Gunther Wolff

Series dead space volume assessed as the mean value of a distribution function

Series dead space (VdS) is assumed to be represented by that volume exhaled until alveolar gas is observed. Phase II of the single breath CO2-diagram contains the (flow, concentration and sequence weighted) distribution off all stationary interfaces (SI) expired before phase III. We describe a new method to estimate the mean value of VdS based on the differentiation of phase II. This approximation of VdS is called the ‘Pre Interface Expirate’ (PIE) and is compared in this study with the integrative approach of Langley. Tidal volume (Vt) and PEEP were varied from 71 to 123% and from 0 to 6 cmH2O respectively.The estimation of VdS by differentiation of phase II (PIE) shows excellent reproducibility and depends only on phase II — not on phase III and IV as does VdS-Langley. PIE does not depend on Vt and PEEP per se but reflects the distension of convective airways due to elevated end-inspiratory airway pressure.Our results confirm the predictions of Paivas model calculations in that the size of VdS is determined by the distension of airways rather than by the altered position of the SI.

Reliable detection of inspiration and expiration by computer

A new computer assisted method is proposed to distinguish between the inspiratory and expiratory phases of breathing. The method is based on the analysis of both gas flow and CO2-concentration. The algorithm is effective and reliable and is most suitable in critical care patients when an uninterrupted sequence of breaths is to be analysed immediately at the bedside.Marked variations in tidal volume such as are seen in intermittent mandatory ventilation or spontaneous breathing during the phase of weaning from the ventilator, artefacts such as mechanical vibrations of the flow transducer or its connecting tubes do not disturb the analysis.

Polymorphous Ventilation: A New Ventilation Concept for Distributed Time Constants

After a laparotomy the arterial PO2 is reduced and only comes back to preoperative values after about 5 days. This was already interpreted in the 1930s as being connected with another postoperative finding, the reduction in pulmonary gas volume (Beecher 1933 a, b; Beecher et al. 1933; Bendixen 1964; Bendixen and Laver 1965; Berggren 1942). In the following years many aspects of the anesthesia, surgical procedure, and mechanical ventilation were discussed as possible causes of this hypoxemia, including among others the altered pattern of diaphragmatic movements (Westbrook et al. 1973; Froese and Bryan 1974; Rehder et al. 1975), a change in the surfactant factor (Clemens et al. 1961), and the shift of blood volume. It is now generally accepted that the above mentioned reduction in pulmonary gas volume has something to do with the cause of hypoxemia. This connection is the rational basis of volume controlled ventilation with PEEP, which in the last two decades has been successfully employed with respect to oxygenation. However, it is only in recent years that an understanding of the chain of events has been conclusively elaborated and the speed at which these events develop worked out (Hedenstierna et al. 1985a, b, 1989; Rehder et al. 1985; Tokics et al. 1987): After intubation and mechanical ventilation a sizable volume of dependent lung regions collapses within 3 min. This occurs even in patients who originally had normal lungs.

Derivation of the Pulmonary Function Indices

Pulmonary function indices are numbers that give a quantitative description of physiological pulmonary functions. They are derived from primary data (e. g. VT or time courses of CO2, pressure, etc., see Chapter 1) using certain model-concepts. The term gas exchange used here means the exchange of CO2 for O2 in the blood. The corresponding mass movements are brought about by pressure differences. Two types of pressure can be distinguished: absolute pressure and partial pressure (Dalton’s law). While absolute pressure is the driving force of convection, partial pressure effects diffusion processes. The transport of gas between the pulmonary capillary blood and the mouth can thus be characterized by the type of driving force (Fig. 2.1) and consequently, the pulmonary function can be divided into breathing mechanics, purely mechanical behavior of the lung and the thorax under the influence of pressure, flow, and volume gas distribution within the lung (section on pulmonary volume and intrapulmonary gas mixing) transport of O2 and CO2 through the alveolar septa to the erythrocytes and blood plasma (section on transpulmonary gas transport) perfusion of the lung.

Application 1: Standard Values During Mechanical Ventilation After Cardiac Surgery

When patients with normal lungs are ventilated, such as for abdominal surgery, the lung volume is already found to be reduced just a few minutes after the anesthesia has been induced. Furthermore, x-ray reveals shadows in the dependent zones, which are interpreted as atelectases (Strandberg 1985). Lung volume reduction has been confirmed by means of N2 washout, He dilution, and computed tomography. Furthermore, the V´A/Q´ ratios have been found to be changed, the arterial O2 tension lowered, and the distensibility of the lung reduced. Thus, the term “normal” cannot be applied to values in patients with previously healthy lungs obtained immediately after the beginning of mechanical ventilation, i. e., even before an operation. Values obtained after surgery deserve being called so even less. Nevertheless, patients with previously normal lungs who are mechanically ventilated immediately after a particular operation find themselves in a typical situation. In this chapter we shall report the standard values that were recorded in patients mechanically ventilated after open heart surgery in whom the postoperative course was uneventful.

Assessment of Pulmonary Function Indices

The gas flow, airway pressure, and gas composition are measured directly at the mouth, i. e., between the respirator and the patient, in the so-called “measuring head” (Fig. 1.1). This consists of a Fleisch pneumotachograph (PT) No.2 and two Teflon adapters, which provide a “soft” transition from the narrow endotracheal tube and the ventilator connector to the diameter of the PT. A catheter to measure the airway pressure (perpendicular to the flow) and the capillary for the mass spectrometer (tip in the center of the gas flow) are inserted into the Teflon adapter which is closer to the patient.

Application II: A Study on Optimizing Mechanical Ventilation

Thanks to the progress made in the fields of surgery and anesthesia, older patients and those in poor general condition can now be successfully operated on. During recovery, however, pneumonia is still the major fatal postoperative complication. Advancements in postoperative treatment have reduced the incidence of this serious complication with the result that every year more and more patients are being mechanically ventilated.

Application III: A Study on Intermittent Mandatory Ventilation (IMV)

Intermittent mandatory ventilation (IMV) is a ventilatory mode with a complex pattern of breaths. An IMV-cycle begins with a mandatory breath (MB) imposed on the patient by the ventilator, which is followed by a sequence of spontaneous breaths (SB). Since pleural pressure during inspiration rises in the MB and falls in the SB, we assumed that the gas exchange in mandatory breaths is different from that in spontaneous breaths. It is difficult, however, to investigate this hypothesis quantitatively by comparing the mean values of a period of IMV with the mean values of a period of CPPV and of CPAP. Mandatory and spontaneous breaths have to be compared separately. Consequently, we had to use methods suitable for a breath-by-breath analysis. In other words, this study demonstrates special advantages of the measuring system introduced in the present volume. We are able to show that each single breath is investigated separately and the results of the breaths of a certain type i.e., spontaneous and mandatory breaths — can subsequently be grouped. Moreover, the comparison of IMV with CPAP provides the opportunity to demonstrate the measuring system as it is applied for spontaneous breathing.

Evaluation of Pulmonary Function in the intensive Care Patient

Beat-to-beat monitoring of electrocardiograms, arterial and central-venous pressure, and even pulmonary arterial pressure (to name just a few methods) have become absolutely routine in intensive care units and in operating theaters. The circulation monitoring devices currently being marketed by the medicalelectronics industry are so sophisticated that problems associated with calibration, zero points, or stability are just about passe. Measuring methods are designed in such a way that for the nursing staff who uses them they are simply child’s play. Readings are shown at bedside on digital and graphic displays, on line and in real-time. And what is more, the pragmatic interpretation of these readings by the clinician is coming closer and closer to the interpretation made in physiological models.