Ronald A. Gabel

Variation in PCO2 between Arterial Blood and Peak Expired Gas during Anesthesia

Arterial PCO2 (PaCO2) can be continuously and noninvasively estimated by monitoring peak expired CO2 tension (PpeCO2). The practice of calibrating the estimate by an initial measurement of PaCO2 assumes that the difference in PCO2 tension between arterial blood and expired gas P(a-pe)co2 remains constant. We examined the stability of P(a-pe)CO2 during anesthesia in 15 patients undergoing major surgery. Mean P(a-pe)CO2 values ranged from 0.8–7.9 torr with maximum P(a-pe)CO2 values ranging from 4.5–13.0 ton. Calibration of P(a-pe)CO2 based on a single initial measurement of PaCO2 often over- or underestimated PaCO2. Mean estimated PaCO2 from calibrated P(a-pe)CO2 varied from - 7.9–6.4 torr with extreme estimates of — 12.8–12.3 torr. No consistent correlation was shown between P(a-pe)CO2 and duration of anesthesia, variations in ventilation, blood pressure, blood-gas tensions, PpeCO2 or temperature. We conclude that estimation of PaCO2 by monitoring PpeCO2 is not invariably reliable.

Algorithms for calculating and correcting blood-gas and acid-base variables.

This is a review of the origins, derivations, and fidelity of available equations for correcting PO2, PCO2, and pH for temperature, and for estimating blood-gas and acid-base variables from measured values: 1. Oxygen saturation (SO2) from PO2, PCO2, pH, and body temperature (T). 2. Oxygen concentration (CO2) from SO2, PO2, and hemoglobin concentration (Hb). 3. Base excess (BE) from pH, PCO2, and Hb. 4. In vivo BE (BE3) from pH and PCO2. 5. Compensated BE3 (BEC) from PCO2. 6. Bicarbonate ([HCO3-]) and carbon dioxide concentrations (CCO2) from pH and PCO2. PHysiologic considerations are emphasized, with mathematical background when it contributes to understanding. Algorithms are variously compared with their graphic counterparts, with the data from which they were derived, and with each other.

Naloxone does not affect ventillatory responses to hypoxia and hypercapnia in rats

Ventilatory responses (tidal volume, respiratory frequency, and minute ventilation) to steady-state hypoxia and steady-state hypercapnia were measured plethysmographically in awake unrestrained adult rats, before and after subcutaneous injection of placebo (saline) naloxone in doses up to 5.0 mg/kg. Naloxone did not alter the ventilatory responses to hypoxia or hypercapnia.

Administration of Nitrous Oxide in Normal Subjects: Evaluation of Systems of Gas Delivery for Their Clinical Use and Hemodynamic Effects

Nitrous oxide (concentrations of 30 to 50%) was administered to 22 healthy volunteer subjects via nasal prongs, rebreathing mask, and an airlines mask to assess the efficiency of systems of delivery and the hemodynamic effects. The end-expired concentration of nitrous oxide, expressed as a percentage of the inspired concentration of nitrous oxide, was 19% for nasal prongs, 34% for the rebreathing mask, and 95% for the airlines mask (most accurate delivery). The pulse rate fell from a mean of 79+/-3 beats per minute to 67+/-2 with nasal prongs, to 64+/-2 with the rebreathing mask, and to 64+/-2 with the airlines mask. Both systolic and diastolic pressures fell from means of 122+/-4 and 74+/-2 mm Hg, respectively, to 98+/-3 mm Hg and 64+/-2 mm Hg, respectively, with the airlines apparatus. The fall in systolic blood pressure (slope, -0.79) exceeded that in the diastolic (slope, -0.35). In additional eight normal subjects, administration of 30% nitrous oxide via airlines mask produced identical changes in blood pressure and pulse rate, but there was no effect from 30 minutes of administration of 30% nitrous oxide on the end-systolic volume index, cardiac index, ejection fraction, normalized wall velocity determined echocardiographically, ejection time, or the ratio of preejection period to ejection time.

Endogenous Opioids and Ventilatory Adaptation to Prolonged Hypoxia in Goats

To investigate whether endogenous opioid peptides mediate time-dependent changes in ventilatory control during prolonged hypoxia, we studied four adult goats at rest during 14 days at simulated high altitude in a hypobaric chamber (PB approximately 450 Torr). Arterial PCO2 fell during the first several hours of hypoxia, remained stable over the next 7 days, and then rose slightly (but without statistical significance) by day 14. Ventilatory responsiveness to CO2 increased during the first week of hypoxia. By day 14, while still greater than control, the ventilatory response to CO2 was less than that observed on day 7. Immunoactive beta-endorphin levels in plasma and CSF did not change during the 14-day period. Administration of naloxone on day 14 did not restore the ventilatory response to CO2 to the level observed during the first week of acclimatization. We conclude that in adult goats, time-dependent changes in ventilatory response to CO2 during acclimatization to prolonged hypoxia are not primarily attributable to alterations in endogenous opioid peptide activity.

Clinical InvestigationsAdministration of Nitrous Oxide in Normal Subjects: Evaluation of Systems of Gas Delivery for Their Clinical Use and Hemodynamic Effects

Nitrous oxide (concentrations of 30 to 50%) was administered to 22 healthy volunteer subjects via nasal prongs, rebreathing mask, and an airlines mask to assess the efficiency of systems of delivery and the hemodynamic effects. The end-expired concentration of nitrous oxide, expressed as a percentage of the inspired concentration of nitrous oxide, was 19% for nasal prongs, 34% for the rebreathing mask, and 95% for the airlines mask (most accurate delivery). The pulse rate fell from a mean of 79+/-3 beats per minute to 67+/-2 with nasal prongs, to 64+/-2 with the rebreathing mask, and to 64+/-2 with the airlines mask. Both systolic and diastolic pressures fell from means of 122+/-4 and 74+/-2 mm Hg, respectively, to 98+/-3 mm Hg and 64+/-2 mm Hg, respectively, with the airlines apparatus. The fall in systolic blood pressure (slope, -0.79) exceeded that in the diastolic (slope, -0.35). In additional eight normal subjects, administration of 30% nitrous oxide via airlines mask produced identical changes in blood pressure and pulse rate, but there was no effect from 30 minutes of administration of 30% nitrous oxide on the end-systolic volume index, cardiac index, ejection fraction, normalized wall velocity determined echocardiographically, ejection time, or the ratio of preejection period to ejection time.

Calculation and correction of blood-gas and acid-base variables with a versatile computer program.

Data generated in the blood-gas laboratory must often be refined before being used by the investigator or sent to the clinician. PO,, Pco,, and pH are customarily adjusted to body temperature if different from that of the measuring electrodes. Corrections should be made for measuring artifacts, such as “suspension effect” for pH and “stirring artifact” for PO,. An estimate of the metabolic component of acid-base balance, either base excess or plasma bicarbonate concentration, is usually determined. Oxygen saturation is sometimes calculated from measured values of PO,, PCO, pH, and body temperature. Although many modern devices for measuring blood gases and pH have built-in capability for performing many of these calculations, not everyone has equipment with this potential. Furthermore, most commercial systems are limited in breadth of available

Adrenergic Blockade Does Not Change Ventilatory Response to CO2 in Awake Resting Goats

Although adrenergic agonists increase resting ventilation and responsiveness to CO2, there are conflicting data about the effect of adrenergic blockade on ventilatory responses. In this study, we investigated the effect of alpha- or beta-adrenergic blockade on the response to hyperoxic CO2 rebreathing in awake goats. In 5 goats, studied before and after intravenous administration of phentolamine or propranolol, there was no difference (compared to control studies) in the mean slope, x-intercept, or ventilation at end-tidal PCO2 (PETCO2) = 70 torr for the CO2 response curves after either drug. When mean inspiratory flow rate (VT/Ti) was plotted against PETCO2, there was also no change in these measurements after propranolol. After phentolamine, there was a slight decrease in the slope and x-intercept, but no change in VT/Ti at PETCO2 = 70 torr. We conclude that acute administration of alpha- or beta-adrenergic blockers does not affect ventilatory response to CO2 inhalation in goats, and suggest that adrenergic activity is not an important modulating influence for CO2 responsiveness in this species.

Auditing of Anesthesia Performance

MONG reasons that can be given for auditing A anesthesia performance, two stand out: (1) to improve patient care, and (2) to satisfy the requirements of the Joint Commission on Accreditation of Hospitals (JCAH). Although we might prefer to be led by the altruistic rather than the pragmatic justification, the two are not mutually exclusive. In fact, the first is admirably served if the second is fulfilled. This is not to suggest that patient care will automatically improve if JCAH-style auditing is carried out; only that the Joint Commission requirements are not a bad place to begin. Recommendations of the JCAH for appraising anesthesia services are found in the Accreditation Manual for Hospitals, where the general mandate for auditing is given: