The influence of different levels of PEEP on peripheral tissue perfusion measured by subcutaneous and transcutaneous oxygen tension

Objective To compare subcutaneous (PscO₂) and transcutaneous (PtcO₂) oxygen tension measurements in relation to hemodynamic variables at different levels of PEEP, and to evaluate the usefulness of these measurements as monitors of peripheral tissue perfusion.Design Prospective trial.Setting Intensive care unit in a university hospital.Patients Seven patients with gastric cancer who where undergoing total gastrectomy.Interventions Silicone catheter was placed in the upper arm and transcutaneous oxygen monitor was placed on the upper part of the chest. A pulmonary artery catheter was placed in the right pulmonary artery.Measurements and results PscO₂ and PtcO₂ together with hemodynamic variables were measured at different levels of PEEP. Progressive increase of PEEP reduced cardiac index (CI) (p<0.05) with a concomitant decrease of PscO₂ (p<0.05) and oxygen delivery (DO₂) (p<0.05). Changes in PtcO₂ parallelled changes in arterial oxygen tension (PaO₂), but no correlation was found between PtcO₂, CI and DO₂.Conclusion PscO₂ is a sensitive indicator of subcutaneous tissue perfusion, which can be used to identify the PEEP level, with optimum peripheral perfusion. PscO₂ seems to be a more reliable indicafor of tissue perfusion than PtcO₂.This study was supported by Tore Nilsons Fund for Medical Research. Lundgrens Stiftelse, Medical Faculty, University of Lund and by the Swedish Medical Research Council Projecr no. B88-17x-00640-24B     

Conjunctival oxygen tension and its relationship to arterial oxygen tension

Using a miniaturized Clark electrode embedded in a polymethylmethacrylate eyepiece, we measured transconjunctival oxygen tension (PcjO₂) in 5 healthy volunteer subjects at multiple levels of steady-state isocapnic hypoxia, normoxia, and hyperoxia. PcjO₂ was linearly related to arterial oxygen tension (PaO₂) as PaO₂ ranged from 28 to 205 mm Hg (PcjO₂=0.59 PaO₂+0.36 mm Hg;r=0.94; standard error of the estimate=7.09 mm Hg). However, the relationships between PcjO₂ and PaO₂ varied significantly among subjects. Whereas the overall mean ratio of PcjO₂ to PaO₂ was 0.59, the mean ratio for subjects ranged from 0.47 to 0.79 and was significantly different among subjects (P<0.0001). The time response of the electrode to a step change in oxygen tension in vitro was exponential, with a 90% response time of 38 seconds after a lag of 3.7 seconds. The time responses to in vivo changes in oxygen tension were also exponential. From hypoxia to normoxia, 90% response time was 45.0 seconds after a lag of 5.1 seconds; from room air to hypoxia, 90% response time was 72.4 seconds after a lag of 30.3 seconds; from room air to hyperoxia, 90% response time was 87.2 seconds after a lag of 6.8 seconds. We conclude that, although PcjO₂ measured by a miniaturized Clark electrode is linearly related to PaO₂ in healthy subjects, variation in the relationship of PcjO₂ to PaO₂ among individuals will prevent precise estimation of PaO₂ for any individual unless subject-specific calibration is performed.     

Transorally obtained oxygen tension as an indicator of arterial oxygen tension

Transcutaneous oxygen electrodes have been used with success in neonates as indicators of arterial oxygenation, but with less success in adults because of differences in skin thickness and vascularity. In this study a prototype transoral oxygen electrode was evaluated to determine if a heated mucous membrane would yield arterialized values of oxygen tension in adults. Using a miniaturized Clark electrode, we measured transoral oxygen tension (PtoO₂) in 29 subjects at steady-state conditions. Simultaneously a sample was anaerobically obtained from a radial artery for measurement of arterial oxygen tension (PaO₂). Data were analyzed using linear regression analysis, Student'st test, and analysis of variance. There was no statistically significant difference between non-white and white subjects or male and female subjects. There was a highly significant difference (P<0.001) between the pooled, matched values for PtoO₂ versus PaO₂, and the regression between the PtoO₂ and the PaO₂ was linear (slope 0.92, y-intercept ?8.37,r=0.62,P<0.003). The calculated ratio of PtoO₂ to PaO₂ was 0.83±0.03 (standard error). We concluded that the PtoO₂ was linearly related to the PaO₂, although its accuracy in reflecting PaO₂ was low. This finding correlates with previously published data that suggested that the PtoO₂ reflects tissue oxygen tension rather than arterialized oxygen tension. Gender and race appeared not to affect the function of the electrode in our study.     

Cerebral perfusion pressure and cerebral tissue oxygen tension in a patient during cardiopulmonary resuscitation

OBJECTIVE: To report on the effects of cardiopulmonary resuscitation (CPR) instituted immediately after a cardiac arrest on cerebral perfusion pressure (CPP) and cerebral tissue oxygen tension (PbrO(2)). DESIGN: Case report. SETTING: ICU of a university hospital. PATIENT: A head-injured 17-year-old man submitted to multimodal neurological monitoring underwent sudden cardiac arrest and successful CPR. INTERVENTIONS: External chest compression, 100% oxygen ventilation, volume expansion and standard ACLS protocols. MEASUREMENTS AND RESULTS: Heart rate, ECG, mean arterial blood pressure (MABP), ETCO(2), PaO(2), intracranial pressure (ICP), CPP and PbrO(2) were continuously monitored during CPR and data recorded at 15-s intervals by a dedicated personal computer. At the onset of the cardiac arrest, PbrO(2) decreased to zero. The institution of CPR resulted in a progressive increase of MABP, CPP and PbrO(2). Assuming, on the basis of previous experimental and clinical reports, 8 mmHg PbrO(2) as a possible ischaemic/hypoxic threshold value, during the first 6.5 min of CPR, PbrO(2) values were below this threshold (range 0-7 mmHg) and CPP values were <25 mmHg for 81.5% of the time. In the following 5.5 min, more efficient CPR generated CPP values >25 mmHg for 77.3% of the time. These values were associated with a PbrO(2) >8 mmHg (range 8-28 mmHg) at all times. CONCLUSIONS: In the clinical setting of a witnessed cardiac arrest, immediate institution of CPR can be effective in generating PbrO(2) values above a supposed ischaemic/hypoxic threshold when CPP is >25 mmHg. PbrO(2) monitoring by the Licox system is sensitive and reliable, even at low values, and can be suitable for evaluating cerebral oxygenation during experimental CPR.     

Continuous fiberoptic arterial oxygen tension measurements in dogs

An experimental study using a new fiberoptic sensor for the continuous intraarterial measurement of oxygen tension is described. This optode sensor uses the phenomenon of fluorescence quenching to determine the oxygen tension of the surrounding medium. To assess the accuracy of this device, we anesthetized 4 dogs and monitored them continuously with arterial catheters and an intraarterial optode probe, and intermittently with arterial blood gas analysis. The inspired oxygen fraction was varied from 1.0 to 0.1, and arterial blood gases were measured for comparison with the optode reading. Two hundred ninety data sets yielded a correlation coefficient of 0.96, with a linear regression slope of 0.98 and intercept of 5.1 mm Hg. In the 72 data sets from the last dog, the bias and precision of the optode arterial oxygen tension values were –10.3 mm Hg and 20.0 mm Hg, respectively. The optode probe was easily inserted through a 20-gauge catheter and did not interfere with continuous arterial pressure measurement or blood sampling. This study suggests that the optode has great potential as a continuous, real-time monitor of arterial oxygen tension.     

The measurement of oxygen tension in tissue

Delivery of oxygen to cells is critical to biochemical and metabolic processes and is particularly important during conditions of illness and recovery. However, accurate measurement of perfusion and tissue oxygen tension, determinants of cellular oxygen supply, has been problematic. Technologic advancements offer promise for the assessment of these significant variables. Instruments utilizing polarographic electrodes have been commonly used for the measurement of tissue oxygen tension. Recently, optical fluorescence has been employed to measure the partial pressure of oxygen. These predominant methods for determining tissue oxygen tension are described and applications for research are discussed.     

Effect of oxygen affinity on systemic perfusion and brain tissue oxygen tension after extreme hemodilution with hemoglobin–starch conjugates in rats

Purpose  

To determine the oxygen affinity for optimal tissue oxygen delivery with a hemoglobin–hydroxyethyl starch conjugate (HRC 101).     

Exceptionally high arterial oxygen tension in diabetic ketoacidosis.

We recently managed a patient with diabetic ketoacidosis who had an exceptionally high arterial oxygen tension of 144 mm Hg while breathing room air. This was higher than the calculated alveolar oxygen tension and suggested the impossible situation of an uphill diffusion gradient. We were surprised to find this is a relatively common phenomenon (occurring in 25% of a retrospective study of similar patients). There are a number of probable mechanisms responsible for this observation, all of which reflect the severe metabolic aberration occurring in these patients.     

Tissue oxygenation in hemorrhagic shock measured as transcutaneous oxygen tension, subcutaneous oxygen tension, and gastrointestinal intramucosal pH in pigs

BACKGROUND AND METHODS: Tissue oxygenation, measured in peripheral tissue as transcutaneous PO2 (PtCO2) and subcutaneous PO2, was compared with the oxygenation in GI mucosa, which was measured as intramucosal wall pH (pHi), during experimental hemorrhagic shock and resuscitation in pigs. The pigs were hemorrhaged stepwise to a BP of 80 and 45 mm Hg, followed by retransfusion. PtCO2 was measured in the groin and subcutaneous PO2 was measured in the hip region. Intraluminal PCO2 was measured in the stomach, in the small intestine, and the sigmoid colon using silicone catheters. A simultaneous determination of arterial blood HCO3 concentration allowed pHi to be calculated using Henderson-Hasselbalch equation. Cardiac output was determined by thermodilution, and oxygen delivery (DO2) was calculated. RESULTS: Early indications of shock were decreases in PtCO2 and intestinal pHi (p less than .01). All measured variables decreased at the second step of bleeding. PtCO2 and subcutaneous PO2 was correlated to DO2 through the entire experiment (r2 = .25 and .49, respectively). Also, the pHi of the small intestine and the sigmoid colon correlated with DO2 (r2 = .36 and .25, respectively). PtCO2 and subcutaneous PO2 correlated with pHi in the small intestine and sigmoid colon. CONCLUSIONS: PtCO2 and pHi in the small intestine and sigmoid colon were the variables that most rapidly indicated blood volume loss. Subcutaneous PO2 and PtCO2, and small intestine and sigmoid colon pHi were correlated to total body oxygen transport. Peripheral tissue perfusion followed intestinal perfusion to some extent.     

Prediction of changing cerebral blood flow by use of the conjunctival oxygen tension/arterial oxygen tension index

Current methods of assessing cerebral blood flow (CBF) are limited in their ability to provide data at the bedside in a timely, inexpensive, and continuous fashion. Since the palpebral conjunctiva is perfused by branches of the internal carotid artery, perfusion of this tissue may reflect global CBF. Conjunctival oxygen tension (PcjO2), PaO2, PaCO2, and pH were measured in ten healthy subjects during normal ventilation and active hyperventilation. CBF was measured simultaneously using positron emission tomography. CBF decreased from an average of 64.3 +/- 15.1 ml x 100 g-1 x min-1 during baseline measurements to 33.2 +/- 8.4 ml x 100 g-1 x min-1 during hyperventilation. The ratio of PcjO2 to PaO2 (the PcjO2/PaO2 index) decreased from 0.53 +/- 0.07 to 0.35 +/- 0.09 in the same time period. The PcjO2/PaO2 index was significantly correlated with CBF (r = .78, p less than .001). We conclude that the PcjO2/PaO2 index may reflect the reduction in CBF induced by hyperventilation in normal humans, and should be investigated further as a method of assessing CBF in other settings which can result in globally reduced cerebral perfusion.     

The impact of arterial oxygen tension on venous oxygen saturation in circulatory failure

Central and mixed venous oxygen saturations have been used to guide resuscitation in circulatory failure, but the impact of arterial oxygen tension on venous oxygen saturation has not been thoroughly evaluated. This observational study investigated the impact of arterial oxygen tension on venous oxygen saturation in circulatory failure. Twenty critically ill patients with circulatory failure requiring mechanical ventilation and a pulmonary artery catheter in an intensive care unit in a tertiary hospital in Western Australia were recruited. Samples of arterial blood, central venous blood, and mixed venous blood were simultaneously and slowly drawn from the arterial, central venous, and pulmonary artery catheter, respectively, at baseline and after the patient was ventilated with 100% inspired oxygen for 5 min. The blood samples were redrawn after a significant change in cardiac index (>or =10%) from the baseline, occurring within 24 h of study enrollment while the patient was ventilated with the same baseline inspired oxygen concentration, was detected. An increase in inspired oxygen concentration significantly increased the arterial oxygen tension from 12.5 to 38.4 kPa (93.8-288 mmHg) (mean difference, 25.9 kPa; 95% confidence interval [CI], 7.5-31.9 kPa; P < 0.001) and the venous oxygen saturation from 69.9% to 76.5% (mean difference, 6.6%; 95% CI, 5.3% - 7.9%; P < 0.001). The effect of arterial oxygen tension on venous oxygen saturation was more significant than the effect associated with changes in cardiac index (mean difference, 2.8%; 95% CI, -0.2% to 5.8%; P = 0.063). In conclusion, arterial oxygen tension has a significant effect on venous oxygen saturation, and this effect is more significant and consistent than the effect associated with changes in cardiac index.     

Relationship of pulse oximetry to arterial oxygen tension in infants

Pulse oximetry is a useful technique for noninvasive oxygen monitoring in sick infants. We simultaneously measured oxygen saturation by pulse oximetry and on arterial blood samples by co-oximetry as well as PaO2 and the relative content of fetal (F) and adult hemoglobin in order to evaluate the reliability of pulse oximetry. Comparisons were made in triplicate in ten infants with acute cardiorespiratory disease less than 7 days of age and in 11 infants with chronic lung disease greater than 28 days of age. Oxygen saturation pulse oximetry and arterial saturation were well correlated over a wide range of saturation values. In infants with chronic lung disease, PO2 derived from pulse oximetry was within 10 torr of measured PaO2 in 73% of comparisons. In contrast, calculated PaO2 was within 10 torr of measured PaO2 in only 50% of comparisons in patients with acute disorders. The chronic infants all had less than 10% hemoglobin F, but in the acute infants, hemoglobin F ranged from 26% to 83%. Nonetheless, correction of oxygen dissociation curves for type of hemoglobin in these acute infants failed to improve the correlation between calculated and measured PaO2. We conclude that pulse oximetry saturations and their derived PaO2 values correlated well with measured arterial saturation and PaO2 obtained from arterial blood samples in neonates with chronic lung disease and prolonged oxygen dependence. In infants with acute cardiorespiratory problems, pulse oximetry unreliably reflects PaO2, but may be useful in detecting clinical deterioration.     

Measurement of tissue oxygen tension: Comparison between two subcutaneous oxygen tonometers

We compared the 95% response time (95% RT) of two tissue oxygen tonometers under two sets of circumstances. We first evaluated the devices during normoxia, hyperoxia, and anoxia in vitro, using a transcutaneous PO₂ electrode (PtcO₂) as the reference. The responses to normoxia and to different grades of hyperoxia were examined in vivo in 8 healthy volunteers to assess the relationship between changes in subcutancous PO₂ and PtcO₂, an estimate of arterial PO₂ (PaO₂). One subcutaneous method (ScA) used a technique based on a polarographic needle electrode in situ connected to an ammeter; the second method (ScB) was based on a blood gas analyzer system first described by Hunt (Lancet 164;2:1370). ScA and PtcO₂ both responded to stepwise changes in ambient oxygen concentration (21–100%) in vitro within 10 seconds; the 95% RT of ScA was 1.39±0.5 to 2.39±0.8 minutes and that of PtcO₂ was 0.32±0.1 to 0.49±0.1 minutes. ScB had a lag of 3 minutes, and the 95% RT was 6.75±0.5 to 8.2±0.8 minutes. In contrast to the results in vitro, the response of ScA to changes in FiO₂ in vivo was delayed compared with the rapid response of PtcO₂, reflecting the physiologic delay of tissue PO₂ in response to increased PaO₂. The time lag and the long 95% RT of ScB were even more evident in vivo. ScA reacted three to four times faster than ScB, both in vitro and in vivo, to changes in the oxygen environment. The in vitro 95% RT of ScA to changes in ambient oxygen varied from 2 to 3.5 minutes. In contrast, the 95% RT of ScB was 8 to 9 minutes. PtcO₂ had the fastest 95% RT, from 0.4 to 0.5 minutes. The results suggest that the subcutaneous needle electrode method (ScA) provides close to real-time assessment of tissue PO₂.Supported by the Paulo Foundation and the Orion Foundation of Research.We thank the local distributor of Radiometer, Oriola Oy, for the loan of TINA.     

Transcutaneous carbon dioxide and oxygen tension measured at different temperatures in healthy adults

Transcutaneous carbon dioxide tension (tc-pco2) at 37, 39, 41, 43, and 45 degrees C, and transcutaneous oxygen tension (tc-po2) at 41, 43, and 45 degrees C were measured simultaneously in 10 healthy adults during hyperventilation and inhalation of O2/CO2 gas. Nine electrodes were applied to each subject: Five CO2 electrodes, one O2 electrode, and three combined O2/CO2 electrodes. The CO2 electrodes had negligible temperature coefficients in the calibration gases, but the O2 electrodes showed an increase in po2 of 4.5% per degree C. With skin application, tc-pco2 increased approximately 4% per degrees C between 37 and 45 degrees C, which is close to the anaerobic temperature coefficient of pco2 in blood. The tc-po2 increases on the skin with increasing temperature appeared to be more dependent on changes in blood flow in skin, but in the temperature range 43 to 45 degrees C, tc-po2 showed the expected decrease in the temperature coefficient with increasing po2. The correlation between transcutaneous and capillary pco2 was close at all transcutaneous electrode temperatures, even 37 degrees C, provided the skin was preheated (via the electrode) to 45 degrees C. For tc-po2, an electrode temperature of at least 43 degrees C was necessary to produce a reasonable correlation between tc-po2 and capillary po2. The combined O2/CO2 electrodes measured slightly higher pco2 values than the single CO2 electrodes, but there were no differences in po2 readings, stabilization time, imprecision, or electrode drift between the two electrode types. The imprecision (CV, %) of tc-pco2 and tc-po2 measurements was approximately twice that of the corresponding capillary blood-gas measurements.     

Noninvasive arterial hemoglobin oxygen saturation versus transcutaneous oxygen tension monitoring in the preterm infant

We found that results from a transcutaneous arterial hemoglobin oxygen-saturation monitor correlated well with those from a co-oximeter. The monitor was not disturbed by differing hematocrit levels, the presence of fetal hemoglobin, or hypotension. We also found that the results of simultaneous transcutaneous arterial hemoglobin oxygen saturation (StcaO2) and transcutaneous oxygen tension (PtcO2) monitoring were predictably correlated over a wide range of hemoglobin saturations in preterm infants. When StcaO2 was between 80% and 95%, PtcO2 was at a safe level of 40 to 80 torr in 94% of the patients studied. StcaO2 monitoring as an index of arterial oxygenation has several advantages for the preterm infant.     

Effects of epinephrine on intestinal oxygen supply and mucosal tissue oxygen tension in pigs

OBJECTIVE: To study the effects of increasing dosages of epinephrine given intravenously on intestinal oxygen supply and, in particular, mucosal tissue oxygen tension in an autoperfused, innervated jejunal segment. DESIGN: Prospective, randomized experimental study. SETTING: Animal research laboratory. SUBJECTS: Domestic pigs. INTERVENTIONS: Sixteen pigs were anesthetized, paralyzed, and normoventilated. A small segment of the jejunal mucosa was exposed by midline laparotomy and antimesenteric incision. Mucosal oxygen tension was measured by using Clark-type surface oxygen electrodes. Microvascular hemoglobin oxygen saturation and microvascular blood flow (perfusion units) were determined by tissue reflectance spectrophotometry and laser-Doppler velocimetry. Systemic hemodynamics, mesenteric-venous acid-base and blood gas variables, and systemic acid-base and blood gas variables were recorded. Measurements were performed after a resting period and at 20-min intervals during infusion of increasing dosages of epinephrine (n = 8; 0.01, 0.05, 0.1, 0.5, 1, and 2 microg x kg(-1) x min(-1)) or without treatment (n = 8). In addition, arterial and mesenteric-venous lactate concentrations were measured at baseline and at 60 and 120 mins. MEASUREMENTS AND MAIN RESULTS: Epinephrine infusion led to significant tachycardia; an increase in cardiac output, systemic oxygen delivery, and oxygen consumption; and development of lactic acidosis. Epinephrine significantly increased jejunal microvascular blood flow (baseline, 267 +/- 39 perfusion units; maximum value, 443 +/- 35 perfusion units) and mucosal oxygen tension (baseline, 36 +/- 2.0 torr [4.79 +/- 0.27 kPa]; maximum value, 48 +/- 2.8 torr [6.39 +/- 0.37 kPa]) and increased hemoglobin oxygen saturation above baseline. Epinephrine increased mesenteric venous lactate concentration (baseline, 2.9 +/- 0.6 mmol x L(-1); maximum value, 5.5 +/- 0.2 mmol x L(-1)) without development of an arterial-mesenteric venous lactate concentration gradient. CONCLUSIONS: Epinephrine increased jejunal microvascular blood flow and mucosal tissue oxygen supply at moderate to high dosages. Lactic acidosis that develops during infusion of increasing dosages of epinephrine is not related to development of gastrointestinal hypoxia.     

The influence of tissue oxygen and perfusion on wound healing

The availability of oxygen (O2) to cells in the wound area and the presence of adequate blood flow are important factors to the healing process. Oxygen plays a critical role in the formation of collagen, the growth of new capillaries, and the control of infection. Perfusion and delivery of O2 to tissue are closely related. Although an adequate blood flow does not guarantee a sufficient supply of O2, without it the provision of O2 to healing tissues will be impaired. Basic scientific studies have clarified how O2 and blood flow influence healing. Recent research has focused on clinical populations and begins to provide direction for additional clinical studies and interventions to support the healing process. Based on existing research, clinical interventions aimed to maintain perfusion and supply of O2 include fluid volume assessments, pulmonary hygiene regimens, postoperative position changes, and ambulation.