Effect of norepinephrine and angiotensin II on intrarenal oxygen availability.

Oxygen availability (O2a) in the cortex, outer medulla, inner medulla, and papilla of the left kidney was measured in 12 unanesthetized rabbits implanted with O2 sensitive electrodes. Each was injected with a single i.v. bolus of either norepinephrine (NE) or angiotensin II (AII) in equipressor doses, and intrarenal O2a was continuously recorded. The NE response consisted of parallel and proportional intrarenal O2a decreases and overshoots in all zones; AII response consisted of progressively smaller O2a decreases from outer to inner zones during maximum response and smaller increases during maximum overshoot. These response differences may be important under various physiologic or pathologic conditions.

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.

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 decreased in CHe from 110 nmol/min/kg to 75 nmol/min/kg (p less than 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 transcutaneous CO2 partial pressure monitoring.

Correlation between transcutaneous and arterial CO2 partial pressure (Ptcco2, and Paco2) under normal and hemorrhagic shock conditions was evaluated in rabbits. Under normal conditions the Paco2-to-Ptcco2 least-squares regression line had a slope of 1.03 an intercept of 4.57 Torr, and a root mean variance of +/- 3.79 Torr. Under hemorrhagic shock conditions the slope remained similar, but the intercept increased, producing a significant difference between arterial and transcutaneous values. The correlation line shifts to the left so that, for a given Paco2, the Ptcco2 value increases. The transcutaneous response time (90%) under conditions produced by breathing 10% CO2 lagged 2.8 +/- 1.4 min behind that of the breathing 10% CO2 lagged 2.8 +/- 1.4 min behind that of the Paco2. The difference between transcutaneous and arterial CO2 observed during hemorrhagic shock and the lag in transcutaneous response time can be altered by topical application of dimethyl sulfoxide, by altering both flow and permeability. These results indicate that good Ptcco2-to-Paco2 correlation exists under normal conditions and that hemorrhagic shock will produce tissue CO2 accumulation and therefore higher than arterial Ptcco2 values.

Clinical aspects of continuous neonatal oxygen monitoring.

Twenty newborn infants (gestational age 30--40 weeks, weight 980--3400 g) were studied in two groups to compare two commercially available systems for continuous in vivo oxygen monitoring: the So2 catheter and the transcutaneous Po2 (TcPo2) electrode, and their respective electronic systems. Measurements from these systems were correlated with determinations made from samples intermittently drawn and measured by conventional So2 and Pao2 in vitro methods , respectively. Information about these two in vivo oxygen monitoring systems was then related to our previous experience with the bare-wire earlobe O2a electrode. Measurements from the two in vivo monitoring techniques studied showed good correlations with their respective in vitro oxygen measurements: So2 catheter, y = x - 3.08, r = 0.98 (range studied 74% to 100%) and transcutaneous electrode, y = 0.98x + 0.57, r = 0.89 (range studied 34 to 92 mm Hg). It was concluded that all three systems give a good reflection of central arterial oxygen (either Sao2 or Pao2)). The system to be used in specific clinical situations should depend on condition of the baby and stage of treatment, need for an umbilical line to measure other variables, equipment available, and training of personnel.

An improved method for inducing hypothermia and rewarming.

A hypothermia and rewarming system combining body surface and ventilatory heat exchange is described. The method utilizes body surface heat exchange through conduction, convection, and black body radiation, and ventilatory heat exchange across the lung surface through conduction, convection, and water evaporation. The system consisted of a chamber in which the temperature was maintained at a desired level (+/- 2.5 degrees C) using a refrigeration-heat pump unit. Chamber temperatures during cooling and rewarming were -15.5 +/- 2.7 degrees C and 43.2 +/- 2.3 degrees C, respectively. Inhalate temperatures during cooling were -8.2 +/- 6.5 degrees C and during rewarming they were 41.5 +/- 0.3 degrees C. Helium (100%) was supplied to the chamber, while the animal was ventilated with 20% O2 + 80% He. Under these conditions, the cooling and rewarming rates were 0.33 +/- 0.06 degrees C/min and 0.20 +/- 0.04 degrees C/min, respectively, at 38--21 degrees C. The system provided for rapid cooling and rewarming with no evidence of any untoward effects.

Effect of inhalate thermal conductivity and high O2 in producing hypothermia.

The effect of an increase in inhalate thermal conductivity and the fraction of inspiratory O2 (FIO2) on the rate of cooling and rewarming using a surface-inhalate heat exchange method was evaluated. Male New Zealand White rabbits were divided into three groups: those ventilated with air, those with 20% O2 + 80% He, and those with 100% O2. All animals were cooled to an esophageal temperature of 22.5 degrees C (or for 180 min maximum). Following a 15-min exposure to room air, the animals were connected to the humidifying and warming system. He-O2 had the highest thermal conductivity and the animals ventilated with it had the fastest cooling rate. One hundred percent O2 and room air had similar thermal conductivities, but the animals ventilated with 100% O2 had significantly lower cooling rates. These data indicate that, while maintaining a constant surface heart exchange, the rate of heat exchange across the lung can be modified by altering the thermal conductivity of the inhalate gas mixture. Total heat exchange can also be modified by hyperoxemia-induced hemodynamic changes.

In vivo evaluation of monopolar intravascular PO2 electrodes.

Ten intravascular electrodes were evaluated on ten rabbits. The mean stabilized 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.

Transcutaneous PO2 monitoring of flap circulation following surgery.

In a series of rabbit experiments, tcPO2 was found to be a very sensitive indicator of flap viability, reflecting minute-to-minute changes in physiology. It is safe, reproducible, and easily verified by comparing flap tcPO2 to a control site and by measuring response to increased FIO2. Transcutaneous PO2 measurements should prove a valuable tool in flap research and in monitoring pedicle flaps and microvascular tissue transfers.

In vitro evaluations of monopolar intravascular oxygen sensors.

Intravascular PO2 electrodes of a commercially available design were tested in vitro to establish their characteristics and to further elucidate on their clinical usability. In addition, four different cathode-anode combinations, utilizing the same geometrical design, were evaluated to establich the best combination. Au-Ag/AgCl exhibited the best characteristics as verified by the evaluation of stabilization time, sensitivity and reproducibility of the response, response time, linearity, drift, effects of temperature, flow, and pH, and the useful life of the sensors. Even though the Au-Ag/AgCl combination exhibited the best characteristics, it fell far below clinically recommended criteria for continuous monitoring of less than 10 Torr/24 h.

Low impedance pH sensitive electrochemical devices that are potentially applicable to transcutaneous PCO2 measurements.

Two cases of low impedance, non-glass membrane electrodes for pH measurement were evaluated: (I) Metal--metal oxide electrodes and (II) Reduction-oxidation electrodes. The fundamental cause of oxygen sensitivity of metal-metal oxide electrodes were examined and three approaches for its suppression were proposed. For the case of Sb--Sb2Ox electordes, 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 tis open-circuit value. The oxygen sensitivity also was observed to 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. The redox electrodes are typified by the Quinhydrone electrode. A membrane Quinhydrome electrode showed a sensitivity of 56 mV/Decade at 37 degree C and no oxygen sensitivity up to 50 kPa and a drift of 1 mV/h over a 24-h period. However, the stability deteriorated over a long-term period.

Tissue oxygen available as a criterion for the effectiveness of continuous positive pressure breathing.

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 breating (CPPB). CPPB was evaluated in 10 normal rabbits (CL = 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.

Electrochemical sensor for continuous transcutaneous PCO2 measurement.

A sensor suitable for continuous transcutaneous PCO2 measurements is described. The sensor consists of an antimony-antimony oxide electrode in combination with a silver-silver chloride reference electrode, bathed in an electrolyte and covered by a Teflon membrane. A servo-controlled heater unit was used to maintain the sensor's temperature and to produce local hyperemia. The resulting oxidation-reduction potential under constant temperature is a linear function of the logarithm PCO2. Response time (95%) to step changes in PCO2 from 27 to 70 mmHg was 2.7 +/- 0.3 min. Following a 12-h "aging" time, the electrode exhibited a minimal drift of 5.2 +/- 2.2 mV for 16 h, representing an average PCO2 drift of 0.5 mmHg/h. This sensor was applied on three rabbits and on five human volunteers, and found satisfactory under normal physiological conditions.

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

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 diamter, 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.

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.