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Description / Table of Contents: Abstract During pressure support ventilation (PSV), the timing of the breathing cycle is mainly controlled by the patient. Therefore, the delivered flow pattern during PSV might be better synchronised with the patient's demands than during volume-assisted ventilation. In several modern ventilators, inspiration is terminated when the inspiratory flow decreases to 25% of the initial peak value. However, this timing algorithm might cause premature inspiration termination if the initial peak flow is high. This could result not only in an increased risk of dyssynchronization between the patient and the ventilator, but also in reduced ventilatory support. On the other hand, a decreased peak flow might inappropriately increase the patient's inspiratory effort. The aim of our study was to evaluate the influence of the variation of the initial peak-flow rate during PSV on respiratory pattern and mechanical work of breathing. Patients. Six patients with chronic obstructive pulmonary disease (COPD) and six patients with no or minor nonobstructive lung pathology (control) were studied during PSV with different inspiratory flow rates by variations of the pressurisation time (Evita I, Drägerwerke, Lübeck, Germany). During the study period all patients were in stable circulatory conditions and in the weaning phase. Method. Patients were studied in a 45° semirecumbent position. Using the medium pressurization time (1 s) during PSV the inspiratory pressure was individually adjusted to obtain a tidal volume of about 8 ml/kg body weight. Thereafter, measurements were performed during five pressurization times (〈0.1, 0.5, 1, 1.5, 2 s defined as T 0.1, T 0.5, T 1, T 1.5 and T 2) in random order, while maintaining the pressure support setting at the ventilator. Between each measurement steady-state was attained. Positive end-exspiratory pressure (PEEP) and FIO2 were maintained at prestudy levels and remained constant during the study period. Informed consent was obtained from each patient or his next of kin. The study protocol was approved by the ethics committee of our medical faculty. Gas flow was measured at the proximal end of the endotracheal tube with a pneumotachometer (Fleisch no. 2, Fleisch, Lausanne, Switzerland) and a differential pressure transducer. Tracheal pressure (Paw) was determined in the same position with a second differential pressure transducer (Dr. Fenyves & Gut, Basel, Switzerland). Esophageal pressure (Pes) was obtained by a nasogastric balloon-catheter (Mallinckrodt, Argyle, NY, USA) connected to a further differential pressure transducer of the same type as described above. The balloon was positioned 2–3 cm above the dome of the diaphragm. The correct balloon position was verified by an occlusion test as described elsewhere. The data were sampled after A/D conversion with a frequency of 20 Hz and processed on an IBM-compatible PC. Software for data collection and processing was self-programmed using a commercially available software program (Asyst 4.0, Asyst Software Technologies, Rochester, NY, USA). Patient's inspiratory work of breathing Wpi (mJ/l) was calculated from Pes/volume plots according to the modified Campbell's diagram. Dynamic intrinsic PEEP (PEEPidyn) was obtained from esophageal pressure tracings relative to airway pressure as the deflection in Pes before the initiation of inspiratory flow Patient's additive work of breathing (Wadd) against ventilator system resistance was calculated directly from Paw/V tracings when Paw was lower than the pressure on the compliance curve. Two-way analysis of variance (ANOVA) was used for statistical analysis, followed by post hoc testing of the least significant difference between means for multiple comparisons. Probability values less than 0.05 were considered as significant. Results. COPD patients had significantly higher pressure support than control patients. With decreasing inspiratory flow, Wpi increased significantly in COPD patients. Additionally, the duct cycle (Ti/Ttot) significantly increased with decreased flow rates which resulted in a higher PEEPidyn compared to the baseline. At T 1.5 and T 2 with lower flow rates, the pre-set pressure support level was not achieved within inspiration in the COPD patients. Wadd increased significantly at T 1, T 1.5 and T 2 in COPD patients and at T 1.5 and T 2 in the control group. In one patient, premature termination of inspiration owing to high initial peak flow was corrected by adjustment of the inspiratory flow. Conclusion. Our results demonstrate that a decreased peak flow during PSV resulted in increased patient's work of breathing in COPD patients. During lower flow, the pre-set pressure support level was not attained and additional work had to be done on the ventilator system. Furthermore, the higher PEEPidyn during lower flow rates indicates a higher risk of dynamic pulmonary hyperinflation in patients with COPD. We conclude that the use of pressurization times ≥1 s to decrease inspiratory peak flow during PSV is of no benefit and should be avoided, particularly in COPD patients. However, in selected cases, slight decrease of inappropriately high peak flows might be useful for optimization of PSV setting to avoid premature termination of inspiration.

Description / Table of Contents: Abstract. Intrinsic positive end-expiratory pressure (PEEPi) occurring during mechanical ventilation depends on expiratory time constants, expiratory volume and expiration time as well as on external flow resistance (tubes, valves, etc.). It is not routinely determined in mechanically ventilated patients, but it is necessary to optimize respirator settings. The aim of the present study was the validation of an automated PEEPi determination method implemented in the respirator EVITA (Drägerwerke, Lübeck) in mechanically ventilated patients with acute lung failure. Patients. The method was validated in ten sedated, myorelaxed patients with respiratory insufficiency of different etiologies (five with restrictive, and five with obstructive pulmonary disease). PEEPi was determined using the volume constant ventilatory mode at ZEEP or at an external PEEP of 5 as well as 10 cm H2O. Method. PEEPi was first determined with the automated method implemented in the EVITA (five measurements at each end-expiratory pressure level; PEEPEvita). Steady-state was attained between each measurement. These values were compared to the results obtained with end-expiratory occlusion (external, computer-controlled valve in the inspiratory limb of the circuit) at the respective pressure levels (PEEPEEO). The average of five measurements at each PEEP level with each method was defined as PEEPi for the particular ventilatory situation. Gas flow was measured at the proximal end of the endotracheal tube with a heated pneumotachometer (Fleisch no. 2, Fleisch, Lausanne, Switzerland) and a differential pressure transducer. Tracheal pressure was determined in the same position with a further differential pressure transducer (Dr. Fenyves & Gut, Basel, Switzerland). After A/D conversion, data were sampled with a frequency of 20 Hz and processed on an IBM compatible PC. Software for data collection and processing as well as for control of the occlusion valve was self-programmed. For the statistical analysis we used the Mann-Whitney U-test or Wilcoxon signed-ranks test; a P value less than 0.05 was considered significant. Results. At the given respiratory setting and without PEEP patients with obstructive lung disease had a higher PEEPi (median: 6.4 cm H2O; range: 5.0 – 9.6 cm H2O) than those with restrictive pulmonary disease (median: 2.3 cm H2O; range: 0.8 – 3.0 cm H2O) (P〈0.05). Increasing external PEEP to 5 or 10 cm H2O significantly decreased the pressure difference between PEEPi and external PEEP (P〈0.05), but was unable to eliminate it completely. There was no statistically significant difference between PEEPEEO and PEEPEvita (P=0.43; Wilcoxon signed-ranks tests). Regression analysis showed a highly significant correlation between PEEPEEO and PEEPEvita values (r=0.985, P〈0.001; y=1.03x−0.18). Discussion. PEEPi occurs during ventilation in patients with obstructive and restrictive lung disease. The difference between external end-expiratory pressure and PEEPi decreases with increasing external PEEP. However, PEEPi may increase with increasing external PEEP in some instances. The reason for this may be that the PEEPi determined at the proximal end of the endotracheal tube represents only a mean value of different PEEPi values of various lung regions. Increasing external PEEP only partially alters this mean value due to an effect on PEEPi values lower than external PEEP. The PEEPi values measured by the EVITA respirator compared with classical end-expiratory occlusion with an external valve were nearly identical. Unfortunately, PEEPi measurement of the EVITA can only be performed during controlled and not during assisting (PSV, BIPAP etc.) ventilation. Optimal respirator settings require a knowledge of PEEPi (i.e., adaption of external PEEP for lowering the work of breathing in COPD patients or prolongation of the expiratory phase to avoid unwanted side effects of an occult PEEPi on the circulation). Since modern microprocessor-controlled respirators can easily be updated with the necessary equipment, measurement of PEEPi should be a part of routine ventilatory monitoring today.

Description / Table of Contents: Zusammenfassung An 16 druckunterstützt beatmeten Patienten, davon acht mit chronisch obstruktiver Lungenerkrankung (COPD) und acht ohne obstruktive Lungenkrankheiten wurde der Einfluß eines PEEP von 5 cmH2O und einer Druckunterstützung von 5 und 10 cmH2O auf die mechanische Atemarbeit und andere atemmechanische Meßgrößen untersucht. Sowohl durch PEEP wie auch durch Druckunterstützung konnte die Atemarbeit gesenkt werden. Die Kombination beider Maßnahmen wirkte additiv. Ein PEEP von 5 cm H2O und eine Druckunterstützung von 10 cmH2O senkte die Atemarbeit im Durchschnitt um mehr als 50% in beiden Patientengruppen. Ohne Druckunterstützung leistet der Patienten mehr als 20% seiner gesamten Atemarbeit auf Widerstände des Beatmungssystems (z.B. Gasflußanlieferung, Triggermechanismus etc.). Durch 10 cmH2O Druckunterstützung war dieser Atemarbeitsanteil nahezu kompensiert und zu vernachlässigen. Ein bestehender intrinsischer PEEP bei COPD-Patienten erhöhte die Atemarbeit und wurde durch Applikation eines externen PEEP vermindert. Die Höhe der Atemarbeit war in unserer Untersuchung interindividuell sehr unterschiedlich. Daher erscheint uns eine individuelle Anpassung von PEEP und Druckunterstützung anhand der gemessenen Atemarbeit sinnvoll.

Description / Table of Contents: Zusammenfassung Spontanatmung unter Biphasic Positive Airway Pressure (BIPAP) oder Airway Pressure Release Ventilation (APRV) führt bei Patienten mit akutem Lungenversagen zu einer Reduktion des Blutflusses zu nicht ventilierten Shuntarealen, der Totraumventilation und einer Zunahme des PaO2. Bedingt durch die Spontanatmung nahm der venöse Rückstrom, das Herzzeitvolumen und die Sauerstofftransportkapazität zu, ohne daß der Sauerstoffverbrauch stieg. Bei Patienten, die unter APRV/BIPAP frühzeitig spontan atmeten, war der Gasaustausch signifikant besser als bei den Patienten, die zunächst drei Tage kontrolliert beatmet und anschließend mittels APRV/BIPAP entwöhnt wurden. Die Dauer der maschinellen Beatmung, der Intubation und des Intensivaufenthaltes waren bei Patienten, die frühzeitig unter APRV/BIPAP spontan atmeten, signifikant kürzer. APRV/BIPAP scheint als primäre Unterstützung einer insuffizienten Spontanatmung vorteilhaft zu sein.