Variation des inspiratorischen Gasflusses unter druckunterstützter Spontanatmung

Zusammenfassung

Bei einigen Respiratoren kann unter Pressure Support Ventilation (PSV) der Inspirationsfluß (V˙ _i ) durch Veränderung der Druckanstiegszeit variiert werden. Über den Einfluß des Inspirationsflußprofils unter PSV auf die Atemarbeit ist besonders bei Patienten mit chronischer Atemwegsobstruktion (COPD) kaum etwas bekannt. Wir untersuchten an 6 COPD-Patienten und 6 Patienten ohne Lungenerkrankung (Kontrollgruppe) die Effekte einer Variation von V˙ _i unter PSV auf atemmechanische Variablen und die mechanische Atemarbeit. Unter individuell eingestellter Druckunterstützung wurde die Druckanstiegszeit in 5 Stufen verändert. Die Verlängerung der Druckanstiegszeit verminderte den initialen V˙ _i in beiden Patientengruppen. Gleichzeitig stieg die Atemarbeit in der COPD-Gruppe bei verringertem V˙ _i signifikant an. Eine Ursache hierfür war, daß bei den COPD-Patienten bei langsamen Druckanstiegszeiten die eingestellte Druckunterstützung nicht mehr erreicht wurde. Der langsamere V˙ _i führte zu einer Verlängerung der Inspiration auf Kosten der Exspirationszeit. Dies verursachte bei COPD-Patienten eine unerwünschte Erhöhung des intrinsischen PEEP. Die Auswirkungen eines niedrigeren V˙ _i unter PSV in der Kontrollgruppe waren klinisch nur wenig relevant. Unsere Ergebnisse zeigen, daß besonders bei Patienten mit COPD unter PSV hohe initiale Inspirationsflüsse zu bevorzugen sind, da ein niedriger Fluß die Patientenatemarbeit erhöht und eine dynamische Lungenüberblähung verstärkt. Allerdings konnte bei einem Patienten ein vorzeitiger Inspirationsabbruch aufgrund eines hohen Initialflusses durch Anpassung des Flußprofils korrigiert werden. In Einzelfällen kann daher eine Verlängerung der Druckanstiegszeit bis auf maximal 1 s sinnvoll sein.

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 F_IO₂ 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 (P_aw) was determined in the same position with a second differential pressure transducer (Dr. Fenyves & Gut, Basel, Switzerland). Esophageal pressure (P_es) 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 W_pi (mJ/l) was calculated from P_es/volume plots according to the modified Campbell's diagram. Dynamic intrinsic PEEP (PEEP_idyn) was obtained from esophageal pressure tracings relative to airway pressure as the deflection in P_es before the initiation of inspiratory flow Patient's additive work of breathing (W_add) against ventilator system resistance was calculated directly from P_aw/V tracings when P_aw 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, W_pi increased significantly in COPD patients. Additionally, the duct cycle (Ti/Ttot) significantly increased with decreased flow rates which resulted in a higher PEEP_idyn 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. W_add 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 PEEP_idyn 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.