Characteristics of non-aqueous electrolytes for transcutaneous oxygen electrodes.

We investigated the characteristics of non-aqueous solvents for tcPO2 electrolytes, using a tcPO2 electrode with three 15 micrometer platinum cathodes set in a polished glass surface, a 17.8 mm2 silver anode covered with a 25 micrometer Telfon membrane, with and without a 12 micrometer cellophane spacer, and polarized at--0.8v. We examined the effect of ethylene glycol (EG) concentration, in water, on O2 sensitivity, stirring effect, in vitro drift, in vitro response time, behaviour on the skin of newborn infants and in vivo response time. We found the following: (1) O2 sensitivity was reduced by increasing EG concentration. (2) phi, the stirring effect (gas/50% glycerine) was 1.01 at 90% EG with cellophane, but higher without cellophane. (3) In vitro drift at 44 degree C electrode temperature, in 43 degree C water (simulating skin), was reduced with 90% EG, 25 micrometer Telfon and no spacer, but increased with a spacer. (4) In vitro response time was increased with EG. (5) Correlations with arterial PO2 in a group of newborns was as excellent with an electrode made with 90% EG and 25 micrometer Telfon, without spacer, as it was with our previous study using aqueous electrolyte and 12 micrometer Telfon wint 12 micrometer cellophane: (tcPO2 = 1.32 + 0.988 PaO2 r = 0.987). In vivo response time was not prolonged. Using EG, the tc PO2 electrode membrane required changing only after 7--9 days, compared with 1--2 days when using aqueous electrolytes. No advantage of using cellophane spacers was identified in these in vivo studies.

A respiratory function jacket for measuring tidal volume and changes in FRC.

We describe a simple application of a mercury-in-rubber strain gauge in a jacket used to measure tidal volume in paralyzed, mechanically ventilated rabbits. The jacket produces a record with a straight base line. Shifts of this base line can be used to assess changes in functional residual capacity (FRC).

Functional residual capacity and lung mechanics at different levels of mechanical ventilation.

We assessed the effects of rapid ventilatory rates (60 to 120 breath/min) and high mechanical ventilation pressures (30/5 to 40/10 cm H2O) on lung mechanics and intravascular pressures in 9 paralyzed, sedated rabbits ventilated with a time-cycled, pressure-limited flow generator (Baby bird). Measurements of tidal volume, ventilator line pressure, tracheal pressure, functional residual capacity (FRC), and arterial and venous blood pressures showed that: 68% of the peak pressure developed by the ventilator was transmitted to the trachea at 60 breath/min, 74% at 120 breath/min, and 87% when ventilation pressures were increased to 40/10 cm H2O; when the ventilatory rate and the PEEP were increased, the end-expiratory pressure in the trachea became progressively greater than that indicated on the ventilator pressure gauge; FRC increased when the PEEP and mean tracheal pressure increased; tidal volume and dynamic compliance decreased and minute ventilation increased as ventilatory rate increased; compliance decreased whenever FRC increased, and increased whenever FRC decreased; and there was little effect on mean central venous or arterial pressure. These data indicate that increasing ventilator rates cause gas trapping within the lung. In normal animals, this may interfere with gas exchange and pulmonary blood flow. In abnormal lungs, the gas trapping may increase FRC and improve gas exchange within the lung.

Age and the solubility of volatile anesthetics in ovine tissues.

To determine the effect of age on the solubility of volatile anesthetics in tissues, we measured the blood/gas and tissue/gas partition coefficients of isoflurane, enflurane, halothane, and methoxyflurane in vitro at 37 degrees C in newborn lambs and postpartum adult sheep. The tissue specimens examined were brain, heart, liver, kidney, muscle, and fat. Hematocrit and serum concentrations of albumin, globulin, cholesterol, and triglycerides were measured. The blood/gas partition coefficients, hematocrit, and the serum concentrations of albumin, globulin, cholesterol, and triglycerides in the newborn lambs did not differ from those in the adult sheep. The tissue/blood partition coefficients [the ratio of (tissue/gas)/(blood/gas)] in newborn lambs were 28% [mean value for the four anesthetics] less than those in the adults. The tissue/blood partition coefficients for enflurane and methoxyflurane in newborn tissues were significantly less (P less than 0.05) than those for halothane and isoflurane. We conclude that the blood/gas partition coefficients in sheep do not change significantly with age, and that the time required for equilibration of volatile anesthetics (particularly enflurane and methoxyflurane) in newborn tissues is probably less than in adult sheep.

Halothane anesthesia reduces pulmonary function in the newborn lamb.

To study the effects of anesthesia on respiratory function of the neonate, the authors investigated the effect of breathing 100% oxygen and of breathing oxygen plus 0.75 MAC halothane on functional residual capacity, lung and airway resistance, expired minute volume, work of breathing, lung compliance, and blood gases and pH in nine 5-8-day-old, 4.6-7.7-kg lambs. Breathing 100% oxygen increased PaO2 but had no effect on PaCO2, minute ventilation, or lung mechanics. Three-fourths MAC halothane depressed minute ventilation 34% +/- 13% (P less than 0.05) and increased PaCO2 50% +/- 5% (P less than 0.05). Lung and airway resistance increased 59% +/- 26% (P less than 0.05); work of breathing decreased (P less than 0.05); and lung compliance was unchanged. Functional residual capacity was reduced 32% +/- 6% (P less than 0.05), which may be due to loss of diaphragm and intercostal muscle function and to an inability to take deep breaths. The authors conclude that 0.75 MAC halothane significantly impairs the pulmonary function of lambs who breathe spontaneously. Similar changes in human infants could account for the hypoxemia and hypercarbia that often are seen during anesthesia.

The association between abnormal glucose tolerance (hyperglycemia and hypoglycemia) and estriol excretion in pregnancy.

In 2,000 consecutive patients having glucose tolerance tests in pregnancy hyperglycemia (greater than or equal to ninety-fifth percentile) was associated with increased placental weight (p less than 0.01) but not with increased fetal birth weight. Patients with hypoglycemia (less than or equal to fifth percentile) were more likely to have small-for-dates babies (p less than 0.01). Perinatal death was related to maternal glucose tolerance, being reduced from 1.3% in the total series to 0.6% when normoglycemia was present (p less than 0.05); it was significantly increased in the presence of maternal hyperglycemia (p less than 0.001) and hypoglycemia (p less than 0.01). A combination of abnormal glucose tolerance and subnormal estriol excretion detected pregnancies with significantly higher incidences of fetal and placental growth retardation, major fetal malformations, and perinatal deaths. Moreover, the combination of normoglycemia and normal estriol excretion (62.3% of patients) was associated with a very favorable pregnancy outcome (0.4% perinatal death rate). Hypoglycemia was at least as significant as hyperglycemia in terms of unfavorable pregnancy outcome, especially when associated with subnormal estriol excretion.

Osmolarity determines the solubility of anesthetics in aqueous solutions at 37 degrees C.

The authors determined whether they could predict accurately the solubility of anesthetics in aqueous solutions at 37 degrees C, knowing the osmolarity and the pH of the solution and the solute composition. The partition coefficients of the four volatile anesthetics, isoflurane, enflurane, halothane, and methoxyflurane, were determined concurrently at 37 degrees C between air and aqueous solutions containing sodium chloride, dextrose, mannitol, or heparin. The osmolarities of these solutions ranged from 0 to 7,000 mOsm/l. The partition coefficients decreased linearly with increasing osmolarity when plotted on a semilogarithmic scale. The effect of osmolarity on the partition coefficient of the alkane anesthetic, halothane, was 20% less (P less than 0.001) than the effect of osmolarity on the partition coefficients of the three methyl-ethyl ether anesthetics, isoflurane, enflurane, and methoxyflurane. The solubility of anesthetics in aqueous solutions did not depend on either the molecular structure of the solute or the pH of the solution. The solubility of volatile anesthetics in aqueous solutions at 37 degrees C is inversely and predictably dependent on the osmolarity of the solutions.

Anesthetic requirements for halothane in young children 0-1 month and 1-6 months of age.

In a previous study, the authors found that infants, in the first 6 months of life, required the highest minimum alveolar concentration (MAC) of any age group (1.09% halothane). Because only two neonates (0-31 days of age) were included in the original study and because profound depression of blood pressure and heart rate have been reported in neonates, the authors determined 1) whether the MAC of halothane in neonates (n = 12) differs from that in infants (1-6 months of age) (n = 12) and 2) whether the blood pressure and heart rate responses in neonates differ from those in infants at approximately 1 MAC. The authors found that the MAC of halothane in neonates, 0.87% +/- 0.03 SEM, was significantly lower (P less than 0.01) than that in infants, 1.20% +/- 0.06 SEM. With induction of anesthesia, the systolic blood pressure decreased 23% in neonates (P less than 0.05) and 34% in infants (P less than 0.005) from awake values. Similarly, the heart rate decreased 12% in neonates and 22% in infants (P less than 0.05). The incidence of hypotension (greater than 30% decrease in systolic blood pressure from awake) in neonates, 33%, was not significantly different from that in infants, 44%. The authors conclude that the MAC of halothane in neonates is 25% less than that in infants and significantly less than was thought previously. The MAC in infants is the highest of any age group. The decrease in blood pressure and the incidence of hypotension in neonates are similar to those in infants at approximately 1 MAC of halothane.

Transcutaneous oxygen tension in sick infants.

The clinical usefulness of the transcutaneous O2 electrode was investigated in 30 sick infants; 159 simultaneous measurements of arterial PO2 and transcutaneous PO2 were made. During the comparisons, arterial blood pressure, heart rate, and thoracic impedance were continuously recorded, and skin axillary and environmental temperatures and hematocrit were noted. Despite a wide range of arterial blood pressure and hematocrits, arterial PO2 and transcutaneous PO2 were similar (slope, 0.963), except for 2 groups of sick infants. Some infants with severe, persistent pulmonary hypertension who were receiving an intravascular infusion of tolazoline and infants with mean arterial blood pressures more than 2.5 SD less than the predicted average had values for transcutaneous PO2 that were lower than PO2. The surface O2 electrode is safe and relatively easy to use and provides data that can help in the management of most sick infants.

Age and solubility of volatile anesthetics in blood.

The more rapid rate of rise of alveolar anesthetic partial pressure in children compared with adults may be explained in part by an increasing solubility of volatile anesthetics in blood with age. To investigate this possibility, the authors measured the blood-gas partition coefficients of isoflurane, enflurane, halothane, and methoxyflurane in four groups of fasting subjects: 10 full-term newborns (at delivery), 11 children (3-7 years old), 11 adults (20-40 years old), and 10 elderly adults (75-85 years old). The blood-gas partition coefficients were greatest in adults: isoflurane 1.46, enflurane 2.07, halothane 2.65, and methoxyflurane 16.0; and least in newborns: 1.19, 1.78, 2.14, 13.3, respectively. The blood-gas partition coefficients in children (1.28, 1.78, 2.39, 15.0, respectively), which were intermediate between those in newborns (P less than 0.005) and those in adults (P less than 0.005), were not significantly different from those in elderly adults (1.29, 1.79, 2.41, 15.0, respectively). The blood-gas partition coefficients of both isoflurane and enflurane correlated directly with the serum albumin and triglyceride concentrations; that of halothane correlated directly with the serum cholesterol, albumin, triglyceride, and globulin concentrations; and that of methoxyflurane correlated directly with the serum cholesterol, albumin, and globulin concentrations. The authors conclude that age significantly affects blood-gas partition coefficients, and the lower blood-gas partition coefficients in children explain in part the more rapid rise of alveolar anesthetic partial pressure in this age group.

Failure of conventional monitoring to detect apnea resulting in hypoxemia.

In summary, our findings support and confirm the concerns of many investigators that present methods of cardiorespiratory monitoring are inadequate for the detection of many forms of apnea. Nurses underrecord both the frequency and duration of apneic episodes. Bradycardia is an unreliable index of hypoxemia. Thoracic impedance monitors are unreliable because they detect only a fixed duration of respiratory pause and are sensitive to many artifacts unavoidable in a clinical setting. Finally, ineffective breathing patterns such as disorganized breathing, obstructive apnea, and paradoxical breathing are undetectable by thoracic impedance monitoring. We warn against the reliance on heart rate and thoracic impedance monitoring alone for infants with recurrent apnea.

Effect of age on the solubility of volatile anesthetics in human tissues.

To determine the effect of age on the solubility of volatile anesthetics in human tissues, the authors measured the solubilities of isoflurane, enflurane, halothane, and methoxyflurane in vitro at 37 degrees C in 35 postmortem human tissue specimens. Specimens were taken from neonates, and young (20-50 yr), middle-aged (50-70 yr), and elderly adults (greater than 70 yr). Brain/gas, heart/gas, and liver/gas partition coefficients for all four anesthetics increased significantly (P less than 0.05) between birth and adulthood, although brain/gas partition coefficients in young adults tended to be higher than those in middle-aged and elderly adults. Heart/gas and liver/gas partition coefficients tended to increase with aging. Muscle/gas partition coefficients for the four anesthetics increased linearly with age. Fat/gas partition coefficients did not change significantly with age. Tissue/blood solubilities for the four anesthetics were of the same order of magnitude for a given tissue and age group. Tissue/blood solubilities for enflurane were 30% lower than those for isoflurane in the same tissue and age group. In summary: the solubility of volatile anesthetics in human tissues increases with age; the lower solubility of anesthetics in neonates partially explains the more rapid increase of alveolar and tissue anesthetic partial pressures in neonates; despite the higher blood solubility of enflurane, its lower tissue solubility may explain a rate of recovery comparable with that of isoflurane.

Dynamics of expiration and gas trapping in rabbits during mechanical ventilation at rapid rates.

Mechanical ventilation at rapid rates may cause gas trapping by decreasing the time available for expiration. The volume of gas trapped within the lungs depends not only on the expiratory time, but also on the rate of emptying of the lungs. Because newborn infants are frequently ventilated at rapid rates, we examined the factors determining the rate of lung emptying, and thereby the development of gas trapping, at ventilatory rates of 40 to 120 breath/min in anesthetized and paralyzed rabbits. We found that flow and volume were related nonlinearly during the first segment of expiration, and linearly during the second segment. Only the second segment could therefore be described with a single time constant. The expiratory resistance of the respiratory system and endotracheal tube was up to 4.5 times greater than the inspiratory resistance, a finding explained by the higher transmural airway pressures during inspiration and by the dynamic effects of the increase in cross-sectional airway area at the junction of endotracheal tube and trachea. This high expiratory resistance lengthened the time constant of the second segment of expiration which, combined with the delay caused by the slow opening of the exhalation valve during the first segment, promoted gas trapping when the expiratory time was shortened.

Clinical limitations and advantages of transcutaneous oxygen electrodes.

We investigated the clinical use and limitations of the transcutaneous oxygen electrode described by Huch, Lübbers and Huch in 30 sick infants. One hundred and fifty-nine measurements of arterial oxygen tension (PaO2) and transcutaneous oxygen tension (tcPO2) were made. During the comparisons, arterial blood pressures, heart rate thoracic impedance were continuously recorded, skin axillary and environmental temperatures, hematocrit and skin thickness were measureed and the degree of peripheral perfusion noted. Despite a wide range of these variables, values of tcPO2 and PaO2 were similar (slope 0.963). Two groups of infants were identified in whom tcPO2 was lower than PaO2. These were infants receiving an intravascular infusion of tolazoline and infants with mean arterial blood pressures more than 2.5 s.d. below the predicted average value. Both of these situations represent extreme alterations in peripheral blood flow and give important information regarding the limitations of the method. Less extreme alterations in flow caused by mild hypotension, hypothermia, anemia, radiant warmers, and bilirubin lights did not adversely affect the transcutaneous PO2--arterial PO2 correlation. Advantages of transcutaneous oxygen monitoring over more conventional monitoring methods were assessed. We conclude that the transcutaneous oxygen electrode is safe, is easy to use, has few limitations and provides data which can help improve the management of most sick infants.