Partitioning of dead space--a method and reference values in the awake human.

Although dead space is often increased in disease, it is not frequently measured in the clinic. This may reflect that an adequate method as well as reference values are missing. Healthy males and females, n=38, age 20-61 yrs, were connected to a pneumotachograph and a fast CO2 analyser after radial artery catheterization. The physiological dead space was partitioned into airway and alveolar dead space using a delineation principle denoted the pre-interface expirate. Physiological dead space was 201+/-41 mL in males and 150+/-34 mL in females. Dead space values were depending upon parameters reflecting lung size (predicted total lung capacity), breathing pattern and age. After multiple correlation the variation decreased and differences between males and females disappeared. The residual SD was then for physiological dead space 18.9 mL. The clinical use of the new method for determination of dead space can be based upon reference values, with a more narrow range than previous data.

Aspiration of airway dead space. A new method to enhance CO2 elimination.

Alveolar ventilation and CO2 elimination during mechanical ventilation can be enhanced by reducing dead-space ventilation. Aspiration of gas from the dead space (ASPIDS) is a new principle, according to which gas rich in CO2 during late expiration is aspirated through a channel ending at the distal end of the tracheal tube. Simultaneously, fresh gas injected into the inspiratory line fills the airway down to the same site. We hypothesized that ASPIDS would allow a reduction of tidal volume (VT) and airway pressure (Paw). To test our hypothesis we studied six anaesthetized and mechanically ventilated pigs (24 +/- 4 kg). The intention was to decrease VT while keeping PaCO2 constant by using ASPIDS. VT was reduced by decreasing the minute ventilation (V E) in two steps, of 1.8 L/min (VE - 1.8) and 2.2 L/min (VE - 2.2), respectively, and by increasing respiratory rate (RR) from 20 to 46 breaths/min. At ASPIDS, peak Paw was reduced by 35% at VE - 1.8 and at VE - 2.2 (p < 0.001), and by 20% at an RR of 46 (p < 0.01). PaCO2 was maintained or reduced at ASPIDS. No intrinsic positive end-expiratory pressure developed. Arterial blood pressure and heart rate were unaffected. The results show that ASPIDS allows a reduction in VT and Paw while PaCO2 is kept constant. ASPIDS does not lead to problems associated with jet streams of gas or with gas humidification, and can be developed as a safe technique.

A single computer-controlled mechanical insufflation allows determination of the pressure-volume relationship of the respiratory system.

OBJECTIVE: To evaluate and further develop a method for determination and mathematical characterisation of the elastic pressure-volume (Pel-V) relationship in mechanically ventilated human subjects during one single modified insufflation with simultaneous determination of resistance of the respiratory system. SUBJECTS: Eight adult non-smoking human subjects without heart, lung, or thoracic cage disease scheduled for non-thoracic surgery. The study was performed in anaesthetised and muscle-relaxed subjects. MEASUREMENTS AND MAIN RESULTS: The Pel-V curve was determined with a computer-controlled Servo Ventilator 900C during a modified insufflation with either constant or sinusoidally varying flow. Pressure and flow were measured with the built-in sensors of the ventilator. Tracheal pressure (Ptr) was calculated by subtracting the pressure drop over the tracheal tube. The elastic recoil pressure in the peripheral lung, Pel, was obtained from the calculated Ptr by subtracting the pressure drop over the airways. Ptr was also directly measured through a catheter. The calculated Ptr gave similar results as the directly measured Ptr, thus indicating the reliability of the signal originating from the ventilator sensor for computation of downstream pressures. The inflection points of the sigmoidal Pel-V curve and the compliance of the linear segment were determined with high reproducibility. CONCLUSIONS: Using one single modified insufflation allows a fast and accurate determination of respiratory mechanics. The Pel-V curves were determined with high reproducibility and were adequately described by a three-segment model of the curve incorporating a linear segment between two asymmetrical non-linear segments.

The static pressure-volume relationship of the respiratory system determined with a computer-controlled ventilator.

The pressure-volume relationship of the respiratory system offers a guideline for setting of ventilators. The occlusion method for determination of the static elastic pressure-volume (Pel(st)/V) relationship is used as a reference and the aim of the study was to improve it with respect to time consumption and precision of recording and analysis. The inspiratory Pel(st)/V curve was determined with a computer-controlled ventilator using its pressure and flow sensors. During an automated procedure, an operator-defined volume history preceded each of a number of study breaths. These were interrupted at different volumes evenly distributed over a predefined volume interval. Total positive end-expiratory pressure (PEEP) was measured and could be separated into its components, external PEEP and auto-PEEP. The volume relationship between the curve and the current tidal volume was defined. An analytical method for definition of a linear segment of the Pel(st)/V curve and determination of its compliance is presented. In eight healthy human anaesthetized subjects duplicate Pel(st)/V curves were studied with respect to compliance and the position along the volume axis of the linear segment. The difference in compliance between measurements was 1.6 +/- 1.3 ml cmH2O(-1) or 1.2 +/- 0.9%. The position of the curve differed between measurements by 15 +/- 10 ml or by 1.1 +/- 0.9%. In a patient with acute lung injury the feasibility of applying a numerical method for a more detailed description of the Pel(st)/V curve was illustrated.

Dynamic properties of body plethysmographs and effects on physiological parameters.

A model incorporating compliance, resistance, inertia, and the thermal time constant of plethysmographs is used to describe the effect of its dynamic properties on measured respiratory parameters. Using numerical simulation we studied the effect of distortion of flow signals from 13 infants in whom flow and esophageal pressure had been recorded. The distortion in amplitude, shape, and timing of the recorded flow patterns was dependent on the dynamic properties of the plethysmograph. For constant-volume "pressure" plethysmographs, errors of derived parameters such as compliance and resistance are very important if the thermal time constant is short. These errors are not corrected by calibrating the plethysmograph at the breathing frequency. Time correction of the flow signals in volume-displacement plethysmographs gives accurate results when the plethysmograph resistance and compliance are low. Overall, a volume-displacement plethysmograph with moderately high resistance of the flowmeter, corrected for internal pressure and inertia, gives the best possible results.

Isocapnic high frequency jet ventilation: dead space depends on frequency, inspiratory time and entrainment.

Twelve healthy pigs were ventilated with high frequency jet ventilation via a Mallinckrodt HiLo jet tube. The expired gas was led to a conventional ventilator and CO2 analyzer which were used to measure CO2 elimination. There was no bias flow, so that the jet entrained only expired gas, i.e. rebreathing occurred. Frequency was varied between 2 and 11 Hz and the duration of inspiration, as a fraction of the ventilatory cycle (Ti/Ttot), from 5 to 20%. The minute ventilation, Vjet, delivered by the jet ventilator was adjusted to maintain a constant PaCO2. At 2 Hz and a Ti/Ttot of 5%, Vjet was of the same magnitude as ventilation during conventional intermittent positive pressure ventilation, and the total dead space fraction, VD/VT was 0.32. Both increasing frequency at a constant Ti/Ttot, and increasing Ti/Ttot at a constant frequency, increased VD/VT which was maximal (0.8) at 11 Hz and a Ti/Ttot of 20%. When entrainment was blocked, tidal jet volume had to be greatly increased. The continuous measurement of CO2 elimination was found to be useful for maintaining isocapnia when the jet ventilator setting was changed.

Measurement of ventilation and respiratory mechanics during continuous positive airway pressure (CPAP) treatment in infants.

A new method has been evaluated for measuring ventilation and lung mechanics in spontaneously breathing infants by means of a face chamber. Airway flow is measured with a pneumotachograph inserted between the face chamber and a stable pressure source. Oesophageal pressure is measured via a water-filled oesophageal catheter. The method is suitable for use in conjunction with continuous positive airway pressure (CPAP) treatment in neonatal intensive care. A flat frequency response curve up to 15 Hz for the two measuring systems (i.e., airway flow and oesophageal pressure), and a time shift between the two respective signals of less than 2 msec are prerequisites for correct evaluation of respiratory mechanics. In preterm infants with chest distortion, the inhomogeneity of pleural pressure affects the significance of resistance and compliance values, as calculated from oesophageal pressure. Supra-diaphragmatic pressure variations reflect the resistive and elastic load on the diaphragm exerted by the lungs and thorax. Thus, oesophageal pressure is still useful in studies of respiratory mechanics in preterm infants.

Gas exchange during controlled ventilation in children with normal and abnormal pulmonary circulation: a study using the single breath test for carbon dioxide.

Carbon dioxide single breath tests (SBT-CO2) were obtained during anesthesia and controlled ventilation in 42 children about to undergo thoracic surgery. The tests were obtained with a computerized system based on the Servo ventilator. The system made on-line corrections for compressed volume, apparatus deadspace, and rebreathing. Children with normal pulmonary circulation had excellent gas exchange with high PaO2 values, a mean alveolar deadspace fraction (VDalv/VTalv) of 0.10, and a gently sloping phase III of SBT-CO2. Children with pulmonary hyperperfusion (left to right shunting) due to an atrial septal defect or a ventrical septal defect had significantly lower PaO2 values, steeper phase III slopes, and a greater spread of values for VDalv/VTalv. Children with pulmonary hypoperfusion due to pulmonary stenosis in combination with intracardiac right to left shunting had extremely low PaO2 values, and "adult" values for VDalv/VTalv. They required increased ventilation to maintain CO2 homeostasis. In the pooled material, the airway deadspace was strongly correlated to height, weight, and age. The airway deadspace was unaffected when tidal volume was increased by 37%, and ventilatory frequency simultaneously reduced by 30%, a maneuver that left alveolar ventilation unchanged. This is probably because an end-inspiratory pause was used; when frequency is reduced the length of the end-inspiratory pause increases, allowing proximal diffusion of the alveolar/fresh gas interface.

An analyzer for in-line measurement of expiratory sulfur hexafluoride concentration.

An infrared analyzer for the inert tracer gas sulfur hexafluoride (SF6) is described and evaluated. The analyzer consists of a transducer and a processor unit. It is designed to operate in a nonrebreathing system with a ventilator and a computer. The transducer, which is placed over a cuvette with windows in the ventilator tubings, reads the SF6 concentration in the airway during the expiratory phase. At the end of the inspiratory phase, the zero level of the instrument is automatically reset. The response time and linearity of the analyzer were tested, and interference by other gases was assessed. Full response was reached within 20 ms after a sudden introduction of 0.5% SF6 into the cuvette. The analyzer-computer system had adequate linearity below 0.5% of SF6. Oxygen, nitrogen, and humid air had no influence on the analyzer signal. One hundred per cent nitrous oxide, 4% enflurane, 4% isoflurane, and 4% halothane caused signals corresponding to 0.010, 0.023, 0.022, and 0.043% SF6, respectively. Due to the method for zero reset, the importance of interference from these gases is greatly reduced when inspired and expired concentration approach each other. The disturbance from CO2 (10% CO2 gave a signal corresponding to 0.020% SF6) can be compensated for by including a CO2 analyzer in the set-up. The rapid response and the high sensitivity of the analyzer may make it useful for studies of pulmonary gas mixing and for measurements of lung volume during mechanical ventilation.