InfluÃncias do esforÃo muscular respiratÃrio e da assincronia paciente-ventilador sobre o âstressâ e o âstrainâ pulmonares em modelo mecÃnico de sÃndrome da angÃstia respiratÃria aguda

Acute Respiratory Distress Syndrome (ARDS) is an inflammatory disease characterized by pulmonary edema, stiff lungs and hypoxemia. Patients with ARDS are more susceptible to VILI (ventilator induced lung injury). Under mechanical ventilation, lung stress and strain are the main determinants of VILI and in patients with muscle effort patient-ventilator asynchrony may enhance this phenomenon. Ventilation modes PCV and VCV with auto-flow can minimize patient-ventilator asynchrony, but then can liberate the offer of flow and tidal volume, compromising the protective ventilatory strategy in ARDS. This study aimed to evaluate the influence of muscle effort and patient-ventilator asynchrony on pulmonary stress and strain in a mechanic lung model of acute respiratory distress syndrome. An experimental bench study was performed, using a lung simulator, ASL 5000TM, in which was configured a lung model with restrictive respiratory mechanics with complacency of 25ml/cmH2O and resistance of 10 cmH2O/L/sec. Muscle effort was adjusted in three situations: no muscular effort (Pmus = 0), with inspiratory muscle effort (Pmus = -5 cmH2O) and inspiratory and expiratory effort (Pmus = -5/+5 cmH2O), all with breathe rate (b) of 20 bpm. Five ventilators were connected to the simulator through and endotracheal tube No 8.0 mm and adjusted on VCV, VCV with Auto-flowTM (in the ventilator in which it was available) and PCV modes, all with tidal volume (VT): 420 ml, PEEP: 10 cmH2O and breath rate set in two situations: b = 15 bpm (lower than b of the respiratory muscle effort) and b = 25 bpm (higher than b of the respiratory muscle effort). Variables analyzed were: maximum VT, alveolar pressure at the end of inspiration, effective PEEP, driving pressure, transpulmonary pressure at the end of inspiration and expiration, average transpulmonary pressure, inspiratory peak flow and analysis of mechanic curves. In the studied lung model the b of the ventilator adjusted higher of the b of the patient and not the muscle effort was the main determinant for the development of patient-ventilator asynchrony, causing large variations of the VT and pulmonary pressures, intensifying the lung stress and strain. The ventilatory modes had similar behavior, although VCV Auto-flowTM and PCV have presented slightly higher values of VT and pulmonary pressures. Thus it is concluded that the proper adjustment of the programed breath rate in the assisted/controlled modes can minimize patient-ventilator asynchrony, reducing lung stress and strain.A SÃndrome da AngÃstia RespiratÃria Aguda (SARA) à uma doenÃa inflamatÃria caracterizada por edema pulmonar, pulmÃes rÃgidos e hipoxemia. Pacientes com SARA estÃo mais suscetÃveis à VILI (ventilator induced lung injury). Sob ventilaÃÃo mecÃnica, o stress e o strain pulmonares sÃo os principais determinantes da VILI e nos pacientes com esforÃo muscular a assincronia paciente-ventilador pode potencializar este fenÃmeno. Os modos ventilatÃrios PCV e VCV com AutoFlow podem minimizar a assincronia paciente-ventilador, mas por outro lado podem liberar a oferta de fluxo e volume corrente, comprometendo a estratÃgia ventilatÃria protetora na SARA. Objetivou-se avaliar as influÃncias do esforÃo muscular e da assincronia paciente-ventilador sobre o âstrainâ e o âstressâ pulmonares em modelo pulmonar mecÃnico de sÃndrome da angÃstia respiratÃria aguda. Foi realizado um estudo experimental de bancada, utilizando um simulador de pulmÃo, ASL 5000 no qual foi configurado um modelo pulmonar com mecÃnica respiratÃria restritiva, com complacÃncia de 25ml/cmH2O e resistÃncia de 10 cmH2O/L/sec. O esforÃo muscular foi ajustado em trÃs situaÃÃes: sem esforÃo muscular (Pmus=0), com esforÃo muscular inspiratÃrio (Pmus= -5cmH2O) e esforÃo inspiratÃrio e expiratÃrio (Pmus= -5/+5 cmH2O), todos com frequÃncia respiratÃria (f) de 20rpm. Ao simulador foram conectados cinco ventiladores atravÃs de um tubo orotraqueal n 8,0 mm e ajustados nos modos VCV, VCV com sistema AutoFlow (no ventilador que tinha o sistema disponÃvel) e PCV, todos com volume corrente (VC): 420 ml, PEEP: 10 cmH2O e frequÃncia respiratÃria programada em duas situaÃÃes: f=15rpm (< que a f de esforÃo muscular respiratÃrio) e f=25rpm (> que a f de esforÃo muscular respiratÃrio). As variÃveis analisadas foram: VC mÃximo, a pressÃo alveolar no final da inspiraÃÃo, PEEP efetiva, driving pressure, pressÃo transpulmonar no final da inspiraÃÃo e expiraÃÃo, pressÃo transpulmonar mÃdia, pico de fluxo inspiratÃrio e anÃlise das curvas de mecÃnica. No modelo pulmonar estudado a f do ventilador pulmonar ajustada acima da f do paciente e nÃo o esforÃo muscular o principal determinante para o desenvolvimento de assincronia paciente ventilador, causando grandes variaÃÃes de VC e pressÃes pulmonares, o que intensificou o stress e strain pulmonares. Os modos ventilatÃrios tiveram comportamento semelhante, embora os modos VCV AutoFlow e PCV tenham apresentado valores discretamente maiores de VC e pressÃes pulmonares. Desta forma conclui-se que o ajuste adequado da frequÃncia programada nos modos assistido/controlado podem pode minimizar a assincronia paciente ventilador reduzindo o stress e strain pulmonares. Palavras