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 (Paco2) 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 (Psqo2) was recorded from a subcutaneous tonometer. An oximeter probe on the upper arm measured muscle oxygen saturation. Cardiac output was monitored noninvasively. Paco2 was adjusted to 20, 30, 40, 50, or 60 mmHg in random order with each concentration being maintained for 45 min.

Results: Increasing Paco2 linearly increased cardiac index and Psqo2: Psqo2 = 35.42 + 0.77 (Paco2), P < 0.001.     

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.     

Laparoscopic surgery impairs tissue oxygen tension more than open surgery

BACKGROUND: Wound infection remains a common and serious complication after colonic surgery. Although many colonic operations are performed laparoscopically, it remains unclear whether this has any impact on the incidence of wound infection. Subcutaneous tissue oxygenation is an excellent predictor of surgical wound infection. The impact of open and laparoscopic colonic surgery on tissue oxygenation was compared. METHODS: Fifty-two patients undergoing elective open and laparoscopic left-sided colonic resections were evaluated in a prospective observational study. Anaesthesia management was standardized and intraoperative arterial partial pressure of oxygen was kept at 150 mmHg in both groups. Oxygen tension was measured in the subcutaneous tissue of the right upper arm. RESULTS: At the start of surgery subcutaneous tissue oxygen tension (PsqO(2)) was similar in both groups (mean(s.d.) 65.8(17.2) and 63.7(23.6) mmHg for open and laparoscopic operations respectively; P = 0.714). Tissue oxygen remained stable in the open group, but dropped significantly in the laparoscopic group during the course of surgery (PsqO(2) after operation 53.4(12.9) and 45.5(11.6) mmHg, respectively; P = 0.012). CONCLUSION: Laparoscopic colonic surgery significantly decreases PsqO(2), an effect that occurs early in the course of surgery. As tissue oxygen tension is a predictor of wound infection, these results may explain why the risk of wound infection after laparoscopic surgery remains higher than expected.     

Mild hypercapnia increases subcutaneous and colonic oxygen tension in patients given 80% inspired oxygen during abdominal surgery

BACKGROUND: Supplemental perioperative oxygen increases tissue oxygen tension and decreases incidence of wound infection in colorectal surgery patients. Mild intraoperative hypercapnia also increases subcutaneous tissue oxygen tension. However, the effect of hypercapnia in patients already receiving supplemental oxygen is unknown, as is the effect of mild hypercapnia on intestinal oxygenation in humans-although the intestines are presumably the tissue of interest for colon surgeries. The authors tested the hypothesis that mild intraoperative hypercapnia increases both subcutaneous tissue and intramural intestinal oxygen tension in patients given supplemental oxygen. METHODS: Patients undergoing elective colon resection were randomly assigned to normocapnia (n = 15, end-tidal carbon dioxide tension 35 mmHg) or mild hypercapnia (n = 15, end-tidal carbon dioxide tension 50 mmHg). Intraoperative inspired oxygen concentration was 80%. The authors measured subcutaneous tissue oxygen tension in the right upper arm and intramural oxygen tension in the left colon. Measurements were averaged over time within each patient and, subsequently, among patients. Data were compared with chi-square, unpaired t, or Mann-Whitney rank sum tests; P < 0.05 was significant. RESULTS: Morphometric characteristics and other possible confounding factors were similar in the groups. Intraoperative tissue oxygen tension in hypercapnic patients was significantly greater in the arm (mean +/- SD: 116 +/- 29 mmHg vs. 84 +/- 25 mmHg; P = 0.006) and colon (median [interquartile range]: 107 [81-129] vs. 53 [41-104] mmHg; P = 0.020). CONCLUSIONS: During supplemental oxygen administration, mild intraoperative hypercapnia increased tissue oxygen tension in the arm and colon. Previous work suggests that improved tissue oxygenation will reduce infection risk via the proposed pathomechanism, although only an outcome study can confirm this.     

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.     

Mild Hypercapnia Increases Subcutaneous and Colonic Oxygen Tension in Patients Given 80% Inspired Oxygen during Abdominal Surgery

Background: Supplemental perioperative oxygen increases tissue oxygen tension and decreases incidence of wound infection in colorectal surgery patients. Mild intraoperative hypercapnia also increases subcutaneous tissue oxygen tension. However, the effect of hypercapnia in patients already receiving supplemental oxygen is unknown, as is the effect of mild hypercapnia on intestinal oxygenation in humans-although the intestines are presumably the tissue of interest for colon surgeries. The authors tested the hypothesis that mild intraoperative hypercapnia increases both subcutaneous tissue and intramural intestinal oxygen tension in patients given supplemental oxygen.

Methods: Patients undergoing elective colon resection were randomly assigned to normocapnia (n = 15, end-tidal carbon dioxide tension 35 mmHg) or mild hypercapnia (n = 15, end-tidal carbon dioxide tension 50 mmHg). Intraoperative inspired oxygen concentration was 80%. The authors measured subcutaneous tissue oxygen tension in the right upper arm and intramural oxygen tension in the left colon. Measurements were averaged over time within each patient and, subsequently, among patients. Data were compared with chi-square, unpaired t, or Mann-Whitney rank sum tests; P < 0.05 was significant.

Results: Morphometric characteristics and other possible confounding factors were similar in the groups. Intraoperative tissue oxygen tension in hypercapnic patients was significantly greater in the arm (mean +/- SD: 116 +/- 29 mmHg vs. 84 +/- 25 mmHg; P = 0.006) and colon (median [interquartile range]: 107 [81-129] vs. 53 [41-104] mmHg; P = 0.020).     

Effect of arterial carbon dioxide tension on regional myocardial tissue oxygen tension in the dog]

We investigated the effects of arterial carbon dioxide tension on the myocardial tissue oxygen tensions of subepicardium and subendocardium in the anesthetized dogs. The study was done in fourteen open-chest mongrel dogs, weighing 13 +/- 1 kg, anesthetized with sodium pentobarbital (30 mg.kg-1 iv), and mechanically ventilated with 100% oxygen to maintain normocapnia. End tidal CO2 fraction (FECO2) was monitored continuously by capnograph. Regional myocardial tissue PO2 was measured using a monopolar polarographic needle electrode. Two pairs of combined needle sensors were carefully inserted, one in the epicardial and the other in the endocardial layer of the beating heart. Electromagnetic blood flow probe was applied on the left anterior descending artery (LAD). After a stable normocapnic ventilation, hypocapnia was induced by increasing the respiratory rate, and this mechanical hyperventilation was kept fixed throughout the experiments. To induce hypercapnia, exogenous carbon dioxide was added to the inspired gas step-wise until FECO2 reached 10%. Hypocapnic hyperventilation (PaCO2: 22 mmHg) invariably resulted in a significant reduction of coronary blood flow (LADBF) and left ventricular myocardial tissue PO2 in both epicardial and endocardial layers, while addition of carbon dioxide to the inspired gas (hypercapnic hyperventilation) reversed the change by increased LADBF and arterial PaCO2 in a dose-dependent manner. These results indicate that injudicious and severe hypocapnic hyperventilation may induce impaired myocardial tissue perfusion and oxygenation although normal cardiac output and arterial blood oxygenation are maintained.     

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.     

Subcutaneous perfusion and oxygen during acute severe isovolemic hemodilution in healthy volunteers

HYPOTHESIS: Acute severe isovolemic anemia (to a hemoglobin [Hb] concentration of 50 g/L) does not decrease subcutaneous wound tissue oxygen tension (PsqO(2)). SETTING: University hospital operating room and inpatient general clinical research center ward. SUBJECTS: Twenty-five healthy, paid volunteers. METHODS: Subcutaneous oxygen tension and subcutaneous temperature (Tsq) were measured continuously during isovolemic hemodilution to an Hb level of 50 g/L. In 14 volunteers (initially well-perfused), "normal" perfusion (Tsq >34.4 degrees C) was achieved by hydration and systemic warming prior to starting isovolemic hemodilution, while in 11 volunteers (perfusion not controlled [PNC]), no attempt was made to control perfusion prior to hemodilution. MAIN OUTCOME MEASURES: Measurements of PsqO(2), Tsq, and relative subcutaneous blood flow (flow index). RESULTS: While PsqO(2), Tsq, and flow index were significantly lower in PNC vs well-perfused subjects at baseline, there was no significant difference between them at the Hb of 50 g/L (nadir). Subcutaneous PO(2) did not decrease significantly in either group. Arterial PO(2) was not different between the groups, and did not change significantly over time; Tsq and flow index increased significantly from baseline to nadir Hb in both groups. CONCLUSIONS: The level of PsqO(2) was maintained at baseline levels during hemodilution to Hb 50 g/L in healthy volunteers, whether they were initially well-perfused or mildly underperfused peripherally. Given the significant increase in Tsq and flow index, this resulted from a compensatory increase in subcutaneous blood flow sufficient to maintain oxygen delivery. Wound healing depends to a large extent on tissue oxygen delivery, and these data suggest that even severe anemia by itself would not be sufficient to impair wound healing. Thus, transfusion of autologous packed red blood cells solely to improve healing in surgical patients with no other indication for transfusion is not supported by these results.     

Hypercapnia increases cerebral tissue oxygen tension in anesthetized rats

PURPOSE: To test the hypotheses that deliberate elevation of PaCO(2) increases cerebral tissue oxygen tension (PBrO(2)) by augmenting PaO(2) and regional cerebral blood flow (rCBF). METHODS: Anesthetized rats were exposed to increasing levels of inspired oxygen (O(2)) or carbon dioxide (CO(2); 5%, 10% and 15%, n = 6). Mean arterial blood pressure (MAP), PBrO(2) and rCBF were measured continuously. Blood gas analysis and hemoglobin concentrations were determined for each change in inspired gas concentration. Data are presented as mean +/- standard deviation with P < 0.05 taken to be significant. RESULTS: The PBrO(2) increased in proportion to arterial oxygenation (PaO(2)) when the percentage of inspired O(2) was increased. Proportional increases in PaCO(2) (48.7 +/- 4.9, 72.3 +/- 6.0 and 95.3 +/- 15.4 mmHg), PaO(2) (172.2 +/- 33.1, 191.7 +/- 42.5 and 216.0 +/- 41.8 mmHg), and PBrO(2) (29.1 +/- 9.2, 49.4 +/- 19.5 and 60.5 +/- 23.0 mmHg) were observed when inspired CO(2) concentrations were increased from 0% to 5%, 10% and 15%, respectively, while arterial pH decreased (P < 0.05 for each). Exposure to CO(2) increased rCBF from 1.04 +/- 0.67 to a peak value of 1.49 +/- 0.45 (P < 0.05). Following removal of exogenous CO(2), arterial blood gas values returned to baseline while rCBF and PBrO(2) remained elevated for over 30 min. The hypercapnia induced increase in PBrO(2) was threefold higher than that resulting from a comparable increase in PaO(2) achieved by increasing the inspired O(2) concentration (34.9 +/- 14.5 vs 11.4 +/- 5.0 mmHg, P < 0.05). CONCLUSION: These data support the hypothesis that the combined effect of increased CBF, PaO(2) and reduced pH collectively contribute to augmenting cerebral PBrO(2) during hypercapnia.     

Hypercapnia improves tissue oxygenation in morbidly obese surgical patients

Risk of wound infection is increased in morbidly obese surgical patients, in part because a major determinant of wound infection risk, tissue oxygenation, is marginal. Unlike in lean patients, supplemental inspired oxygen (Fio2) only slightly improves tissue oxygenation in obese patients. Mild hypercapnia improves tissue oxygenation in lean patients but has not been evaluated in obese patients. We thus tested the hypothesis that mild hypercapnia markedly improves tissue oxygenation in morbidly obese patients given Fio2 80% during major abdominal surgery. Thirty obese patients (body mass index 61.5 +/- 17 kg/m2) scheduled for open gastric bypass were randomly assigned to normocapnia (n = 15, end-tidal Pco2 35 mm Hg) or hypercapnia (n = 15, end-tidal Pco2 50 mm Hg); Fio2 was 80%. Anesthetic management and other confounding factors were controlled. Tissue oxygen tension was measured subcutaneously at the upper arm using a polarographic probe in a silastic tonometer. Demographic characteristics, cardiovascular measurements, and Pao2 (222 +/- 48 versus 230 +/- 68 mm Hg in normocapnic versus hypercapnic; mean +/- sd; P = 0.705) were comparable in the groups. Tissue oxygen tension, however, was greater in hypercapnic than in normocapnic patients (78 +/- 31 versus 56 +/- 13 mm Hg; P = 0.029). Mild hypercapnia increased tissue oxygenation by an amount believed to be clinically important and could potentially reduce the risk of surgical wound infection in morbidly obese patients.     

Perioperative high-dose oxygen therapy in vascular surgery

BACKGROUND: Patients undergoing infrainguinal bypass surgery have reduced baseline tissue oxygen tension and high rates of wound infections. The hypoxaemia worsens during surgery, potentially reducing the ability to combat bacterial lodgement. We investigated whether high-dose perioperative oxygen administration to patients undergoing infrainguinal arterial surgery results in increased tissue oxygenation. METHODS: Ten consecutive patients undergoing infrainguinal arterial surgery had transcutaneous partial pressure of oxygen (TcpO(2)) measured preoperatively, intraoperatively after arterial clamps applied, postoperatively and at discharge. Measurements were taken with inspired oxygen concentration (F(i)O(2)) of 30% then 80%. Arterial blood gases were measured at the same times. RESULTS: Tissue oxygenation showed no difference intraoperatively while arterial clamps were in place, but significantly higher tissue oxygenation was seen with use of high-dose oxygen (F(i)O(2) 80%) postoperatively (P<0.05). Carbon dioxide levels in tissue increased while arterial clamps were in place (P<0.01) and pH fell intraoperatively and following reperfusion (P<0.05). CONCLUSION: The administration of high-dose oxygen to vascular surgical patients undergoing lower-limb arterial surgery results in increased tissue oxygen concentrations when perfusion is not reduced by the presence of arterial clamps. These results suggest the administration of high-dose oxygen intraoperatively may be beneficial in reducing wound infections, but further research is required.     

Supplemental oxygen and carbon dioxide each increase subcutaneous and intestinal intramural oxygenation

Oxidative killing by neutrophils, a primary defense against surgical pathogens, is directly related to tissue oxygenation. We tested the hypothesis that supplemental inspired oxygen or mild hypercapnia (end-tidal PCO2 of 50 mm Hg) improves intestinal oxygenation. Pigs (25 +/- 2.5 kg) were used in 2 studies in random order: 1) Oxygen Study: 30% versus 100% inspired oxygen concentration at an end-tidal PCO2 of 40 mm Hg, and 2) Carbon Dioxide Study: end-tidal PCO2 of 30 mm Hg versus 50 mm Hg with 30% oxygen. Within each study, treatment order was randomized. Treatments were maintained for 1.5 h; measurements were averaged over the final hour. A tonometer inserted in the subcutaneous tissue of the left upper foreleg measured subcutaneous oxygen tension. Tonometers inserted into the intestinal wall measured intestinal intramural oxygen tension from the small and large intestines. Oxygen 100% administration doubled subcutaneous oxygen partial pressure (PO2) (57 +/- 10 to 107 +/- 48 mm Hg, P = 0.006) and large intestine intramural PO2 (53 +/- 14 to 118 +/- 72 mm Hg, P = 0.014); intramural PO2 increased 40% in the small intestine (37 +/- 10 to 52 +/- 25 mm Hg, P = 0.004). An end-tidal PCO2 of 50 mm Hg increased large intestinal PO2 approximately 16% (49 +/- 10 to 57 +/- 12 mm Hg, P = 0.039), whereas intramural PO2 increased by 45% in the small intestine (31 +/- 12 to 44 +/- 16 mm Hg, P = 0.002). Supplemental oxygen and mild hypercapnia each increased subcutaneous and intramural tissue PO2, with supplemental oxygen being most effective.     

Tissue oxygenation response to mild hypercapnia during cardiopulmonary bypass with constant pump output

Background. Tissue oxygenation is the primary determinant ofwound infection risk. Mild hypercapnia markedly improves cutaneous,subcutaneous (s.c.), and muscular tissue oxygenation in volunteersand patients. However, relative contributions of increased cardiacoutput and peripheral vasodilation to this response remainsunknown. We thus tested the hypothesis that increased cardiacoutput is the dominant mechanism. Methods. We recruited 10 ASA III patients, aged 40–65yr, undergoing cardiopulmonary bypass for this crossover trial.After induction of anaesthesia, a Silastic tonometer was inserteds.c. in the upper arm. S.C. tissue oxygen tension was measuredwith both polarographic electrode and fluorescence-based systems.Oximeter probes were placed bilaterally on the forehead to monitorcerebral oxygenation. After initiation of cardiopulmonary bypass,in random order patients were exposed to two arterial CO₂ partialpressures for 30 min each: 35 (normocapnia) or 50 mm Hg (hypercapnia).Bypass pump flow was kept constant throughout the measurementperiods. Results. Hypercapnia during bypass had essentially no effecton , mean arterial pressure, or tissue temperature. and pH differed significantly. S.C. tissue oxygenation was virtuallyidentical during the two periods [139 (50–163) vs 145 (38–158), P=0.335] [median(range)]. In contrast, cerebral oxygen saturation (our positivecontrol measurement) was significantly less during normocapnia[57 (28–67)%] than hypercapnia [64 (37–89)%, P=0.025]. Conclusions. Mild hypercapnia, which normally markedly increasestissue oxygenation, did not do so during cardiopulmonary bypasswith fixed pump output. This suggests that hypercapnia normallyincreases tissue oxygenation by increasing cardiac output ratherthan direct dilation of peripheral vessels.     

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.     

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.     

Effect of Acute Respiratory Acidosis on the Limits of Oxygen Extraction during Hemorrhage

Background: Hypercapnia can impair cells' capacity to maintain energy status anerobically and enhances the risk of hypoxic injury when oxygen availability is reduced. The ability to maintain tissue oxygenation is determined by both bulk blood flow and the efficiency of oxygen extraction. Bulk blood flow is maintained during hypercapnia through increased sympathetic activity. The effect of hypercapnia on oxygen extraction, however, is unknown. This study evaluates the effect of hypercapnia on cells' capacity to adapt to reductions in oxygen availability by increasing oxygen extraction.

Methods: In three groups of paralyzed, mechanically ventilated dogs that were anesthetized with alpha-chloralose, the concentration of carbon dioxide in the inhaled gas mixture was adjusted to achieve normocapnia, moderate hypercapnia (PaCO2 = 72 +/- 3 [SE] mmHg) or severe hypercapnia (PaCO2 = 118 +/- 4 [SE] mmHg). Stepwise hemorrhage was induced until each dog's blood pressure was destabilized. At each stage in the hemorrhage protocol, the oxygen delivery, oxygen consumption, and oxygen extraction ratios (ratio of arteriovenous oxygen content difference to arterial oxygen content) were determined.

Results: At the point of onset of delivery dependence of oxygen consumption, the oxygen delivery rate (critical oxygen delivery) was 7.8 +/- 1.5 (SE) ml [dot] kg sup -1 [dot] min sup -1 and the oxygen extraction ratio (critical oxygen extraction ratio) was 0.72 +/- 0.04 (SE) in the normocapnic dogs. Moderate hypercapnia had no effect on these parameters. In the severely hypercapnic dogs, the critical values for oxygen delivery and extraction ratios were 12.5 +/- 1.8 (SE) ml [dot] kg sup -1 [dot] min sup -1 and 0.54 +/- 0.035 (SE), respectively (P < 0.05 for differences from the normocapnic dogs).     

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.     

Effects of carbon dioxide (hypocapnia and hypercapnia) on tissue blood flow and oxygenation of liver, kidney and skeletal muscle in the dog

We investigated the effects of carbon dioxide on the splanchnic visceral organs (liver and kidney) as well as skeletal muscle in the anesthetized dog. Thirty two adult mongrel dogs were anesthetized with sodium pentobarbital, intubated and ventilated mechanically with 100% oxygen to maintain normocapnia. After laparotomy, miniature Clark-type polarographic oxygen electrodes were placed on the surfaces of liver, kidney and rectus femoris muscle. Electromagnetic blood flow (BF) probes were also applied to hepatic artery (HA), portal vein (PV), left renal artery (RA) and left femoral artery (FA). After a stable normocapnic ventilation, the hypocapnia was produced by increasing respiratory rate, and the hypercapnia was induced by adding the exogenous carbon dioxide. Results: Hyperventilation resulted in a significant decrease in HABF, PVBF, liver surface PO2 and kidney surface PO2 in parallel with the decreased PaCO2, but these parameters increased dose dependently when the carbon dioxide was added to the inspired gas (hypercapnic hyperventilation). On the contrary, FABF and skeletal muscle surface PO2 increased by hypocapnia and decreased during hypercapnia. Neither PaCO2 or cardiac output showed any significant change during the entire experiment. Arterial PCO2 appears to exert significant effects on both splanchnic and skeletal muscle perfusion as well as corresponding changes in tissue oxygenations. It is possible that injudicious and prolonged hypocapnic hyperventilation may seriously compromise splanchnic organ perfusion and oxygenation.     

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.