Georg Mols

Extracorporeal membrane oxygenation: a ten-year experience

BACKGROUND Extracorporeal membrane oxygenation (ECMO) is a supportive therapy used for severe acute respiratory distress syndrome (ARDS). We present outcome, clinical parameters, and complications in a cohort of 245 ARDS patients of whom 62 were treated with ECMO. METHODS Data of all ARDS patients were prospectively collected between 1991 and 1999. Outcome and clinical parameters of patients treated with and without ECMO were evaluated. RESULTS One hundred thirty-eight patients were referred from other hospitals, 107 were primarily located in our hospital. About one fourth of these patients were treated with ECMO. The survival rate was 55% in ECMO patients and 61% in non-ECMO patients. CONCLUSIONS ECMO is a therapeutic option for patients with severe ARDS, likely to increase survival. However, a randomized controlled study proving its benefit is still awaited. Until the development of a causal or otherwise superior therapy ECMO should be used in selected patients.

Respiratory comfort of automatic tube compensation and inspiratory pressure support in conscious humans

ObjectiveTo compare the new mode of ventilatory Support, which we call automatic tube compensation (ATC), with inspiratory pressure support (IPS) with respect to pereeption of respiratory comfort. ATC unloads the resistance of the endotracheal tube (ETT) in inspiration by increasing the airway pressure, and in expiration by decreasing the airway pressure aecording to the non-linear pressure-flow relationship of the ETT.DesignProspective randomized single blind cross-over study.SettingLaboratory of the Section of Experimental Anaesthesiology (Clinic of Anaesthesiology; University of Freiburg).SubjectsTen healthy volunteers.InterventionsThe subjects breathed spontaneously through an ETT of 7.5 mm i. d. Three different ventilatory modes, each with a PEEP of 5 cmH2O, were presented in random order using the Dräger Evita 2 ventilator with prototype software:(1)IPS (10 cmH2O,1 s ramp),(2)inspiratory ATC (ATC-in),(3)inspiratory and expiratory ATC (ATC-in-ex).Measurements and main resultsImmediately following a mode transition, the volunteers answered with a hand sign to show how they perceived the new mode compared with the preceding mode in terms of gain or loss in subjective respiratory comfort: “better”, “unchanged” or “worse”. Inspiration and expiration were investigated separately analyzing 60 mode transitions each. Flow rates were continuously measured. The transition from IPS to either type of ATC was perceived positively, i.e. as increased comfort, whereas the opposite transition from ATC to IPS was perceived negatively, i. e. as decreased comfort. The transition from ATC-in to ATC-in-ex was perceived positively whereas the opposite mode transition was perceived negatively in expiration only. Tidal volume was 1220 ± 404 ml during IPS and 1017 ± 362 ml during ATC. The inspiratory peak flow rate was 959 ± 78 ml/s during IPS and 1048 ± 197 ml/s during ATC.ConclusionsATC provides an increase in respiratory comfort compared with IPS. The predominant cause for respiratory discomfort in the IPS mode seems to be lung over-inflation.

Volume-dependent compliance in ARDS: proposal of a new diagnostic concept

Objective: Adaptation of ventilator settings to the individuals respiratory system mechanics requires information about the pressure-volume relationship and the change of compliance which is dependent on inflated volume. Unfortunately, established methods of obtaining this information are invasive and time-consuming, and, therefore, not well suited for clinical routine. We propose a new standardized diagnostic concept based on the recently developed slice method. This multiple linear regression method (MLR) determines volume-dependent respiratory system compliance (CSLICE) within the tidal volume (VT) during ongoing mechanical ventilation. The impact of a ventilator strategy, recommended by a consensus conference, on the course of compliance within VT was investigated in patients with the acute respiratory distress syndrome (ARDS) or acute lung injury (ALI).¶Design: Prospective observational study.¶Setting: Intensive care unit of a university hospital.¶Patients: 14 ARDS patients, 2 patients with ALI.¶Interventions: None.¶Measurements and results: After measurement of flow and airway pressure and calculation of tracheal pressure, CSLICE was determined. The resulting course of CSLICE within VT was estimated using a mathematical algorithm. CSLICE data were compared to those obtained by standard MLR. We found decreasing CSLICE mainly in the upper part of VT in all patients. In 7 patients, we found an additional increasing CSLICE mainly in the lower part of VT.¶Conclusions: CSLICE was not constant in patients with ARDS/ALI whose lungs were ventilated according to consensus conference recommendations. The proposed diagnostic concept may serve as a new tool to obtain a standardized estimation of respiratory system compliance within VT non-invasively without interfering with ongoing mechanical ventilation.

Respiratory system inertance: investigation in a physical inertance model

This paper provides a description and phenomenological interpretation of the Sutherland Family Network, a volunteer home visiting project in the Sutherland Shire, Sydney, Australia. Home visitation is undergoing a revival in health and welfare systems. Research and experience has shown home visitation to be an effective strategy in attempts to prevent child abuse and neglect, in all its forms. Programmes utilizing volunteers are just one form of home visitation. They do, however, raise a number of industrial, socio-political and gender issues, which have been addressed in this study. Phenomenological analysis was used to describe the experiences of the coordinator, the volunteers, and the mothers who have been involved in the project. The study is based on interviews conducted with the coordinator, 10 volunteers and 15 mothers between February and July 1996. The Sutherland Family Network provides service to new mothers, of any parity, in families at low risk of child abuse and neglect. Befriending was their main role as volunteers. Volunteers were trained not to give advise or counselling. Training also ensured against volunteer exploitation. A unique approach of this volunteer project was a focus on the new mother, rather, than the children. New mothers became involved in the Network mainly due to either emotional or physical isolation, or depression. Major functions of the Network were to provide a service of friendship, and to link mothers with other community groups and professionals.

Extubation after breathing trials with automatic tube compensation, T-tube, or pressure support ventilation

Background: Automatic tube compensation (ATC) is a new option to compensate for the pressure drop across the endotracheal or tracheostomy tube (ETT), especially during ventilator‐assisted spontaneous breathing. While several benefits of this mode have so far been documented, ATC has not yet been used to predict whether the ETT could be safely removed at the end of weaning, from mechanical ventilation.

Continuous calculation of intratracheal pressure in the presence of pediatric endotracheal tubes.

Objective: To measure the pressure‐flow relationship of pediatric endotracheal tubes (ETTs) in trachea models, to mathematically describe this relationship, and to evaluate in trachea/lung models a method for calculation of pressure at the distal end of the ETT (Ptrach) by subtracting the flow‐dependent pressure drop across the ETT from the airway pressure measured at the proximal end of the ETT. Design: Trachea models and trachea/lung models. Setting: Research laboratory in a university medical center. Interventions: The pressure‐flow relationship of pediatric ETTs (inner diameter, 2.5‐6.5 mm) was determined using a physical model consisting of a tube connector, an anatomically curved ETT, and an artificial trachea. The model was ventilated with sinusoidal gas flow (12‐60 cycles/min). The coefficients of an approximation equation considering ETT resistance and inertance were fitted separately to the measured pressure‐flow curves for inspiration and expiration. Calculated Ptrach was compared with directly measured Ptrach in mechanically ventilated physical trachea/lung models. Measurements and Main Results: The pressure‐flow relationship was considerably nonlinear and showed hysteresis around the origin caused by the inertia of accelerated gas. ETT inertance ranged from 0.1 to 0.4 cm H2O/L·sec2 (inner diameter, 6‐2.5 mm). The abrupt change in cross‐sectional area at the tube connector caused an inspiration‐to‐expiration asymmetry. Calculated and measured Ptrach were within ± 1 cm H2O. Correspondence between measured and calculated Ptrach is improved even further when the ETT inertance is taken into account. Conclusions: Ptrach can continuously be monitored in the presence of pediatric ETT by combining ETT coefficients and the flow and airway pressure continuously measured at the proximal end of the ETT.

Breathing pattern associated with respiratory comfort during automatic tube compensation and pressure support ventilation in normal subjects

Background: Automatic tube compensation (ATC) is a new option to support spontaneously breathing tracheally intubated patients. We have previously demonstrated an increased respiratory comfort compared to pressure support ventilation (PSV) in volunteers. Here we characterized the breathing pattern during ATC associated with respiratory comfort in comparison to PSV. Furthermore, we studied whether ATC can be substituted by a simple modification of PSV.

Detection of endotracheal tube obstruction by analysis of the expiratory flow signal.

Objective: Acute obstruction of endotracheal tubes (ETT) increases airway pressure, decreases tidal volume, increases the risk of dynamic hyperinflation by prolonging the duration of passive expiration, and prevents reliable calculation of tracheal pressure. We propose a computer-assisted method for detecting ETT obstruction during controlled mechanical ventilation. The method only requires measurement of the expiratory flow. Design: Computer simulation; prospective study in two cases; retrospective study in one case and in seven patients with the adult respiratory distress syndrome (ARDS). Setting: Laboratory of the Section of Experimental Anaesthesiology (University of Freiburg); surgical adult intensive care units in a university hospital (University of Basel) and in a university affiliated hospital (Zentralklinikum Augsburg). Patients: 3 patients with partial ETT or bronchial obstructions and 7 ARDS patients. Measurements and results: Expiratory flow was measured using a pneumotachograph and integrated to obtain expiratory volume. The time-constant of passive expiration (τE) as a function of expired volume [τE(VE) function] was calculated from the expiratory volume/flow curve. We investigated the τE(VE) function of data obtained from: (1) computer simulation of mechanically ventilated homogeneous and inhomogeneous lungs intubated with ETTs of different sizes; (2) one patient with an artificial ETT obstruction of 7.5 and 25 % of the cross-sectional area of the ETT (case 1); (3) one patient with ETT obstruction due to secretions (case 2); (4) one patient with acute bronchial constriction (case 3); (5) seven ARDS patients who showed an increase in airway resistance of more than 2 cm H2O · s/l. It was found that an ETT obstruction caused an increase in τE in early expiration (at high flow), whereas τE in late expiration was virtually unchanged. The reason for this is the flow dependency of the increase in ETT resistance produced by ETT obstruction. Unlike ETT obstruction, an increase in pure airway resistance produced an increase in τE throughout expiration. Conclusions: An ETT obstruction can be reliably distinguished from an increase in pure airway resistance by a characteristic pattern change in the τE(VE) function, which can be detected easily even by an automated pattern recognition system.

Volume-dependent compliance and ventilation-perfusion mismatch in surfactant-depleted isolated rabbit lungs.

ObjectiveVolume-dependent alterations of lung compliance are usually studied over a very large volume range. However, the course of compliance within the comparably small tidal volume (intratidal compliance-volume curve) may also provide relevant information about the impact of mechanical ventilation on pulmonary gas exchange. Consequently, we determined the association of the distribution of ventilation and perfusion with the intratidal compliance-volume curve after modification of positive end-expiratory pressure (PEEP). DesignRepeated measurements in randomized order. SettingAn animal laboratory. SubjectsIsolated perfused rabbit lungs (n = 14). InterventionsSurfactant was removed by bronchoalveolar lavage. The lungs were ventilated thereafter with a constant tidal volume (10 mL/kg body weight). Five levels of PEEP (0–4 cm H2O) were applied in random order for 20 mins each. Measurements and Main Results The intratidal compliance-volume curve was determined with the slice method for each PEEP level. Concurrently, pulmonary gas exchange was assessed by the multiple inert gas elimination technique. At a PEEP of 0–1 cm H2O, the intratidal compliance-volume curve was formed a bow with downward concavity. At a PEEP of 2 cm H2O, concavity was minimal or compliance was almost constant, whereas higher PEEP levels (3–4 cm H2O) resulted in a decrease of compliance within tidal inflation. Pulmonary gas exchange did not differ between PEEP levels of of 0, 1, and 2 cm H2O. Pulmonary shunt was lowest and perfusion of alveoli with a normal ventilation-perfusion was highest at a PEEP of 3–4 cm H2O. Deadspace ventilation did not change significantly but tended to increase with PEEP. ConclusionsAn increase of compliance at the very beginning of tidal inflation was associated with impaired pulmonary gas exchange, indicating insufficient alveolar recruitment by the PEEP level. Consequently, the lowest PEEP level preventing alveolar atelectasis could be detected by analyzing the course of compliance within tidal volume without the need for total lung inflation.

Large increase in cardiac output in a patient with ARDS and acute right heart failure during inhalation of nitric oxide

Background Inhaled nitric oxide (NO), a selective pulmonary vasodilator, reduces pulmonary artery pressure in patients with acute respiratory distress syndrome (ARDS). In spite of the reduction of right ventricular afterload, the effect of NO on cardiac output remains unclear.