/- 0.05 and returned to the baseline value with transfusion. Brain PCO2 did not change significantly during the experiment. Cerebral blood flow increased from 37 +/- 3 to 66 +/- 15 mL x 100 g(-1) x min(-1) during hemodilution and returned to baseline after infusion of red blood cells. There was some loss of EEG amplitude and the calculated cerebral metabolic rate (CMRO2) decreased from 4.3 +/- 0.6 to 1.9 +/- 0.3 mL x 100 g(-1) x min(-1) at the most profound level of anemia. This is the first report of which the authors are aware of continuous monitoring of brain tissue pH, PCO2, and PO2 during profound hemodilution and transfusion. Hemodilution results in a decrease in brain tissue PO2. Increases in CBF and oxygen extraction can only partially compensate for the decreased oxygen carrying capacity of the blood. Decreases in brain tissue PO2, pH, CMRO2, and a loss of EEG amplitude suggest that the maximum tolerable limit of hemodilution was achieved in this study.     

Increased FiO2 improves intrathecal oxygenation during thoracic aortic cross-clamping in pigs

OBJECTIVES: To investigate the effect of 100% oxygen ventilation on cerebrospinal fluid (CSF) oxygenation in 11 pigs during thoracic aortic cross-clamping. DESIGN: An aorto-aortic shunt was used for control of central hemodynamics and study of hypoperfusion by exsanguination. CSF PO2, PCO2 and pH were continuously monitored before and during clamping. The changes in hemodynamic parameters and intrathecal gas tensions in response to variations in proximal mean aortic pressure and fraction of inspired oxygen (FiO2) were recorded. RESULTS: Baseline CSF PO2 decreased from 4.8 +/- 1.9 to 2.6 +/- 2.2 kPa following aortic occlusion. Increasing FiO2 to 1.0 resulted in a significant increase in CSF PO2 to 4.1 +/- 3.0 with a return to 2.7 +/- 2.1 kPa after reducing FiO2 to 0.4 again. The same variations in FiO2 did not induce any significant changes in CSF PO2 during hypotension. CONCLUSION: Increased FiO2 during experimental thoracic aortic cross-clamping with stable proximal arterial pressure helps to maintain CSF PO2, whereas severe hypotension could not be compensated for by hyperoxemia.     

Effects of remifentanil and fentanyl on intraocular pressure during the maintenance and recovery of anaesthesia in patients undergoing non-ophthalmic surgery

BACKGROUND AND OBJECTIVE: To compare the effects of remifentanil and fentanyl on intraocular pressure during the maintenance and recovery of anaesthesia in patients undergoing elective non-ophthalmic surgery. METHODS: Thirty-two patients (ASA I-II) were randomized into two groups to receive either a continuous infusion of remifentanil (0.25-0.5 microg kg(-1) min(-1), n =16, Group R) or an intermittent bolus of fentanyl (2-5 microg kg(-1), n = 16, Group F) during the maintenance of anaesthesia. For the induction of anaesthesia, Group R received remifentanil 1 microg kg(-1) and Group F received fentanyl 2 microg kg(-1); both groups then received propofol 2 mg kg(-1) with vecuronium 0.1 mg kg(-1). Anaesthesia in both groups was maintained with a continuous infusion of propofol 4-8 mg kg(-1) h(-1). Ventilation of the lungs was controlled to a constant end-tidal PCO2 of 4.7-5.4 kPa. Blood pressure, electrocardiography, heart rate and oxygen saturation were monitored throughout anaesthesia. Intraocular pressure was determined before surgery, during the maintenance of anaesthesia, 2 min after emergence and in the recovery room using a Perkins hand-held applanation tonometer by an ophthalmologist blinded to the anaesthetic technique. RESULTS: After induction of anaesthesia, a significant decrease in intraocular pressure in the remifentanil group from 13.6 +/- 2.6 to 7.1 +/- 3.1 mmHg (P < 0.001) and in the fentanyl group from 13.7 +/- 2.2 to 9.7 +/- 3.4 mmHg (P < 0.001) was observed and maintained during anaesthesia. Thirty minutes after the end of anaesthesia, intraocular pressure returned to baseline values in both groups (remifentanil: 13.9 +/- 2.8 mmHg, P = 0.28; fentanyl: 13.6 +/- 2.3 mmHg, P = 0.59). The intraocular pressure and haemodynamic variables did not differ significantly between the two groups (intraocular pressure, P = 0.7327; blood pressure, P = 0.1295; heart rate, P = 0.8601). CONCLUSIONS: Remifentanil maintains intraocular pressure at an equally reduced level compared with fentanyl.     

Effects of Temperature on Cerebral Tissue Oxygen Tension, Carbon Dioxide Tension, and pH during Transient Global Ischemia in Rabbits

Background: A decrease in brain temperature (Tbrain) causes a decrease in the cerebral metabolic rate for oxygen (CMRO2) and provides potent neuroprotection against ischemic damage. In the present study, the effects of mild to moderate hypothermia on cerebral tissue oxygen tension (PO2 brain), carbon dioxide tension (PCO2 brain), and pH (pHbrain) were monitored during short episodes of global cerebral ischemia.

Methods: After approval by the Animal Care and Use Committee, 10 New Zealand white rabbits were anesthetized (1% halothane in air) and mechanical ventilation was adjusted to maintain the arterial carbon dioxide tension at 35 mmHg (alpha-stat). A sensor to measure PO2 brain, PCO2 brain, pHbrain, and Tbrain was inserted into the brain through a burr hole in the skull. Tbrain was adjusted to 38 [degree sign] Celsius, 34.4 [degree sign] Celsius, and 29.4 [degree sign] Celsius in a random sequence in each animal. PO2 brain, PCO sub 2 brain, and pHbrain (all variables are reported at the actual Tbrain) were recorded every 10 s during a 5-min baseline, 3 min of cerebral ischemia induced by inflation of a neck tourniquet, and 10 min of reperfusion at each level of Tbrain. Analysis of variance and Dunnett's test were used for statistical analysis. Data are presented as means +/- SD.

Results: During ischemia, PO2 brain decreased from 56 +/- 3 to 33 +/- 2 mmHg at 38 [degree sign] Celsius, from 58 +/- 3 to 32 +/- 3 mmHg at 34.4 [degree sign] Celsius, and from 51 +/- 2 to 32 +/- 2 mmHg at 29.4 [degree sign] C (p = NS). PCO2 brain increased by 6.7 +/- 2 mmHg at 38 [degree sign] Celsius, by 5.1 +/- 1.4 mmHg at 34.4 [degree sign] Celsius, and by 2.3 +/- 0.8 mmHg at 29.4 [degree sign] Celsius. pH sub brain inversely followed the trend of PCO2 brain.     

Origin and behaviour of emphysematous bullae.

Giant emphysematous bullae are believed to produce symptoms of pulmonary compression and collapse by containing gas under pressure that has been generated through valvular feeding airways. To examine this hypothesis, we have measured oxygen and carbon dioxide tensions (PO2, PCO2) in four patients and pressure within the bullae in three patients immediately before surgery. During spontaneous tidal respiration PO2 in the bulla was higher than arterial PO2 but did not rise as fast during the breathing of oxygen. The intra-bulla pressure during inspiration was negative (-5.5 to -19 cm H2O) and similar to pleural pressure in phase and degree. During intermittent positive pressure ventilation in two patients airway pressures were transmitted to the bulla with the development of a positive end expiratory pressure within the bulla. Histological examination of the walls of the bullae in the four patients and adjacent lung tissue in an additional patient failed to identify any valvular mechanism. The available information suggests that bullae develop after retraction and collapse of surrounding lung away from a region of weakness.     

Accuracy and clinical performance of a continuous intra-arterial blood- gas monitoring system during thoracoscopic surgery

Accuracy and performance of the only currently available intra-arterial blood-gas monitoring system (Paratrend 7, PT7) were assessed in 23 patients during thoracoscopic surgery using one-lung ventilation. Over a wide range of values for arterial PO2 (6.1-61.1 kPa), PCO2 (4.1-9.5 kPa) and pH (7.19-7.50), 138 arterial blood-gas values obtained by PT7 were compared with corresponding in vitro laboratory blood-gas measurements. We found good clinical performance with the PT7 and good agreement between PT7 values and in vitro measurements for arterial PO2 (bias (1.96 SD) = 0.38 (9.52) kPa), PCO2 (0.31 (0.76) kPa) and pH (- 0.017 (0.065)). Also, the bias for sequential changes between two, consecutive times was not significantly different from the ideal value of 0. We conclude that the PT7 is helpful in monitoring patients during thoracoscopy.      

Brain tissue PO(2), PCO(2), and pH during cerebral vasospasm

BACKGROUND: The purpose of the present study was to assess brain tissue monitoring for detection of ischemia due to vasospasm in aneurysmal subarachnoid hemorrhage (SAH) patients. METHODS: After obtaining informed consent, a burr hole was made in 10 patients and a Neurotrend 7 probe was inserted ipsilateral to the region of SAH. In eight patients the probe was inserted during surgery for clipping the aneurysm and in two patients the probe was inserted in the neurosurgery ICU. Brain tissue gases and pH were collected over 6-hour periods for 7 to 10 days until the termination of monitoring. The onset of vasospasm was confirmed by angiography and xenon computed tomography (Xe/CT) cerebral blood flow studies. RESULTS: Seven patients did not develop vasospasm during monitoring and were considered as controls. In this group, brain tissue oxygen pressure (PO(2)) remained above 20 mmHg, carbon dioxide pressure (PCO(2)) stabilized at 40 mmHg and pH remained between 7.1 and 7.2. In three patients who developed vasospasm during monitoring, PO(2) was not different from the control group. However, PCO(2) increased to 60 mmHg and pH decreased to 6.7 (p < 0.001). CONCLUSION: In this study, patients with SAH who developed vasospasm had significantly lower brain tissue pH and higher PCO(2) compared to controls. However, there was no significant change in PO(2) levels associated with vasospasm. Brain tissue monitoring can provide an indication of ischemia during vasospasm.     

Effect of intra-operative end-tidal carbon dioxide partial pressure on tissue oxygenation

Postsurgical infection risk is correlated with subcutaneous tissue oxygenation. Mild hypercapnia augments cutaneous perfusion. We tested the hypothesis that peripheral tissue oxygenation increases as a function of arterial PCO2 in surgical patients. Twenty patients were randomly assigned to intra-operative end tidal PCO2 of 3.99 (control) or 5.99 kPa (hypercapnia). All other anaesthetic management was per protocol. Tissue oxygen partial pressure, transcutaneous oxygen tension, cerebral oxygen saturation, and cardiac output were measured. Mean (SD) subcutaneous tissue oxygen tension was 8.39 (1.86) kPa in control and 11.84 (2.53) kPa hypercapnia patients (p = 0.014). Cerebral oxygen saturation was 55 (4)% for control vs. 68 (9)% for hypercapnia (p = 0.004). Neither cardiac index nor transcutaneous tissue oxygen tension differed significantly between the groups. Mild intra-operative hypercapnia increased subcutaneous and cerebral oxygenation. Increases in subcutaneous tissue oxygen partial pressure similar to those observed in patients assigned to hypercapnia are associated with substantial reductions in wound infection risk.     

Positive-pressure ventilation at moderately high frequency in newborn infants with respiratory distress syndrome (IRDS)

In 24 seriously ill newborn infants with respiratory distress syndrome (IRDS) and ensuing respiratory failure, high-frequency positive-pressure ventilation was administered. The mean gestational age of the infants was 32 +/- 3 weeks. In the infant ventilator employed, the compressible volume had been reduced in order to give higher flow rates but lower intratracheal pressures. The ventilation frequency was kept constant at 60-66 per min and the insufflation period at 32-35% of the ventilatory cycle. A positive end-expiratory pressure (PEEP) of 0.2-0.6 kPa was used. Arterial PCO2 was maintained at 4.0-5.3 kPa and PO2 at 8.5-10.5 kPa by adjusting the gas flow through the patient circuit, the peak tubing pressure, the PEEP and the oxygen concentration in inspired gas. High-frequency positive-pressure ventilation improved oxygenation and gave adequate alveolar ventilation in all infants, in most cases at a low peak pressure. Only one infant developed pneumothorax during intermittent positive pressure ventilation, and in no infant did bronchopulmonary dysplasia or retrolental fibroplasia occur. One infant died from intracranial hemorrhage during the ventilation period, giving a survival rate of 96%.     

Continuous conjunctival oxygen tension (Pcjo2) monitoring for assessment of cerebral oxygenation and metabolism during carotid artery surgery

The clinical value of noninvasive continuous monitoring of conjunctival oxygen tension for assessment of cerebral perfusion during carotid endarterectomy performed under general anaesthesia has been evaluated. The patients (n = 17; mean age 62.5 +/- 1.7 years) were monitored as follows: conjunctival oxygen tension (PcjO2); internal jugular venous oxygen tension at the skull base level (PcijvO2); arterial blood pressure; arterial and internal jugular venous blood gases; acid-base data and lactate, pyruvate levels; end-tidal CO2 concentration. The mean preanaesthetic PcjO2 level of 4.86 +/- 0.40 kPa was significantly lower than PaO2(PcjO2)/PaO2 ratio of 0.48). Following anaesthesia, a larger PcjO2-PaO2 gradient (ratio 0.32) was seen in spite of the hyperoxic situation (FiO2 = 0.40) due to vasoconstriction induced by slight hypocapnia (reduction of PaCO2 from 5.13 +/- 0.08 to 4.64 +/- 0.10 kPa). The carotid artery crossclamping resulted in a rapid and pronounced decrease of PcjO2, while PcijvO2 remained unchanged. No relationship between PcjO2 and stump pressure was found, while a significant correlation (P less than 0.02) between PcjO2 and lactate in effluent venous blood from the brain was demonstrable. It is concluded that PcjO2 monitoring seems a clinically useful trend indicator of cerebral perfusion in the individual patient. Due to large interindividual variations in basal PcjO2 readings and in PcjO2 changes during carotid artery clamping, however, transconjunctival oxygen tension monitoring does not seem to allow early and accurate recognition of impending cerebral ischaemia during carotid endarterectomy, and its routine use therefore seems of limited value.     

Ventilatory compensation for inspiratory resistive loads during anaesthesia with halothane or isoflurane

We have analysed the ventilatory response to sustained inspiratory resistive loads in 14 patients, while awake and during halothane (n = 7) or isoflurane (n = 7) anaesthesia. Patients breathed halothane or isoflurane in oxygen at 1.2 minimum alveolar concentration (MAC). Inspiratory resistances of 0, 12 and 37 cm H2O litre-1 s were applied. Tidal volume (VT) was maintained with the greater loads. At the greatest resistance, a significant reduction in minute ventilation occurred in both awake (-18.9%) and anaesthetized states, with both halothane (-10.4%) and isoflurane (-14.5%). Ventilatory frequency decreased significantly from mean 14.6 (SD 4.7) to 12.5 (4.3) bpm in the awake state and during anaesthesia, with increasing inspiratory resistance (29.5 (3.6) to 23.7 (7.2) bpm and 25.8 (3.3) to 23.4 (4.0) bpm, respectively, for halothane and isoflurane) because inspiratory time (TI) was significantly longer (P < 0.01). End-tidal PCO2 increased by 0.3 kPa, on average, from baseline to the highest level of resistance (P < 0.01). Inspiratory occlusion pressure at 100 ms increased significantly with increased loading in all situations (P < 0.001). We found a similar pattern of ventilatory adaptation to sustained inspiratory flow resistive loads both in awake and anaesthetized states. VT was maintained at increased loads because of an increase in inspiratory neuromuscular output and inspiratory duration.      

Carbon dioxide tensions in infants during mask anaesthesia with spontaneous ventilation

Carbon dioxide tensions (PCO2) in arterialized capillary blood samples were measured in 39 infants anaesthetized for minor paediatric surgery. Anaesthesia was induced and maintained with oxygen, nitrous oxide and halothane, using a Mapleson-D system with spontaneous ventilation and a Rendell-Baker face mask. The duration of anaesthesia was between 15 and 95 min. Two capillary blood samples were obtained during stable anaesthesia before and after surgery. The PCO2 values varied between 3.7 and 8.0 kPa. The highest values were found in infants aged 15-30 days, 6.6 +/- 0.7 kPa (mean +/- s.d.), compared to 5.9 +/- 0.7 in infants aged 31-60 days, 5.6 +/- 0.8 in infants aged 61-180 days and 5.5 +/- 0.7 in infants aged 181-300 days. Comparison between measurements before and after surgery did not in any group indicate a progressive hypoventilation or a correlation between the length of the anaesthesia and the PCO2. It is concluded that anaesthesia with oxygen, nitrous oxide and halothane with spontaneous mask ventilation is a satisfactory method for minor procedures in infants over 1 month of age, while in younger infants controlled ventilation with intubation may be a safer choice of method.     

Monitoring brain PO2, PCO2, and pH during graded levels of hypoxemia in rabbits.

Brain ischemia and hypoxia are of concern when they occur following traumatic brain injury because they frequently result in potentially preventable secondary brain damage. In this study, we examined the ability of an implantable catheter (Paratrend 7; Diametrics Medical, St. Paul, MN) to continuously measure brain tissue pH, PCO2, and PO2 during graded levels of hypoxia. Values obtained from this catheter were compared with simultaneous measurements of arterial and sagittal sinus blood. As expected, there was a good correlation between the changes in pH, PCO2, and PO2 in brain tissue and sagittal sinus blood. Brain tissue PO2 was numerically lower than sagittal sinus blood at all inspired levels of oxygen. These data suggest that the Paratrend 7 may be useful in monitoring brain tissue oxygen tension in patients at risk for regional cerebral ischemia and hypoxia.     

Tissue gas tensions in patients with necrotising fasciitis and healthy controls during treatment with hyperbaric oxygen: a clinical study.

OBJECTIVE: To investigate the effect of hyperbaric oxygen (HBO) treatment on tissue oxygen and carbon dioxide tensions in patients with necrotising fasciitis and healthy volunteers. DESIGN: Clinical study in patients and healthy controls. SETTING: University hospital, Finland. SUBJECTS: 6 patients with necrotising fasciitis and 3 healthy volunteers. INTERVENTIONS: Subcutaneous tissue PO2 and PCO2 tensions were measured directly in patients with necrotising fasciitis and in healthy volunteers during normobaric and hyperbaric conditions. Arterial blood PO2 and PCO2 tensions were measured only in the patients. Tissue gas tensions were measured with a Silastic tube tonometer implanted in the brachial subcutaneous tissue of both patients and controls as well as in the subcutaneous tissue of the patients in the immediate vicinity of the necrotising process. The diagnosis of necrotising fasciitis was made on the basis of the presence of typical clinical signs and symptoms, intraoperative findings, and microbiological observations. MAIN OUTCOME MEASURES: Arterial and subcutaneous tissue PO2 and PCO2 tensions. RESULTS: In patients with necrotising fasciitis the arterial PO2 rose about 7-fold whereas the arterial PCO2 increased only slightly during exposure to 2.5 absolute atmospheres (ATA) of oxygen. During HBO the subcutaneous tissue PO2 increased four to five fold from the baseline and CO2 tensions also increased, but to a lesser degree, in both healthy and infected tissues. In patients with necrotising fasciitis, the PO2 was higher, but not significantly so, in the vicinity of the infected area than in the healthy tissue. CONCLUSION: Under hyperbaric conditions the subcutaneous PO2 in patients with necrotising fasciitis rose higher in the vicinity of the infected area than in the healthy tissue, which may be the result of vasodilatation and increased microcirculation induced by the inflammatory process associated with infection or it may be the result of decreased local tissue oxygen utilisation, or both. The tissue PCO2 values tended to rise during HBO probably from hypoventilation or reduced CO2 washout from tissue because venous blood haemoglobin was fully saturated with oxygen.     

Effects of desflurane and isoflurane on intestinal tissue oxygen pressure during colorectal surgery

Volatile anaesthetics differ in the effects they have on splanchnic haemodynamics and oxygenation. The aim of this study was to evaluate the effects of desflurane and isoflurane as part of a balanced anaesthetic technique on intestinal tissue oxygenation during colorectal surgery. Data were analysed from 44 patients randomly assigned to receive either desflurane (desflurane group, n = 20), or isoflurane (isoflurane group, n = 24) for inhalational anaesthesia. Tissue oxygen pressure (P(tiss)O2) was measured on the serosal side of the large intestine prior to colonic resection (T1) and following the completion of the bowel anastomosis (T2). In addition, haemodynamic and oxygenation parameters were assessed. No difference in mean P(tiss)O2 was observed between the groups at T1 [desflurane group: 8.1 (2.9) kPa vs. isoflurane group: 7.7 (2.7) kPa]. Following completion of the anastomosis (T2) mean P(tiss)O2 was higher in the isoflurane group [9.6 (2.9) kPa] than the desflurane group [7.7 (2.4) kPa, p = 0.025]. During surgery no difference between the groups could be observed with regard to haemodynamics and global oxygenation parameters. The lack of a difference between the groups in P(tiss)O2 before resection of the colon suggests that, under normal conditions, desflurane and isoflurane have comparable effects on intestinal blood flow and oxygenation. However, following local ischaemia, the reactive hyperaemia seems to be better preserved during isoflurane anaesthesia indicated by a local increase in P(tiss)O2blank(p = 0.013) following completion of the bowel anastomosis.