New concepts of ventilation.

A short survey is given of established methods for ventilatory support. The merits of intermittent mandatory ventilation (IMV) and the new mode of mandatory minute ventilation (MMV) are discussed and compared. The importance of gasflowpressure patterns are analyzed as based on measurements on an awake, non-medicated intubated subject. It can be demonstrated that an inspiratory-assist function minimizes the work of breathing associated with increased airway resistance in the peripheral airways. A metabolic computer system for automatic continuous measurements of oxygen uptake and carbon-dioxide elimination during ventilatory care is described. This metabolic computer is designed to be used with the Engström Erica ventilator and makes it possible to evaluate the caloric demand of the patient as well as the effects of various compositions used for parenteral nutrition.

Airway closure and distribution of inspired gas in the extremely obese, breathing spontaneously and during anaesthesia with intermittent positive pressure ventilation.

Airway closure (closing capacity, CC), FRC, total efficiency of ventilation (lung clearance index, LCI) and distribution of inspired gas (nitrogen washout declay percentage, NWOD) were determined by nitrogen washout techniques and arterial PO2and PCO2 measured by standard electrodes in 10 extremely obese subjects, prior to an during anaesthesia and artifical ventilation. CC was normal, but because of small FRC, airway closure occurred within a tidal breath in 9 out of 10 subjects during spontaneous breathing, when awake. PO2 was reduced, the hypoxaemia correlating to the magnitude of airway closure. LCI was normal, but NWOD was borderline. During anaesthesia, CC was unaltered by FRC was further reduced, so that in nine subjects sirway closure occurred above FRC and tidal volume together. A marked increase in relative hypoxaemia was recorded. LCI and NWOD rose, indicating less efficient and less even ventilation. It is concluded that airway closure reasonably explains the marked hypoxaemia in obese subjects during anaesthesia, and that it may also be the reason for the uneven distribution of inspired gas.

Effects of enflurane on haemodynamics and oxygen uptake with special reference to the influence of surgical stress.

The influence on central haemodynamics of enflurane, in uniform anaesthetic concentration (1.5 MAC), was studied in 10 normocapnic patients undergoing upper abdominal surgery. The patients were studied awake, during anaesthesia prior to surgery, and during surgery. On institution of anaesthesia, cardiac output (QT) fell from 5.05 +/- 0.51 to 4.12 +/- 0.15 1/min and systemic vascular resistance (SVR) decreased from 2.36 +/- 0.22 to 1.93 +/- 0.18 kPa min. 1(-3). The arteriovenous oxygen content difference (AVD) did not change. On commencement of surgery, AVD diminished from 38.5 +/- 2.6 to 30.8 +/- 1.9 ml/min and QT rose to 5.82 +/- 0.46 1/min, while SVR remained unchanged. It is concluded that the fall in QT seen during enflurane anaesthesia is caused by a diminished tissue oxygen demand. It is also suggested that enflurane acts as an alpha-blocking agent.

Airway closure in each lung of anesthetized human subjects.

Airway closure and functional residual capacity (FRC) were assessed for each lung separately in the anesthetized subject by means of a double-lumen tracheal catheter. Airway closure was studied by argon-bolus and nitrogen-washout techniques, and FRC was calculated from single-breath nitrogen washout. Recordings were done with subjects in the supine and lateral postures. In the supine position, closing capacity (CC) exceeded FRC in each lung. Airway closure occurred synchronously in the two lungs. Argon CC was 0.05-0.1 liter larger than nitrogen CC of either lung. Minor gas trapping occurred during the vital capacity (VC) maneuver, so that inspired VC exceeded expired VC by 3%. In the left lateral posture, CC remained unaltered in either lung, whereas FRC was markedly increased in the nondependent and reduced in the dependent lung. Airway closure occurred asynchronously in the two lungs, and its distribution was discontinuous between them. Onset of airway closure in the dependent lung caused an early (60% VC) upstroke on the overall tracer gas recording (sampling of mixed expirate at the mouth), whereas onset of airway closure in the nondependent lung caused an additional upstroke at 10% VC. Gas trapping was more marked in the dependent lung than in the supine position, but some gas was released (expired VC greater than inspired VC) n the nondependent lung.

Gas exchange during ventilator treatment: a validation of a computerized technique and its comparison with the Douglas bag method.

In recent years a number of commercial instruments for on-line gas exchange measurements has been introduced. One of them, the Engström Metabolic Computer (EMC), is here clinically validated as compared to the standard Douglas bag method for gas sampling and a Centronic Mass Spectrometer (CMS) for gas analysis. VO2, VCO2 and RQ were simultaneously measured and calculated with both methods. Twenty individual gas exchange determinations were made at different times on 12 critically ill patients at the intensive care unit. There was a small but significant difference (P less than 0.025) of 4.3% +/- 8.4% (s.d.) between the two methods when they were used to measure VO2 (the EMC giving the lower value). The corresponding value of VCO2 was 2.4% +/- 9.1% (s.d.), and for RQ the difference was -1.98% +/- 7.1% (s.d.). These differences are not significant at the 95% level of significance. Determinations of oxygen uptake during ventilator treatment with standard methods usually involve technical difficulties and are associated with errors of method around 10%. Our results indicate a probable error of about 8.5% for VO2 in clinical situations. Considering the difficulties involved in determinations of gas exchange, the EMC method seems to be a valuable technique with an accuracy within acceptable limits.

Regional differences in lung function during anaesthesia and intensive care: clinical implications.

Anaesthesia and most frequently acute respiratory failure are accompanied by a lowered functional residual capacity (FRC). This lowering promotes airway closure in dependent lung units and forces ventilation to non-dependent regions. Perfusion, on the other hand, is forced towards dependent lung units. A ventilation-perfusion mismatch is created and hypoxaemia may develop. General PEEP counters airway closure, but impedes cardiac output and forces perfusion further to dependent regions. In addition, barotrauma may occur. Improved matching of ventilation and perfusion can be achieved by: (1) positioning the subject in the lateral posture; (2) ventilating each lung separately in proportion to its perfusion (differential ventilation); and (3) applying PEEP only to the dependent lung (selective PEEP). Because of less overall intrathoracic pressure and lung expansion, interference with the total lung blood flow and the danger of barotrauma should be less than with general PEEP. Improved gas exchange with a 50-100% increase in PaO2 has been observed in a limited number of patients with acute bilateral lung disease studied so far during differential ventilation and selective PEEP.

Airway closure during anaesthesia, and its prevention by positive end expiratory pressure.

Airway closure, functional residual capacity (FRC) and the transpulmonary pressure volume relationship of each lung were studied in the anaesthetized subject in the supine and the left lateral positions. In the supine posture, FRC was of approximately the same size in each lung as was closing capacity (CC). CC exceeded FRC in either lung. In the left lateral position, FRC was increased by 0.91 in the non-dependent lung and was reduced by 0.21 in the dependent lung, while CC was unaltered in either lung. Consequently, FRC exceeded CC in the non-dependent lung and was further lowered beneath CC in the dependent lung. Airway closure did not occur in the non-dependent lung until an average of 0.51 of gas had been expelled after the dependent lung had ceased to empty. The addition of positive end-expiratory pressure (PEEP) in the range 0.5-2 kPa, increased FRC more in the non-dependent than the dependent lung. The findings suggest that airway closure is evenly distributed in the horizontal level, while it has a discontinuous distribution between the dependent and non-dependent lung. Moreover, the increase in lung volume caused by PEEP has a distribution that is by no means ideal for the purpose of countering airway closure.

Thermocamera, a macroscopic method for the study of pollution with nitrous oxide in operating theatres.

Nitrous oxide (N2O) is used in high concentrations in inhalation anaesthesia and can serve as a tracer of other, more potent anaesthetic agents polluting the air of operating theatres. It has the quality of absorbing infra-red light with a characteristic peak of 4.5 micrometer in the absorption spectrum. N2O in the operating-room atmosphere will absorb infra-red light emitted from a heat screen, and can therefore be registered by an infra-red-camera equipped with a filter eliminating waves outside the 4.5 micrometer waveband. The method was tested during paediatric inhalation anaesthesia. The infra-red-camera measurements are semi-quantitative and sensitive to an extinction of about 1000 ppmcm, comparable to a N2O concentration of 100 ppm measured by an infra-red N2O monitor. It was demonstrated that major pollution occurs during mask anaesthesia and after extubation. The polluting gas is insufficiently evacuated by the operating theatre ventilation. The method makes it possible to visualize the dispersion of spilled or leaking N2O, and is therefore of value when constructing and evaluating new scavenging equipment and in producing educational material.