Anthony V. Beran

In Vivo Determination of Renal Tissue Oxygenation During Pulsatile and Nonpulsatile Left Heart Bypass

Abstract Data favoring pulsatile over nonpulsatile left heart bypass are conflicting. To study this problem, renal cortical and medullary tissue oxygen availability (O 2 a) and total renal oxygen flow were compared with total renal oxygen consumption during four hours of pulsatile and nonpulsatile bypass in dogs. Although there was no difference in renal tissue O 2 a during the first two hours, thereafter the decrease in renal O 2 a was greater during nonpulsatile bypass; total renal blood flow was greater with pulsatile bypass, and at the given renal oxygen flow, renal oxygen consumption was greater during pulsatile bypass. These data, along with previous work, support the superiority of pulsatile bypass.

An Improved Sensor and a Method for Transcutaneous CO2 Monitoring

Since CO2 diffuses readily through intact skin along a reasonably short diffusion path, measurement of transcutaneous CO2 (tcPco2) is possible, provided the CO2 consumption of the electrode is minimal and epidermal capillary blood flow and stratum corneum permeability are maintained well.

Capillary pH and blood gas determinations in asthmatic children

Earlobe capillary pH and blood gases in asthmatic children were compared to arterial values. Hyperemia of the earlobe was produced by applying thurfyl nicotinate (Trafuril). Approximately 15 min later capillary blood was obtained simultaneously with a radial arterial sample. Earlobe capillary PO2, PO2, and pH correlated well with arterial values. This technique is simple, safe, and appears to be a satisfactory substitute for arterial blood in normotensive asthmatic children.

Low Impedance pH Sensitive Electrochemical Devices That are Potentially Applicable to Transcutaneous PGo2 Measurements

Two classes of low impedance, non‐glass membrane electrodes for pH measurement were evaluated: (I) Metal‐metal oxide electrodes and (II) Reduction‐oxidation electrodes. The fundamental causes of oxygen sensitivity of metal‐metal oxide electrodes were examined and three approaches for its suppression were proposed. For the case of Sb‐Sb2Ox electrodes, oxygen sensitivity can be attenuated partially by cell loading, either directly across the reference electrode or indirectly across a third slave electrode. In a Po2 range of 8–54 kPa, more than 95% of the Po2 response can be suppressed by loading the cell emf to half of its open‐circuit value. The oxygen sensitivity also was observed to be diminished by grinding the metal‐metal oxide and pressing it under high pressure into a pellet electrode. Other metal‐metal oxide electrodes that have promise in transcutaneous measurement are the Pd‐Pdo2 electrodes.

Hypothermia and Rewarming by Peritoneal Dialysis and Temperature-Controlled Inhalate

In 12 rabbits hypothermia and rewarming were induced with temperature-controlled circulating peritoneal dialysis in combination with temperature-controlled hypoxic and hypercapnic gas mixtures. The average cooling time necessary for the esophageal temperature to decrease from 37.7 degrees +/- 0.7 to 20.6 degrees +/- 1.0 degrees C was 81 +/- 34 minutes with a range of 41 to 150 minutes. The average warming time for esophageal temperature to increase from 20.6 degrees +/- 1.0 degrees C to 35.2 degrees +/- 1.8 degrees C was 90 +/- 35 minutes. Time of cooling was related to the proportions of inspired carbon dioxide and oxygen. In contrast to surface and bypass methods, esophageal and muscular temperatures agreed very closely, suggesting an absence of regional temperature gradients.

CONTINUOUS MONITORING OF ARTERIAL PaCO2 BY NON-INVASIVE TRANSCUTANEOUS METHOD

Antimony oxide (Sb-SbOx) and pH-sensitive glass electrodes, combined with reference electrodes, surrounded by electrolyte and covered with gas permeable membrane (gpm) were used for transcutaneous PCO2(PtcCO2) measurement. A servo-controlled heater unit maintained sensor temp, and produced local hyperemia. A thermistor was placed under the gpm to measure true sensor temp. O2 sensitivity of Sb-SbOx electrode was reduced greatly in the 65-175 mm Hg PO2 range by cell loading, but below 65 mm Hg, O2 sensitivity was significant. The pH-sensitive glass electrode was preferred due to its insensitivity to O2. After the sensor was placed on the skin of rabbits, it was calibrated in situ and its temp, coefficient established. This was repeated at the end of the 6-hr. experiment. For the best correlation between PaCO2 and PtcCO2, the skin surface temp, was maintained between 40-42°C, as lower temps, made ptcCO2 higher. Based on in situ calibration, the fitted regression line had a slope of 1.029, intercept of -4.39, and SE of 4.14 mm Hg PCO2. In the range of 21-75 mm Hg, PtcCO2 was 2.2 - 3.8 mm Hg below PaCO2. Based on in situ calibration with one point PaCO2, calibration, the line had a slope of 1.0, intercept of 0.6, SE of 3.4 mm Hg PCO2, and linear regression coefficient of 1.0. These data indicate the usefulness of PtcCO2 monitoring to indicate PaCO2 changes under normal physiologic conditions.

Tissue Oxygen Available as a Criterion for the Effectiveness of Continuous Positive Pressure Breathing

Summary: As little as 3–5 cm H2O increase in proximal airway pressure applied to normal lung reduces cardiac output. It is postulated that decreased pulmonary compliance in respiratory distress syndrome (RDS) acts as a barrier thus offsetting this effect. Since cardiac output is not routinely measured, severe reduction in it could accompany regression of disease while maintaining -the same airway pressure. This study was undertaken to determine whether tissue oxygen available (O2a) could be used to detect changes in perfusion during continuous positive pressure breathing (CPPB). CPPB was evaluated in 10 normal rabbits (C1 = 9.5 ± 1.8 cc/g at 25 cm H2O) and in 10 pulmonary-damaged rabbits (CL = 5.5 ± 1.4 cc/g at 25 cm H2O) produced by subjecting them to 100% O2. Airway pressure was increased from 0–15 cm H2O in 3 cm H2O increments at 10-min intervals. O2a and PaO2 were monitored continuously. In the normal group, O2a decreased at 3 cm H2O airway pressure, reaching 22% of control at 12 cm H2O, at which pressure PaO2 decreased. Breathing 100% O2 at this airway pressure increased PaO2 to 408 mm Hg, whereas O2a returned to 45% of control. In the experimental group, O2a decreased at 9 cm H2O airway pressure, at 12 cm H2O it was 36% of control at which pressure PaO2 decreased slightly. Breathing 100% O2 at this airway pressure increased PaO2 to 316 mm Hg, and increased O2a to 200% of control. These data indicate that with excessive airway pressure, muscle hypoxia may exist during systemic hyperoxemia and that a low compliance lung exerts a protective effect on O2a. Since changes in cardiac output during CPPB are compliance dependent, and since O2a is perfusion dependent, tissue oxygen available could provide a means of selecting optimal airway pressure during CPPB.Speculation: This study indicates that in the presence of normal PaO2 or every hyperoxemia, an excessive increase in airway pressure can produce a decrease in cardiac output and local oxygen supply to the tissues. The airway pressure required to produce optimal oxygenation without interference with cardiac output is lung compliance dependent. For the most efficient application of CPPB in clinical situations, measurement of physiologic variables which would reflect changes in perfusion in addition to systemic oxygenation are suggested.

Effect of umbilical artery catheters on blood flow and oxygen supply to extremities.

Extract: The effect of catheter placement in the abdominal aorta on the blood flow and oxygenation of lower extremities was evaluated in 30 New Zealand rabbits, weighing 0.55–3.5 kg, by inserting 3.5 and 5.0 French polyvinyl chloride catheters, of the type commonly used for umbilical artery catheterization, through the femoral artery, advancing 15–20 cm, and leaving in place for 10–30 min. Arterial blood pressure (BP), common iliac artery blood flow (BF), gracilis muscle tissue oxygen availability (O2a), and subcutaneous temperature (T) in the foot were continuously monitored before and during catheter placement and after withdrawal. There were no changes in the physiologic variables measured in the contralateral leg when the catheter remained below the aortic bifurcation; however, when the catheter was advanced 15–20 cm into the abdominal aorta, a decrease in lower extremity BF, O2a, and T occurred. Because the length of catheter insertion was maintained constant in each animal, the decreases in BF, O2a, and T are related to the relative dimensions of the vessel and the catheter. The ratio of catheter to vessel diameter, in addition to the site of catheter placement, should be considered during the clinical application of arterial catheters. Reduction in blood flow could be detected by continuous differential monitoring of core and extremity temperature or extremity muscle oxygen availability.Speculation: Placement of catheters in the aortas of animals of physical sizes comparable to those of newborn infants may produce an appreciable obstruction to aortic flow. Until a suitable noninvasive method of assessing oxygenation in newborns is available, umbilical artery catheters should be of the smallest possible outside diameter and placed below the bifurcation of the abdominal aorta.

A Noninvasive Method For Monitoring Intestinal Ischemia: Changes in the Pulmonary Clearance of Helium Instilled into the Colon as an Index of Colonic Blood Flow

ABSTRACT.: To evaluate the concept that changes in colonic blood flow will predictably alter the absorption of colonic gas, we measured the pulmonary clearance rate of helium (CHe) which was instilled rectally into the colon of rabbits at a dose of 2 ml/kg. CHE reached a plateau after 20 min at 109 nmol/min/kg. Using hypoxemia as a cause for bowel ischemia, at PAO2 = 38 torr, we noted a marked decrease in CHe from 110 nmol/min/kg to 75 nmol/min/kg (p < 0.025). Because helium absorption from the colon is diffusion limited, a model can be developed relating “subvillus” colonic blood flow to pulmonary helium clearance. From this model we would predict the hypoxemia induced change in CHe to be secondary to colonic hypoperfusion. This type of indirect monitoring could be useful in detecting patients with bowel ischemia.

In vivo evaluation of monopolar intravascular PO2 electrodes.

Summary: Ten intravascular electrodes were evaluated on ten rabbits. The mean stabilization time of 118 ± 52 min was excessively long for clinical use. The in vitro cal factor was not reproducible nor accurate. The differences between the PO2 values measured by the electrode and those obtained from intermittent blood samples were within ±10 mm Hg during the first 12 hr using the in vivo calibration after electrode stabilization. After 12 hr, the electrode PO2 values using the in vitro cal factor or based upon the in vivo calibration were increased significantly indicating a possible membrane rupture. This occured more rapidly in the animal experiments than in the in vitro studies using a tonometer, suggesting that continuous flow and pressure changes inside of the aorta contribute greatly to mechanical breakage of the membrane. Histologic evaluation indicated the electrode catheter system was thrombogenic at the level of the electrode tip and this thrombogenicity was primarily related to intimal injury caused by the electrode tip movement as a result of flow and pressure changes inside the aorta.Speculation: In the present configuration, this system is not ready for clinical use due to an excessively long stabilization time, need for frequent recalibrations, and possible injury to the blood vessel wall.