Expired Gas Concentrations Research Articles

Emergence from isoflurane/N2O or isoflurane anaesthesia

The first goal of anaesthetic recovery is return of the patient's ability to independently maintain respiratory and circulatory functions. Nitrous oxide remains popular due to minor effects on the cardiovascular and respiratory systems. However, diffusion hypoxaemia can occur during recovery and there is a potential advantage of providing the patient with only a potent vaporised agent. This randomised study of 20 gynaecological patients evaluated respiratory and circulatory variables during emergence after anaesthesia with equipotent mixtures of isoflurane/nitrous oxide or isoflurane. Inspired, end-tidal and mixed expired gas concentrations, expired minute volume, pulse oximetry saturation and arterial blood gases were registered. Monitoring of cardiac output was performed by transthoracic bioimpedance. Patients anaesthetised with isoflurane/N2O resumed their spontaneous breathing 16 min earlier and were extubated 22 min earlier than those anaesthetised with only isoflurane. At extubation, total MAC and end-tidal CO2 were similar in both groups, 0.22-0.26 and 5.5-5.9 vol%, respectively. The isoflurane/ N2O group had greater minute ventilation and CO2 excretion rates than the isoflurane group throughout the emergence period. There were no significant differences between the groups in blood gas variables or in heart rate, mean arterial blood pressure or cardiac index. Cardiac index was between 3.4 and 3.9 l m(-2) min(-1) throughout the emergence period in both groups. Patients anaesthetised with only isoflurane had a longer delay until resumption of spontaneous breathing and extubation in the emergence period. Minute ventilation and carbon dioxide elimination were also significantly more suppressed throughout emergence after anaesthesia with isoflurane as compared with isoflurane/N2O.

Carbogen Breathing in Patients with Glioblastoma Multiforme Submitted to Radiotherapy Assessment of gas Exchange Parameters

It has been reported that carbogen breathing yields a remarkable increase of radiosensitivity in murine tumour models. Hence, application of carbogen might be promising in radiotherapy of human tumours. We describe a method to increase arterial oxygenation and to ensure stability of O2 and CO2 during carbogen breathing in patients with malignant disease. We measured in 6 patients with histologically proven intracranial glioblastoma multiforme arterial blood gases, inspired and expired gas concentrations and vital signs either baseline and during carbogen breathing. The highest values of arterial oxygenation were achieved after 10 min of carbogen breathing and they remained stable up to 15 min. In none of our patients was N2 wash-out from the lungs completed in 15 min of carbogen breathing. In conclusion, carbogen breathing increased arterial oxygenation in patients with intracranial malignant diseases. The system used is reliable and of practical use. Monitoring of expired gas concentrations is highly recommended.

A Prospective, Comparative Trial of Three Anesthetics for Elective Supratentorial Craniotomy

Different anesthetic agents have different effects on cerebrovascular physiology. However, the importance of these differences in neuroanesthetic practice are unclear. In an effort to determine whether important clinical differences are present, the authors compared three anesthetic techniques in 121 adults undergoing elective surgical removal of a supratentorial, intracranial mass lesion. Patients were assigned randomly to one of three groups. In group 1 (n = 40), anesthesia was induced with propofol and maintained with fentanyl (approximately 10 micrograms/kg load, 2-3 micrograms.kg-1.h-1 infusion) and propofol (50-300 micrograms.kg-1.min-1). In group 2 (n = 40), anesthesia was induced with thiopental and maintained with isoflurane and nitrous oxide. Up to 2 micrograms/kg fentanyl was given after replacement of the bone flap. In group 3 (n = 41), anesthesia was induced with thiopental and maintained with fentanyl (approximately 10 micrograms/kg load, 2-3 micrograms.kg-1.h-1 infusion), nitrous oxide, and low-dose isoflurane, if required. Blood pressure, heart rate, expired gas concentrations, and ventilatory parameters were recorded automatically in all patients. Epidural intracranial pressure (ICP) was measured via the first burr hole, brain swelling was rated at the time of dural opening, and emergence was monitored closely. Preoperative computed tomography or magnetic resonance imaging scans were evaluated, and pre- and postoperative neurologic exams were performed by a neurosurgeon unaware of group assignments. Total hospital stay (days) and total hospital cost (exclusive of physician charges) also were reviewed. During induction, higher heart rates were seen in isoflurane/nitrous oxide patients, whereas mean arterial pressure was approximately 10 mmHg less during the maintenance phase (compared with both other groups). Otherwise, there were few intergroup hemodynamic differences. While there were no clinically important intergroup differences in mean ICP (+/- SD)-group 1, ICP = 12 +/- 7 mmHg; group 2, 15 +/- 12 mmHg; group 3, ICP = 11 +/- 8 mmHg-more isoflurane/nitrous oxide patients (nine, group 2) had an ICP > or = 24 mmHg than in the other groups (two each). Emergence was, overall, more rapid with fentanyl/nitrous oxide. For example, the median time until the patient could be awakened by quiet verbal command, e.g., "Open your eye," was 5 min, versus 10 min in the other groups. There were no relationships between ICP and any measurement of emergence (e.g., time to response to commands). Seven of 41 (17%) fentanyl/nitrous oxide patients vomited in the early postoperative period, compared with only 1 of 40 (2.5%) of those given propofol/fentanyl and 2 of 40 (5%) receiving isoflurane/nitrous oxide (P = 0.03). There were no differences in the incidence of new postoperative deficits, total hospital stay, or cost. Although there are modest differences among the three tested anesthetics, short-term outcome was not affected. These results indicate that, despite their respective cerebrovascular effects, all of the anesthetic regimens used were acceptable in these patients undergoing elective surgery.

Expirograms of O 2, CO 2 and intravenously infused C 2H 2 and Freon-22 during panting in dogs

To study pulmonary gas transport in panting, expirograms of several inert and respiratory gases were simultaneously measured in panting dogs. The experiments were performed on 5 conscious dogs (mean body weight 34.4 kg) provided with a chronic tracheostomy. Panting at a mean frequency of 312/min (5.2 Hz) was induced by elevated room temperature (mean 28.1°C). Isotonic saline equilibrated with 50% acetylene and 50% Freon-22 was infused intravenously at a constant rate (4 ml/min). Fractional concentrations in the tracheostomy tube were measured by a respiratory mass spectrometer, using a special sampling system designed for quasi-continuous analysis of rapidly changing gas concentrations. Air flow was monitored by an ultrasonic transit-time flowmeter. A tracing of expired gas concentrations versus expired volume showed no alveolar plateau, displaying a steep increase of Freon-22, acetylene and CO 2 (decrease of O 2) up to the onset of inspiration. The small but statistically highly significant differences between the expirograms of CO 2 and O 2, and of Freon-22 and acetylene, could be qualitatively explained by ventilation-perfusion innequalities with sequential emptying, by Taylor dispersion and by reversible solution in airway mucosa in the course of the respiratory cycle.

Intralipid-Induced Decrease in Oxygenation in Neonates Is Related to Changes in the Respiratory Quotient

Intravenous lipid infusion has been previously reported to be associated with hypoxemia. Different mechanisms have been proposed, but none have explored the decrease in respiratory quotient (RQ) following lipid administration as a possible factor. Ten neonates without respiratory problems, breathing room air and on total parenteral nutrition were studied. Arterialized capillary blood gases, transcutaneous PO2 (TcPO2) measurements and expired gas concentrations were obtained, prior to and 1 g/kg of 10% lipid emulsion was infused over 6 h. Following lipid infusion, the TcPO2 decreased from 72 +/- 8 to 65 +/- 8 mm Hg and the RQ decreased from 0.94 +/- 0.08 to 0.86 +/- 0.5 (p less than 0.05), while the pH, PaCO2 and the alveolar-arterial oxygen tension difference did not change. The decrease in TcPO2 significantly correlated with the decrease in alveolar oxygen tension. Seven of the 10 infants had a significant decrease in TcPO2 with a significant decrease in RQ from 0.98 +/- 0.05 to 0.85 +/- 0.06 (p less than 0.01). In 3 patients with no significant change in TcPO2 following intralipid administration, the RQ was initially significantly lower than the rest of the group (0.85 +/- 0.04; p less than 0.05) and did not change at the end of the infusion (0.88 +/- 0.03). These data suggest that the changes in PaO2 following lipid infusion in neonates without lung disease and breathing room air, are related to the decrease in alveolar oxygen tension secondary to the change in RQ.

A standardised method of estimating KCO on exercise.

This study was designed to standardise a progressive exercise test for the assessment of change in carbon monoxide transfer coefficient (KCO) with exercise and to examine the variation between subjects and the reproducibility within subjects. Normal subjects exercised on a bicycle ergometer while ventilation, heart rate, and expired gas concentrations were recorded continuously. Preliminary studies showed that reduction of the breath hold time to six seconds made measurements of KCO during heavy exercise more comfortable without affecting the result. When KCO was measured immediately after exercise it was lower than when measured during exercise. KCO was measured in 50 normal subjects at rest and at three different work loads maintained for three minutes with a pause of five minutes between each. The relationships between KCO and both oxygen consumption and work load were linear in all subjects but the relationship between KCO and heart rate was distorted by high resting heart rates in some subjects. The mean slope of the relationship between KCO and oxygen consumption (VO2) was steeper in women than in men (mean slopes 0.627 and 0.348 mmol min-1 kPa-1 l-1 per 1 min-1 respectively), and the same was true for the relationship between KCO and work rate. The heart rate rose more steeply in relation to VO2 in women, so that the relationship of KCO to heart rate was similar in men and women (mean slope 0.01 mmol min-1 kPa-1 l-1 per beat min-1). Repeat studies on five occasions in five individuals gave coefficients of variation for the slopes of the relationships between KCO and VO2, work rate, and heart rate of 5-10%.

Evaluation of a microprocessor-controlled exercise testing system

We evaluated a new exercise-testing system (Beckman Horizon MMC), incorporating a microprocessor that controls the acquisition of data, corrects for time delays, applies calibration factors, ensures quality control, and presents results in a variety of formats. Precision of measurements of ventilation (VE) and mixed expired gas concentrations was high. In steady-state exercise (n = 100) VO2 was measured with a precision (+/- SD) of 66 ml/min (4.3%), (r = 0.991); there was a small (4.62%) systematic underestimation of VCO2, but precision was comparable with VO2, with SD being 67 ml/min (4.55%) (r = 0.993). Good agreement was obtained between measurements made in progressive incremental exercise in healthy subjects with correlation coefficients of 0.997 for VE, 0.995 for VO2, and 0.994 for VCO2. Agreement in patients with cardiorespiratory disorders (n = 10) was similar, except in three patients in whom a variable pattern of breathing limited strict comparisons. Comparison with a breath-by-breath analysis system (n = 5) showed that rapid changes in VE, VCO2, and VO2 were followed accurately; the half time for a change in VO2 was not systematically different between the two systems (SD, 3.34 s, r = 0.951). The incorporation of microprocessor-controlled calibration procedures, which are simple to carry out frequently, was judged to be an important feature of this system.

Computer-based evaluation of cardiopulmonary function for the optimization of ventilatory therapy in the adult respiratory distress syndrome.

A computer based evaluation system has been developed for the assessment of respiratory pressure flow dynamics, pulmonary gas exchange and ventilation perfusion interrelationships. This system is based on the acquisition of primary data on-line from intubated and ventilated patients consisting of airway pressures, ventilatory flows and mass spectrometric quantification of inspired and expired gas concentrations. The system has been applied to the study of patients with the adult respiratory distress syndrome (ARDS) following trauma and/or sepsis. The programs developed for evaluation of these data permit an interactive graphic capability to be used by the physician or the respiratory technician in a specific patient to determine the nature of the abnormalities in respiratory function. By use of this system for quantification of the extent and complications of the ARDS condition the specific ventilatory or cardiodynamic therapy can be tailored to meet the patient's physiologic needs. Techniques for optimization of conventional ventilator therapy are described and its application to the specific instances of combined high frequency ventilation or differential lung ventilation are presented. The clinical experience with this unit in a major trauma center suggests that quantitative analysis of an individual patients' respiratory dysfunction permits a precise, accurate and more effective approach to determining corrective therapy in ARDS.

Factors affecting expired waveform for carbon monoxide.

We previously presented a method based on a computer lung model for determining the distribution of both specific ventilation and specific diffusing capacity. These argon and carbon monoxide (CO) washin and washout studies were obtained in 12 normal subjects and 24 patients with varying degrees of obstructive lung disease. In addition to end-tidal and mixed expired gas concentrations, the expired waveform for both gases was sampled. In patients we found that this method failed to adequately describe CO dynamics during the early part of expiration; predicted concentrations were higher than actual data. Modifications of the original model that satisfy all data are presented. This new model suggests that CO uptake occurs in spaces with ventilatory properties of dead space. The accuracy and reliability of these observations were established by computer simulation studies as well as by repeated testing in one subject. These proved to be highly reproducible over a period of 5 mo. Standard parameter sensitivity tests showed parameters to vary by less than 10% and to be stable even when realistic levels of noise were added to the data. We conclude that studies involving ventilation of insoluble gases are insufficient to describe gas exchange in the lung. The addition of an exchangeable gas adds significant understanding of lung function, particularly in disease.

Gas exchange after pulmonary thromboemoblization in dogs.

Gas exchange following experimental pulmonary thromboembolization was studied with an inert gas elimination technique in 17 dogs. Pulmonary arterial, systemic arterial, and expired gas concentrations of six gases infused intravenously were measured before embolization and 5, 10, 15, 30, 60, and 120 minutes after embolization. Distributions of ventilation-perfusion (VA/Q) ratios were derived from the measured concentrations. In all dogs, embolization caused an increase in blood flow and ventilation to VA/Q ratios less than 1. There were no lung units with VA/Q ratios between 10 and 100 before embolization, but, in two-thirds of the dogs, such regions developed after embolization. Ventilation to unperfused lung showed a transient increase of 2-6%. Radioisotope studies of lobes removed post-mortem indicated that thromboemboli rarely caused complete abolition of lobar blood flow. Pulmonary embolization did not cause arteriovenous shunts to appear. The hypoxemia caused by embolization could be accounted for by the changes in VA/Q distributions. Over the 2-hour period after embolization, lung function improved as the distributions partially returned toward the preembolization patterns.

Effect of obesity on ventilatory adjustment to exercise.

The ventilatory response to treadmill exercise was studied in normal and obese subjects, with an analysis of both the steady and unsteady states of exercise. Ventilation, oxygen consumption, carbon dioxide production, respiratory quotient, and alveolar and mixed expired gas concentrations were measured directly or computed. During the steady state of exercise obese subjects increased their ventilation sufficiently to maintain normal alveolar carbon dioxide tensions. During the first 2 min of exercise hypoventilation was more pronounced in obese subjects and in certain individuals resulted in mild reductions in alveolar oxygen tensions. Obese individuals exercised less efficiently than nonobese as manifested by excessive energy expenditure in relation to weight. Steady-state exercise PaCoCo2 values were higher in those subjects previously shown to be relatively insensitive to inhalation of 5% CO2 but failed to correlate with the speed of ventilatory responsiveness.