Effect of different cycling off criteria and positive end-expiratory pressure during pressure support ventilation in patients with chronic obstructive pulmonary disease R2 郭彥麟

Objective During pressure support ventilation, ventilator inspiration ends when inspiratory flow drops to a given percentage of the peak inspiratory flow cycling off criteria. This study evaluated the effect of two different cycling off criteria on breathing pattern, respiratory effort, and gas exchange in patients with chronic obstructive pulmonary disease.

Introduction Pressure support minimizes work of breathing Cycling off criteria (Coc): predetermined fraction of the peak inspiratory flow  inspiration to expiration Ideally,ventilator cycling should coincide with the end of Pt’s inspiratory effort.

Introduction In COPD, the inspiratory flow decay is less steep, because of increase in airway resistance, lung compliance, dynamic hyperinflation Ventilator condtinued to delivery inspiratory flow into neural expiration  reduce time for expiration,increase dynamic hyperventilation and work of breathing

Introduction Adjust the Coc should change the duration of ventilator inflation  improve synchrony Acute lung injury(high Coc): reduce tidal volume(Vt), increase respiratory rate(RR) and WOB COPD(high Coc):reduce Vt,incrase (RR),reduce WOB and dynamic hyperinflation

Introduction Hypothesized:that a high value of Coc could reduce the inspiratory time, VT, and minute ventilation associated with a reduction in the dynamic intrinsic positive end-expiratory pressure (PEEPi) and in the respiratory effort.

introduction the effects of different Coc with and without the application of an external PEEP at two levels of pressure support on breathing pattern,respiratory effort, and gas exchange in COPD patients with mild to severe exacerbation of acute respiratory failure.

Material and methods Patients: Thirteen mechanically ventilated patients with acute exacerbation of chronic obstructive pulmonary disease primarily due to pneumonia (PaO2/FIO mm Hg, PaCO mm Hg).

Material and methods Interventions: Two cycling off criteria (5% and 40% of the peak inspiratory flow) at two levels of pressure support (5 and 15 cm H2O) with and without the application of an external positive end-expiratory pressure (6 and 0 cm H2O) were applied.

Material and methods Measurement: Patient–ventilator time delay of cycling off : the difference between the end of inspiratory flow and the lowest value of inspiratory esophageal pressure. Inspiratory effort: the work of breathing, the pressure time product partitioned into the total pressure time product and the pressure time product due to the dynamic intrinsic positive end-expiratory pressure.

Result Effect of cycling off criteria: At 5 and 15 cm H2O of pressure support, the Coc 40% raised the inspiratory flow at which the ventilator stopped inspiration and reduced the patient -ventilator time delay

Result At 5 cm H2O of pressure support, the Coc 40% significantly reduced VT, inspiration time,minute ventilation, and dynamic PEEPi The inspiratory effort (WOBI,PTPtotal, and PTPpeepi) were significantly lower the expiratory WOB did not change.

Result At 15 cm H2O of pressure support, the Coc 40% significantly reduced the TI, but RR and VT were reduced only with a PEEP of 6 cm H2O. The dynamic PEEPi and PTP due to dynamic PEEPi were significantly reduced at the Coc 40%

Result Effect of External PEEP. At both Coc, the application of PEEP significantly reduced the dynamic PEEPi, and the inspiratory effort (WOBI, PTPtotal, and PTPpeepi). The application of 6 cm H2O of PEEP caused a reduction in RR and higher VT only at 15 cm H2O of pressure support with the Coc 5%

Result Effect of Pressure Support Levels The increase in pressure support from 5–15 cm H2O significantly decreased the RR,minute ventilation, dynamic PEEPi, and inspiratory effort (WOBI, PTPtotal, and PTPpeepi) At 15 cm H2O of pressure support, carbon dioxide and physiologic dead space were lower compared with 5 cm H2O of pressure support the arterial

Discussion main finding a) at both levels of pressure support ventilation the Coc 40% reduced the patient–ventilator time delay of cycling off and the dynamic hyperinflation; b) at 5 cm H2O of pressure support the Coc 40% reduced the VT, minute ventilation, and inspiratory effort c) the change in Coc did not affect the gas exchange.

Discussion: Effect of cycling off criteria Adjustments to the Coc change the percentage of inspiratory flow at which the ventilator stops and consequently the duration of ventilator inspiration Premature termination  double triggering and increase WOB Delayed termination  reduce time for lung emptying  dynamic hyperinflation  activate expiratory muscle to terminate ventilator flow

Discussion: Effect of cycling off criteria Dr. Tassaux and colleagues: Intubated patients during pressure support ventilation that the ventilator cycling always occurred after the end of a patient’s inspiratory effort( excess pressurisation: s ) Obstructive patients the excess duration of pressurization was still longer( 0.25 to1.26)

Discussion: Effect of cycling off criteria This excess of pressurisation increased as the severity of airway obstruction worsened The mismatch between the patient and ventilator was always present in all of the patients, indicating that a significant part of inspiration is passively driven by the ventilator alone, regardless of the level of pressure support

Discussion: Effect of cycling off criteria In obstructive pt highest Coc: reduction in the Ti and in Vt. The RR decreased or did not change in nonobstructive pt a too high Coc caused an excessive reduction in Vt with rapid shallow breathing due to a premature cycling

Discussion: Effect of cycling off criteria The modification of Coc did not have significant effect on the expiratory time. In nonobstructive patients a delayed ventilator cycling significantly prolonged the expiratory time.  in obstructive patients, the mechanoreceptor reflex and the vagal reflex, which mediate this response, are both weak

Discussion: Effect of cycling off criteria In obstructive patients, the increase in flow resistance often associated with expiratory flow limitation is one of the major mechanisms leading to dynamic hyperinflation and dynamic PEEPi. The breathing pattern (a high VT or RR) also can significantly increase the amount of dynamic hyperinflation and dynamic PEEPi. In the present study, the COC 40% significantly reduced dynamic PEEPi, primarily due to a reduction in VT and minute ventilation.

Discussion: Effect of cycling off criteria Inspiratory effort was measured by WOB(elastic,tissue and airway resistaance ) and PTP PTP: a good estimation of the metabolic work or of the oxygen consumption of the respiratory muscles PTP:isometirc contraction(counterbalance dynamic PEEPi)+isotonic contration(produce flow and volume )

Discussion: Effect of cycling off criteria Unlike a previous study in which the modification of Coc did not affect the inspiratory effort  at both levels of pressure support the Coc 40%,by reducing the dynamic PEEPi, significantly reduced the PTP component due to the dynamic PEEPi Only at 5 cm H2O of pressure support, the Coc 40% caused a reduction in the inspiratory effort not due to the dynamic PEEPi.

Discussion: Effect of cycling off criteria Hypothesize for the reduction in the inspiratory effort : that a reduction in pulmonary hyperinflation reduced the diaphragmatic tension required to produce a given transdiaphragmatic pressure because of a better lung compliance and length– tension relationship

Discussion:effect of external PEEP In the presence of an air flow limitation, the use of a moderate level of PEEP during assisted ventilation can reduce the muscle inspiratory effort without increase in the lung volume. In the present study, at both levels of pressure support the application of an external PEEP significantly reduced the inspiratory effort.

Discussion: Effect of pressurt level By increasing the level of pressure support, the RR and inspiratory effort were decreased, while contrary to the study of Dr. Nava and colleagues the VT did not increase, avoiding the risk of hyperinflation and ineffective muscle efforts

Discussion:limitation not stratify the severity of COPD only one type of mechanical ventilator Patients were not ready to be weaned from mechanical ventilation, because at baseline they presented a high level of dynamic PEEPi and needed high levels of pressure support, thus the reduction of inspiratory effort was found only at 5 cm H2O of pressure support.

Conclusion In COPD patients with mild to severe exacerbation of acute respiratory failure, primarily due to pneumonia, Coc 40% may reduce the muscle work of breathing independently of any level of external PEEP, though only at a low level of pressure support. However, further studies will be necessaryto better clarify the role of Coc on the outcome.

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