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.     

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.     

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.     

Directly measured tissue oxygen tension and arterial oxygen tension assess tissue perfusion

Mean subcutaneous tissue PO2 (PsqO2) measurements were obtained in dogs with an unheated electrode placed in an implanted Silastic tonometer, while PaO2 was increased in increments from 40 to 600 torr during normal, increased, and reduced blood volume. These changes reflect that the mean PsqO2 is approximately 10 torr below the PO2 of venous blood draining that tissue. Since PaO2 was already known, the oxygen content of arterial and venous blood entering and leaving this tissue could be determined by reference to blood-oxygen dissociation curves. Therefore, relative changes in blood flow could be calculated using the Fick principle. After a 20% blood loss, the PsqO2 measured during breathing of room air fell to 20% of baseline, corresponding to an 80% fall in sc blood flow; it remained low until the shed blood was returned despite compensatory changes in cardiac output. Rapid infusion of electrolyte solutions in normovolemic animals produced a temporary increase in local blood flow. Subcutaneous oximetry seems capable of quantifying peripheral perfusion and may be clinically useful.     

Effect of Intralipid-induced lipaemia on the arterial oxygen tension in preterm infants.

10% Intralipid was infused (1 g/kg body wt.) intravenously for 15 min with Razel pumps to each of eight selected newborn preterm infants whose general condition and pulmonary status were stable for at least 24 h before study. Six of the eight infants showed greater than 10 mmHg reduction in oxygen tension of umbilical arterial blood. These reductions were correlated with elevated triglyceride concentrations. The findings suggest that Intralipid-induced lipaemia has the potential to lower the oxygen tension of the blood and hence patients with pre-existing pulmonary insufficiency may be at risk from hypoxaemia.     

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 effect of nonnutritive sucking on transcutaneous oxygen tension in noncrying, preterm neonates.

The relationship between transcutaneous oxygen tension (tcpO_2:) and sucking opportunities in noncrying, preterm neonates was investigated. Twenty-six measurements of nonnutritive sucking and its relationship to tcpO₂ were collected on 11 neonates who were monitored continuously for tcpO₂. Of the 26, 10 sets of measurements were recorded on 6 neonates receiving assisted ventilation, and 16 were recorded on 4 neonates breathing room air. One neonate was measured one time on assisted ventilation and two times breathing room air. The design consisted of three 8-minute time periods: pretreatment, treatment (sucking opportunities). and posttreatment. There was a significant relationship between the pretreatment-treatment and treatment-posttreatment tcpO₂ levels (both at p < .01). These results suggest that sucking facilitates more adequate oxygenation in noncrying, preterm neonates.     

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.     

Extrapulmonary factors influencing the ratio of arterial oxygen tension to inspired oxygen concentration in burn patients.

The ratio of arterial oxygen tension to inspired oxygen concentration (PaO2/FIO2) as an index of respiratory function was evaluated in 22 patients with body surface area burns of 15--80%. These results indicate that this ratio is limited in its applicability because extrapulmonary factors, such as cardiac output, oxygen consumption, and arterial oxygen content, can affect this index by alterations in the amount of venous desaturation. Useful estimates of intrapulmonary right to left shunt (Qs/Qt) from PaO2/FIO2 were obtained only when arteriovenous oxygen content differences (avDO2) were between 3--5 ml/dl. There were avDO2 values above and below 3--5 ml/dl in at least 35% of the observations. Under these circumstances, PaO2/FIO2 did not correctly reflect changes in Qs/Qt. Blood gases from central venous catheters did not mirror changes in true mixed venous blood and, thus, can lead to erroneous estimations of Qs/Qt. Rational therapy of reduced arterial oxygen saturation requires measurement of both extra- and intrapulmonary factors contributing to arterial desaturation. Measurement of PaO2/FIO2 alone will not estimate these factors.     

The effect of ciprofloxacin on antipyrine metabolism

The effect of multiple-dose ciprofloxacin on antipyrine metabolism was studied in patients suffering from bacterial infections. The patients were given antipyrine 15 mg/kg intravenously before and after ciprofloxacin treatment. The dosage of ciprofloxacin was 500 mg bd by mouth for 8-10 days. Blood samples were taken at 0, 2, 4, 6, 10 h. Antipyrine total clearance was significantly decreased after ciprofloxacin treatment (0.85 +/- 0.45 vs. 0.52 +/- 0.24 ml/min/kg): elimination rate constants for antipyrine were decreased in all patients after ciprofloxacin, whereas no change in volume of distribution was observed. The average half-life of antipyrine was increased from 9.45 +/- 3.74 h to 14.92 +/- 3.32 h. In two males with advanced chronic hepatic failure the antipyrine half-lives were extremely prolonged. Our results support the hypothesis that ciprofloxacin inhibits intrinsic hepatic drug-metabolizing capacity and may be a source of clinically important drug interactions, particularly in patients with liver disease.     

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.     

Effect of arterial revascularization on transcutaneous oxygen tension of the ischemic extremity

In 20 patients (24 limbs) with peripheral occlusive arterial disease involving the lower extremities, foot and chest transcutaneous oxygen tension (tcPO2) and ankle and arm systolic blood pressures were measured, with the patient's legs horizontal and with them in an elevated position, before and after revascularization procedures. Eighteen of the procedures were unilateral and were performed to alleviate severe ischemia; the three bilateral procedures were done to relieve intermittent claudication. Regional perfusion index (RPI) was calculated for each foot (RPI = tcPO2 foot/tcPO2 chest) with the legs horizontal and with the legs elevated for 3 minutes (RPI3). The ankle/brachial index was calculated from ankle and arm systolic blood pressures. Limbs with severe ischemia had considerably decreased RPI and RPI3 before revascularization, whereas limbs affected by claudication had only a modest decrease in RPI but a pronounced decrease in RPI3. The tcPO2, RPI, and RPI3 increased substantially after revascularization.     

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.     

Transcutaneous versus arterial oxygen tension during exercise in chronic pulmonary disease

Summary. Simultaneous transcutaneous (P_tc O₂) and arterial (P_a O₂) oxygen tension measurements were obtained at rest and during exercise in 15 subjects with known or suspected chronic lung disease. The overall correlation coefficient between P_tc O₂ and P_a O₂ was 0·74, and the mean difference between the two variables was — 1·9kPa (14 mmHg). The correlation coefficient was somewhat, but not significantly, higher during exercise (r=0·86) than at rest (r=0·58). Large random discrepancies between simultaneous observations of P_tc O₂ and P_a O₂ as well as between changes in P_tc O₂ and changes in P_a O₂ in individual subjects were observed. It is concluded that P_tc O₂ cannot be substituted for P_a O₂ in exercise testing applied in the diagnosis and assessment of lung disease.     

Effects of endotracheal tube suctioning on arterial oxygen tension and heart rate variability

The purpose of this study was to examine the autonomic mechanisms underlying changes in heart rate (HR) and systolic blood pressure (SBP) responses to endotracheal tube (ETT) suctioning and to compare the open versus closed methods of ETT suctioning on these measures and on arterial oxygen tension. Eighteen orally intubated participants, 33 to 82 years of age (M = 60 years), were randomized for the order of suctioning method. Arterial oxygen tension (PaO2) was measured before suctioning and 30 s and 5 min following suctioning. Beat-to-beat HR and arterial blood pressure data were collected for 10-min periods before and after suctioning. HR and SBP measures were analyzed before suctioning and 1 min and 5 min following suctioning. Although there were no significant effects of ETT suctioning on the autonomic mechanisms of HR modulation and no significant differences between the two methods of suctioning, ETT suctioning resulted in an increase in HR, SBP, and PaO2. However, there was a decrease in the parasympathetic nervous system indicator of HR variability (HRV) following open suctioning. All patients in this study maintained a PaO2 level 80 mm Hg, which may account for our lack of significant autonomic changes. This suggests that hyperoxygenation with 100% oxygen for a minimum of 1 min (or 20 breaths), as delivered by preoxygenation modes available on most microprocessor ventilators, should be the method of choice for all hyperoxygenation procedures to avoid a decrease in PaO2 following suctioning.