Hypercapnia improves tissue oxygenation

BACKGROUND: Wound infections are common, serious, surgical complications. Oxidative killing by neutrophils is the primary defense against surgical pathogens and increasing intraoperative tissue oxygen tension markedly reduces the risk of such infections. Since hypercapnia improves cardiac output and peripheral tissue perfusion, we tested the hypothesis that peripheral tissue oxygenation increases as a function of arterial carbon dioxide tension (PaCO(2)) in anesthetized humans. METHODS: General anesthesia was induced with propofol and maintained with sevoflurane in 30% oxygen in 10 healthy volunteers. Subcutaneous tissue oxygen tension (PsqO(2)) was recorded from a subcutaneous tonometer. An oximeter probe on the upper arm measured muscle oxygen saturation. Cardiac output was monitored noninvasively. PaCO(2) was adjusted to 20, 30, 40, 50, or 60 mmHg in random order with each concentration being maintained for 45 min.(2) (2) RESULTS: Increasing PaCO(2) linearly increased cardiac index and PsqO(2) : PsqO(2) = 35.42 + 0.77 (PaCO(2)), < 0.001. CONCLUSIONS: The observed difference in PsqO(2) is clinically important because previous work suggests that comparable increases in tissue oxygenation reduced the risk of surgical infection from -8% to 2 to 3%. We conclude that mild intraoperative hypercapnia increased peripheral tissue oxygenation in healthy human subjects, which may improve resistance to surgical wound infections.     

Local heat increases blood flow and oxygen tension in wounds

The effect of local hyperthermia on subcutaneous tissue oxygen tension (Psqo2) and perfusion was investigated in eight patients (13 trials) using a subcutaneously implanted oxygen tonometer. Application of heat increased subcutaneous tissue temperature and Psqo2. Mean Psqo2 during oxygen breathing rose by 39.5 mm Hg, an 80% increase over the average baseline Psqo2. The corresponding mean subcutaneous temperature increased 4.0 degrees C. A significant linear correlation was found between the change in Psqo2 and subcutaneous temperature. There was an average threefold increase in local perfusion estimated by using the Fick principle. The data reaffirm the value of local hyperthermia in treating contaminated wounds and suggest a mechanism for its ability to ameliorate infections. The mechanism implies that local heat may have prophylactic value as well.     

Obesity Decreases Perioperative Tissue Oxygenation

Background: Obesity is an important risk factor for surgical site infections. The incidence of surgical wound infections is directly related to tissue perfusion and oxygenation. Fat tissue mass expands without a concomitant increase in blood flow per cell, which might result in a relative hypoperfusion with decreased tissue oxygenation. Consequently, the authors tested the hypotheses that perioperative tissue oxygen tension is reduced in obese surgical patients. Furthermore, they compared the effect of supplemental oxygen administration on tissue oxygenation in obese and nonobese patients.

Methods: Forty-six patients undergoing major abdominal surgery were assigned to one of two groups according to their body mass index: body mass index less than 30 kg/m2 (nonobese) or 30 kg/m2 or greater (obese). Intraoperative oxygen administration was adjusted to arterial oxygen tensions of approximately 150 mmHg and approximately 300 mmHg in random order. Anesthesia technique and perioperative fluid management were standardized. Subcutaneous tissue oxygen tension was measured with a polarographic electrode positioned within a subcutaneous tonometer in the lateral upper arm during surgery, in the recovery room, and on the first postoperative day. Postoperative tissue oxygen was also measured adjacent to the wound. Data were compared with unpaired two-tailed t tests and Wilcoxon rank sum test; P < 0.05 was considered statistically significant.

Results: Intraoperative subcutaneous tissue oxygen tension was significantly less in the obese patients at baseline (36 vs. 57 mmHg; P = 0.002) and with supplemental oxygen administration (47 vs. 76 mmHg; P = 0.014). Immediate postoperative tissue oxygen tension was also significantly less in subcutaneous tissue of the upper arm (43 vs. 54 mmHg; P = 0.011) as well as near the incision (42 vs. 62 mmHg; P = 0.012) in obese patients. In contrast, tissue oxygen tension was comparable in each group on the first postoperative morning.     

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.     

Obesity decreases perioperative tissue oxygenation

BACKGROUND: Obesity is an important risk factor for surgical site infections. The incidence of surgical wound infections is directly related to tissue perfusion and oxygenation. Fat tissue mass expands without a concomitant increase in blood flow per cell, which might result in a relative hypoperfusion with decreased tissue oxygenation. Consequently, the authors tested the hypotheses that perioperative tissue oxygen tension is reduced in obese surgical patients. Furthermore, they compared the effect of supplemental oxygen administration on tissue oxygenation in obese and nonobese patients. METHODS: Forty-six patients undergoing major abdominal surgery were assigned to one of two groups according to their body mass index: body mass index less than 30 kg/m2 (nonobese) or 30 kg/m2 or greater (obese). Intraoperative oxygen administration was adjusted to arterial oxygen tensions of approximately 150 mmHg and approximately 300 mmHg in random order. Anesthesia technique and perioperative fluid management were standardized. Subcutaneous tissue oxygen tension was measured with a polarographic electrode positioned within a subcutaneous tonometer in the lateral upper arm during surgery, in the recovery room, and on the first postoperative day. Postoperative tissue oxygen was also measured adjacent to the wound. Data were compared with unpaired two-tailed t tests and Wilcoxon rank sum test; P < 0.05 was considered statistically significant. RESULTS: Intraoperative subcutaneous tissue oxygen tension was significantly less in the obese patients at baseline (36 vs. 57 mmHg; P = 0.002) and with supplemental oxygen administration (47 vs. 76 mmHg; P = 0.014). Immediate postoperative tissue oxygen tension was also significantly less in subcutaneous tissue of the upper arm (43 vs. 54 mmHg; P = 0.011) as well as near the incision (42 vs. 62 mmHg; P = 0.012) in obese patients. In contrast, tissue oxygen tension was comparable in each group on the first postoperative morning. CONCLUSION: Wound and tissue hypoxia were common in obese patients in the perioperative period and most pronounced during surgery. Even with supplemental oxygen tissue, oxygen tension in obese patients was reduced to levels that are associated with a substantial increase in infection risk.     

Supplemental perioperative fluid administration increases tissue oxygen pressure

BACKGROUND: Wound infections are common and serious surgical complications. Wound perfusion delivers oxygen, inflammatory cells, growth factors, and cytokines to injured tissues. Hypoperfused regions experience low oxygen tensions that do not support adequate oxidative killing or wound healing. Clinicians may fail to recognize clinically important hypovolemia because hemodynamic stability and urine output are maintained after peripheral perfusion is compromised. We tested the hypothesis that supplemental fluid administration during and after elective colon resection increases tissue perfusion and tissue oxygen pressure. METHODS: Fifty-six patients undergoing colon resection were randomly assigned to conservative (8 mL x kg(-1) x h(-1), n = 26) or aggressive (16 to 18 mL x kg(-1) x h(-1), n = 30) fluid management. Anesthetic technique was standardized. We used 60% nitrous oxide in 40% oxygen. During surgery and postanesthetic recovery, subcutaneous oxygen tension (P(sq)O(2)) was measured by using a polarographic sensor implanted subcutaneously into 1 upper arm. Capillary blood flow was evaluated postoperatively with a thermal diffusion system. Data were analyzed with 2-tailed t tests; P value less than.05 was considered statistically significant. RESULTS: Hemodynamic and renal responses were similar in the groups. Intraoperative tissue oxygen tension was significantly greater in patients given supplemental fluid: 81 +/- 26 vs 67 +/- 18 mm Hg, P =.03. Postoperative P(sq)O(2) (77 +/- 26 vs 59 +/- 15 mm Hg, P =.009) and capillary blood flow (69 +/- 12 vs 53 +/- 12, P <.001) were also greater in the supplemental fluid patients. CONCLUSIONS: Supplemental perioperative fluid administration significantly increases tissue perfusion and tissue oxygen partial pressure. Optimizing tissue perfusion will require providing more fluid than indicated by normal clinical criteria or use of invasive monitoring to guide treatment. The actual effect of supplemental fluid administration on incidence of wound infection requires further investigation.     

Influence of positive end-expiratory pressure ventilation on peripheral tissue perfusion evaluated by measurements of tissue gases and pH. An experimental study in pigs with oleic acid lung injury.

Measurements of subcutaneous oxygen tension (PscO(2)), subcutaneous carbon dioxide tension (PscCO(2)) and subcutaneous pH (pHsc) were used for evaluation of peripheral oxygenation in pigs subjected to oleic acid-induced lung injury during ventilation with increasing levels of positive end-expiratory pressure (PEEP). Lung injury resulted in a decrease of arterial oxygen tension (PaO(2)) from 93 to 37 mm Hg (p<0.01) with maintained cardiac output. PscO(2) decreased from 45 to 17 mm Hg (p<0.01) and pHsc from 7.47 to 7.39 (p<0.05), and PscCO(2) increased from 46 to 59 mm Hg (p<0.05). Increase of PEEP level between 5 and 20 cm H(2)O resulted in a continuous increase of PaO(2) from 45 to 145 mm Hg and a decrease of cardiac output from 4.1 to 2.0 liters/min (p<0.01). PscO(2) increased up to a PEEP level of 15 cm H(2)O, reaching 26 mm Hg. Further increase of PEEP level up to 20 cm H(2)O resulted in an increase of PscCO(2) from 65 to 71 mm Hg (p<0.05) and a decrease of pHsc from 7.31 to 7.29 (p<0.05). In conclusion: measurements of tissue gases and pH can be used to evaluate optimum peripheral tissue oxygenation during titration of PEEP level. Whether these measurements can be used as the only indicator to guide therapy in an individual case remains to be studied.     

Circulatory effects of carbon dioxide in experimental endotoxic shock

In seven anaesthetized dogs the central circulatory effects of changes in Paco2 were studied before and after an intravenous injection of E. coli endotoxin 1.0 mg kg-1. The animals were on controlled ventilation with constant minute volumes, and Paco2 was changed by variations in the inspired gas mixture. Before endotoxin was given, cardiac output, mean aortic pressure, mean pulmonary artery pressure, total peripheral resistance and stroke volume were little affected by the changes in Paco2 from 3.9 +/- 0.4 to 7.2 +/- 0.4 kPa (mean +/- s.e.mean). Only heart rate decreased significantly (P less than 0.05). The intravenous endotoxin injection resulted in a decreased cardiac output (P less than 0.01), a decreased mean aortic pressure (P less than 0.01) and a decreased stroke volume (P less than 0.05). Mean pulmonary artery pressure, total peripheral resistance and heart rate showed only minor changes. In endotoxic shock an increase in Paco2 from 4.4 +/- 0.4 to 7.8 +/- 0.3 kPa (mean +/- s.e.mean) resulted in a significant increase in cardiac output (P less than 0.05), stroke volume (P less than 0.05) and mean pulmonary artery pressure (P less than 0.05), while the other parameters remained unchanged. It can be concluded that the carbon dioxide tension is of importance for the cardiac performance in experimental endotoxic stock in a manner not seen in control animals. The mechanisms behind these findings need further investigation.     

The effects of epidural and general anesthesia on tissue oxygenation

The risk of wound infections is inversely related to subcutaneous tissue oxygen tension. General anesthesia increases local blood flow by direct vasodilation and central inhibition of thermoregulatory vasoconstriction. Epidural anesthesia can increase perfusion in blocked regions by decreasing sympathetic tone. We therefore tested the hypothesis that epidural anesthesia increases tissue oxygen tension in awake and anesthetized subjects. Fifteen healthy volunteers underwent epidural, general, and combined epidural and general anesthesia. Subcutaneous tissue oxygen tension was measured using tonometers in the lateral upper arm and the lateral thigh. Epidural anesthesia to a T10 level was maintained with 0.75% mepivacaine. General anesthesia was maintained with 1.5% sevoflurane in 30% oxygen; 30% inspired oxygen was given via a sealed facemask during baseline and epidural anesthesia. Baseline subcutaneous tissue oxygen tensions for arm and thigh were 57 +/- 11 and 54 +/- 8 mm Hg, respectively. Epidural anesthesia significantly increased tissue oxygenation in the thigh by 9 mm Hg, to 63 +/- 7 mm Hg, without increasing arm oxygenation. Tissue oxygenation in the arm and thigh were similar during general anesthesia alone, 58 +/- 11 and 63 +/- 12 mm Hg. Arm oxygenation remained unchanged with the addition of epidural anesthesia; however, thigh subcutaneous oxygen partial pressure increased 8 +/- 3 mm Hg, from 63 +/- 12 to 71 +/- 9 mm Hg. Although epidural anesthesia increased tissue oxygenation significantly with and without general anesthesia, the magnitude of this increase might be of marginal clinical importance in regard to surgical wound infections. IMPLICATIONS: Epidural anesthesia significantly increased subcutaneous tissue oxygenation in the thigh both with and without general anesthesia. Although each increase was statistically significant, previous work suggests that the magnitude of these changes is unlikely to markedly reduce the risk of surgical wound infection.     

Thoracic epidural anesthesia increases tissue oxygenation during major abdominal surgery

Intraoperative surgical stress may markedly increase adrenergic nerve activity and plasma catecholamine concentrations, which causes peripheral vasoconstriction and decreased tissue oxygen partial pressure possibly leading to tissue hypoxia. Tissue hypoxia is associated with an increased incidence of surgical wound infections. Thoracic epidural anesthesia blocks afferent neural stimuli and inhibits efferent sympathetic outflow in response to painful stimuli. Consequently, we tested the hypothesis that supplemental thoracic epidural anesthesia during major abdominal surgery improves tissue perfusion and subcutaneous oxygen tension. Thirty patients were randomly assigned to two groups: general (n = 15) or combined general and epidural anesthesia (n = 15). Anesthesia technique and fluid management were standardized. Subcutaneous tissue oxygen tension was measured continuously in the upper arm with a Clark type electrode. Data were compared with unpaired, two-tailed t-tests, Wilcoxon's ranked sum test, or repeated-measures analysis of variance and Scheffé F tests as appropriate; P < 0.05 was considered statistically significant. After 60 min, intraoperative tissue oxygen tension was significantly larger during combined anesthesia than during general anesthesia (54.3 +/- 7.4 mm Hg versus 42.1 +/- 8.6 mm Hg; P = 0.0002). Subcutaneous tissue oxygen tension remained significantly higher in the combined general/epidural anesthesia group throughout the observation period. Hemodynamic responses and global oxygen variables were similar in the groups. Thoracic epidural anesthesia improved intraoperative tissue oxygen tension outside the area of the epidural block. Thus, our results give evidence that supplemental neural nociceptive block blunts generalized vasoconstriction caused by surgical stress and adrenergic responses. IMPLICATIONS: Thoracic epidural anesthesia blunts the decrease of subcutaneous tissue oxygen tension caused by surgical stress and adrenergic vasoconstriction during major abdominal surgery. Consequently, combined general and epidural anesthesia helps to provide sufficient tissue oxygenation.     

Oxygen utilization during surface-induced deep hypothermia

Oxygen utilization during surface-induced deep hypothermia under ether anesthesia and respiratory alkalosis, with and without 30 minutes of circulatory arrest, was studied in 12 dogs. Oxygen consumption and saturation, hemoglobin, hematocrit, Po₂, Pco₂, and pH of arterial and mixed venous blood were measured, and oxygen content, arteriovenous oxygen differences, and cardiac output were calculated.There were slightly decreased but persistent arteriovenous oxygen differences during cooling until low cardiac output developed around 18°C., which would suggest continued unloading of oxygen from hemoglobin despite the presence of severe alkalosis. The oxygen debt developed during total circulatory occlusion or from low cardiac output was repaid in the early rewarming period when circulation was reestablished. Venous Po₂ became progressively lower below 25°C. Tissue oxygen uptake is presumably accomplished by lowering tissue oxygen tension, but this drop apparently does not grossly impair tissue function since all dogs tolerated the procedure well and are long-term survivors.     

Changes in canine conjunctival and cerebral oxygen tension during hemorrhagic hypotension

On-line conjunctival oxygen tension (PcjO2) and cerebral cortical oxygen tension (PcxO2) were measured simultaneously using polarographic oxygen sensors during hemorrhagic hypotension in dogs. Mean arterial pressure (MAP) decreased from a control value of 119 +/- 7 to 52 +/- 1 (SEM) mmHg during initial bleeding of 30 minutes, and then this level of MAP was maintained for another 150min by adjusting the height of the reservoir. During the early phase of arterial hypotension, PcjO2 fell sharply, and this was accompanied by a parallel decrease of carotid artery blood flow and cardiac output, whereas PcxO2 remained unaffected with this level of hypovolemic hypotension which was possibly due to the cerebral tissue autoregulatory mechanism. Thus, it was demonstrated that despite the anatomical similarity and proximity of their blood supply, the conjunctival tissue responded differently to the intracranial cerebral tissues when compared during hemorrhagic hypotension. The study also suggests that monitoring the PcjO2 during surgery may be a useful monitoring tool in detecting early signs of tissue ischemia and hypoxia during hypovolemic shock.     

Microvascular Oxygen Delivery and Interstitial Oxygenation during Sodium Pentobarbital Anesthesia

Background: Anesthesia may represent a considerable bias in experimental medicine, particularly in conditions of stress (such as hemorrhage). Sodium pentobarbital (PB), widely used for cardiovascular investigations, may impair oxygen delivery by hemodynamic and respiratory depression. The critical issue, however, is whether the microcirculation can still maintain tissue oxygenation during anesthesia. To answer this question, the authors studied the effect of PB anesthesia on subcutaneous microvascular oxygen delivery and interstitial oxygenation in Syrian golden hamsters.

Methods: Sodium pentobarbital anesthesia was induced by intravenous injection (30 mg/kg body weight) and maintained by a 15-min infusion (2 mg [center dot] kg-1 [center dot] min-1), with animals breathing spontaneously (PB-S) or ventilated with air (PB-V). Systemic parameters evaluated were mean arterial pressure (MAP), heart rate, cardiac index (CI), arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2), base excess, and pH. Microvascular and interstitial oxygen tension (PO2), vessel diameter, red blood cell velocity (upsilonRBC), and blood flow (Qb) were measured in a dorsal skinfold preparation. Microcirculatory PO2 values were determined by phosphorescence decay.

Results: Sodium pentobarbital anesthesia significantly decreased CI, MAP, upsilonRBC, and Qb. During PB infusion, PaO2 values were 56 +/- 12.8 mmHg (PB-S) and 115.9 +/- 14.6 mmHg (PB-V) compared with 69.4 +/- 18.2 mmHg and 61.4 +/- 12.6 mmHg at baseline. However, microvascular PO2 was reduced by 25-55% in both groups, resulting in an interstitial PO2 decrease from 23.9 +/- 5.6 mmHg (control) to 13.1 +/- 9.1 mmHg (PB-S) and 15.2 +/- 7 mmHg (PB-V). Microcirculatory PO2 values were restored 30 min after PB infusion, even though hemodynamic depression and a light anesthetic plane were maintained.     

Cardiovascular and Metabolic Response to Acute Normovolemic Anemia: Effects of Anesthesia

Background: The maintenance of adequate tissue oxygenation during acute anemia depends on an increase in both cardiac output and tissue oxygen extraction. This study tested the hypothesis that anesthesia blunts the cardiac output response associated with acute normovolemic hemodilution.

Methods: Forty patients undergoing major abdominal surgery were prospectively randomized to undergo acute normovolemic hemodilution (ANH) either awake (awake group, n = 20) or with fentanyl-nitrous oxide-isoflurane anesthesia (anesthetized group, n = 20). Radial and pulmonary artery catheters were placed in all patients. After hemodynamic measurements were taken, patients in the two groups underwent hemodilution to decrease their hemoglobin concentration from 13 to 8 g/dl. A total of 1,875 +/- 222 ml (mean +/- SD) of blood was collected and simultaneously replaced by the same volume of medium molecular weight hydroxyethylstarch in both groups.

Results: In the awake group, ANH resulted in a significant increase in cardiac index (from 3.1 +/- 0.5 to 4.8 +/- 1.0 l [middle dot] min-1 [middle dot] m-2) related to both an increase in heart rate and stroke index. Oxygen delivery remained unchanged, but oxygen consumption increased significantly, resulting in an increase in oxygen extraction ratio. In the anesthetized group, ANH resulted in a significantly smaller increase in cardiac index (from 2.3 +/- 0.5 to 3.1 +/- 0.7 l [middle dot] min-1 [middle dot] m-2) related solely to an increase in stroke index. Oxygen delivery decreased but oxygen consumption was maintained as oxygen extraction increased.     

Inhibition of the cardiovascular effects of lysine acetylsalicylate by propranolol in dogs during halothane anaesthesia

The cardiovascular effects of lysine acetylsalicylate and/or propranolol were studied in 26 dogs. All animals were maintained under anaesthesia with halothane 0.75 per cent, supplemented by the intravenous administration of succinylcholine to allow controlled ventilation during a two hour period of monitoring. Cardiac output, stroke volume, heart rate, mean arterial pressure, pulse pressure, central venous pressure, total peripheral resistance, pH, Paco2, pao2 and base deficit were measured in each dog. Lysine acetylsalicylate 50 mg . kg-1, administered alone as a single bolus, significantly (P less than 0.05) increased the cardiac output and stroke volume and significantly decreased the heart rate, central venous pressure and total peripheral resistance in dogs under halothane anaesthesia. Propranolol hydrochloride 0.5 mg . kg-1 as a single intravenous bolus was followed by a significant decrease in cardiac output, heart rate and mean arterial pressure and a significant increase in central venous pressure and total peripheral resistance. The administration of propranolol prior to lysine acetylsalicylate resulted in a significant decrease in cardiac output and heart rate. Pretreatment with propranolol was effective in inhibiting the positive inotropic effect of lysine acetylsalicylate.     

Effect of hypotensive anesthesia on tissue oxygen tension of the heart, kidney and liver]

The effects of hypotensive anesthesia by prostaglandin E1 (PGE1: 8 dogs) or trimetaphan (TMP: 8 dogs) on tissue oxygenation were studied in 16 mongrel dogs anesthetized with pentobarbital. Mean blood pressure (MBP), heart rate (HR), cardiac output (CO), blood gases (BG), the blood flow and tissue oxygen tension of the heart, the kidney and the liver were measured. The blood flow and oxygen tension were measured by electromagnetic flowmeters and by polarographic oxygen electrodes respectively. PGE1 or TMP was injected intravenously to decrease MBP by 30%. MBP, CO, HR and BG of PGE1 were not significantly different with those of TMP. Coronary blood flow decreased for 12% with PGE1 and for 33% with TMP. Though blood flows of the renal and the hepatic arteries were well maintained with PGE1, they decreased for 36% and 34% respectively with TMP. Oxygen tensions of the myocardium (both outer and inner layers) and the liver were well maintained with PGE1. But with TMP, oxygen tension decreased for 23% in outer layer, for 16% in inner layer and for 31% in the liver. Oxygen tension of the kidney remained unchanged with PGE1 and TMP. The results suggest that PGE1 is more useful for the maintenance of the tissue oxygenation than TMP during hypotensive anesthesia.     

CARDIOVASCULAR EFFECTS OF PANCURONIUM IN MAN

We have investigated the effects of pancuronium bromide (0.07mg/kg body weight) on heart rate, mean arterial blood pressure,cardiac output and calculated total peripheral resistance inten artificially ventilated patients, anaesthetized with 60per cent nitrous oxide in oxygen plus phenoperidine (1 mg/15kg body weight). End-tidal Pco₂ was maintained constant at 30±2mm Hg. There was a marked and statistically significant increaseof heart rate of about 25 per cent, accompanied by lesser, butstill statistically significant, incerases of cardiac outputand mean arterial blood pressure. Total peripheral resistancewas unchanged, suggesting that pancuronium has little ganglion-blockingactivity.     

Tissue oxygen tension in random pattern skin flaps during normovolemic hemodilution

We investigated the effect of progressive normovolemic hemodilution with Dextran 70 on subcutaneous tissue oxygen tension (PscO2) in canine random pattern flaps and in adjacent intact skin. PscO2 was measured polarographically in implanted oxygen-permeable Silastic catheters. The mean PscO2 of 5.7 kPa in intact subcutis at hematocrit (HCT) 40 was maintained or insignificantly improved during hemodilution to a HCT of 14. The mean flap PscO2 of 3.1 kPa at HCT 40 was maintained down to HCT 20. Most flap measuring sites exhibited a PscO2 improvement in an individual HCT range at some point during hemodilution. The increment in PscO2 tended to be relatively greater and to occur at lower Hct values in locations with more severe ischemia initially. Central hemodynamics were characterized by a lowered total peripheral resistance and an increased cardiac output, whereas systemic and pulmonary arterial blood pressures and central venous pressure remained essentially unchanged.     

Noninvasive Positive Pressure Ventilation Using a Helmet in Patients with Acute Exacerbation of Chronic Obstructive Pulmonary Disease: A Feasibility Study

Background: Noninvasive positive pressure ventilation (NPPV) with a facemask (FM) is effective in patients with acute exacerbation of their chronic obstructive pulmonary disease. Whether it is feasible to treat these patients with NPPV delivered by a helmet is not known.

Methods: Over a 4-month period, the authors studied 33 chronic obstructive pulmonary disease patients with acute exacerbation who were admitted to four intensive care units and treated with helmet NPPV. The patients were compared with 33 historical controls treated with FM NPPV, matched for simplified acute physiologic score (SAPS II), age, Paco2, pH, and Pao2:fractional inspired oxygen tension. The primary endpoints were the feasibility of the technique, improvement of gas exchange, and need for intubation.

Results: The baseline characteristics of the two groups were similar. Ten patients in the helmet group and 14 in the FM group (P = 0.22) were intubated. In the helmet group, no patients were unable to tolerate NPPV, whereas five patients required intubation in the FM group (P = 0.047). After 1 h of treatment, both groups had a significant reduction of Paco2 with improvement of pH; Paco2 decreased less in the helmet group (P = 0.01). On discontinuing support, Paco2 was higher (P = 0.002) and pH lower (P = 0.02) in the helmet group than in the control group. One patient in the helmet group, and 12 in the FM group, developed complications related to NPPV (P < 0.001). Length of intensive care unit stay, intensive care unit, and hospital mortality were similar in both groups.     

Centrally and locally mediated thermoregulatory responses alter subcutaneous oxygen tension

Mild perianesthetic hypothermia decreases resistance to infections. Decreased resistance likely results in part from direct immune inhibition. However, decreased tissue oxygen partial pressure also decreases resistance to infection by impairing oxidative killing by neutrophils and collagen deposition. Thermoregulatory vasoconstriction decreases skin blood flow and may also decrease subcutaneous tissue oxygen tension. Accordingly, we determined the influence of centrally and locally mediated thermoregulatory vasomotion on subcutaneous oxygen tension. We also compared subcutaneous oxygen tension to other potential markers of tissue perfusion: laser Doppler flowmetry and transcutaneous oxygen tension. Arterial oxygen tension was maintained near 325 mm Hg in five volunteers. Control subcutaneous oxygen tension values were recorded after 1 hour of euthermia (no sweating or vasoconstriction). Volunteers were then cooled with a circulating-water mattress positioned under the trunk and legs. After 1.5 hours of cooling sufficient to produce shivering, the right upper arm was covered for 1 hour with a small circulating water blanket set to 40 degrees C while systemic cooling continued. The volunteers were then systematically warmed to produce sweating, and the right arm was locally cooled. There was no correlation among laser Doppler flowmetry, transcutaneous oxygen tension, and subcutaneous oxygen tension. Systemic cooling significantly decreased subcutaneous oxygen tension, but subcutaneous oxygen tension in the right arm returned to control values during local heating. Systemic warming significantly increased subcutaneous oxygen tension, and 1 hour of local cooling failed to fully reverse the increase. These data indicate that thermoregulatory vasoconstriction significantly decreases tissue oxygen availability. Decreased subcutaneous oxygen tension may be one mechanism by which mild perianesthetic hypothermia facilitates development of surgical wound infections.