机械通气吸气触发时的呼吸力学分析

Respiratory mechanics analysis of inspiratory trigger in mechanical ventilation

目的:通过观察呼吸机吸气触发波形特点,找出触发点对应的回路压力与流量,结合呼吸机工作原理及呼吸力学定律,确定以肺泡内压等作为反映吸气触发努力程度的指标,建立与其相关的数学公式,分析其影响因素与逻辑关系。方法:将模拟肺分别连接到PB840型呼吸机和SV600型呼吸机的呼吸回路。呼吸机调整为流量触发模式,呼气末正压(PEEP)设为5 cmH 2O(1 cmH_(2)O≈0.098 kPa),分别在流量触发灵敏度(V_(Trig))为3 L/min、5 L/min时,分别以慢速、中速、快速3种触发力度手动牵拉模拟肺触发1次有效送气。调整呼吸机为压力触发模式,分别在压力触发灵敏度(P Trig)为2 cmH_(2)O、4 cmH_(2)O时重复上述过程。通过调节呼吸机显示屏内曲线的刻度,观察不同触发情况下触发点对应的回路压力与流量。以肺泡内压Pa作为研究对象,应用呼吸力学方法分析达到有效触发时刻(T T)所需要的Pa(即Pa -T),以及Pa在触发期间的压力变化幅度(ΔP)及时间跨度(ΔT)。 结果:① T T时刻压力与流量的对应关系:流量触发模式下,慢速、中速、快速触发时,吸入流量为V Trig,回路压力分别对应为PEEP、PEEP-P n、PEEP-P n'(P n、P n'为下降幅度,其中P n'>P n);压力触发模式下,吸入流量为偏流(PB840型呼吸机为1 L/min,SV600型呼吸机可能为2 L/min),回路压力为PEEP-P Trig。② Pa -T的计算:流量触发模式下,慢速触发时Pa -T=PEEP-V_(Trig)R(R为气道阻力),中速触发时Pa -T=PEEP-P n-V TrigR,快速触发时Pa -T=PEEP-P n'-V TrigR;压力触发模式下,Pa -T=PEEP-P Trig-1R。③ ΔP的计算:流量触发模式下,慢速触发时,如无内源性PEEP(PEEPi),则ΔP=V _(Trig)R;如有PEEPi,则ΔP=PEEPi-PEEP+V _(Trig)R。中速触发时,如无PEEPi,则ΔP=P n+V_(Trig)R;如有PEEPi,则ΔP=PEEPi-PEEP+P n+V _(Trig)R。快速触发时,如无PEEPi,则ΔP=P n' +V _(Trig)R;如有PEEPi,则ΔP=PEEPi-PEEP+P n'+V TrigR。压力触发模式下,如无PEEPi,则ΔP=P _(Trig)+1R;如有PEEPi,则ΔP=PEEPi-PEEP+P_(Trig)+1R。④ Pa的压力时间变化率(F P)的计算:设F P=ΔP/ΔT,在ΔP相同的情况下,ΔT越短,说明触发越快;同样,在ΔT相同的情况下,ΔP越大,意味着触发能力或触发努力程度越大,F P更能反映患者的触发能力或触发努力程度。 结论:以触发所需要的最低肺泡内压、肺泡内压的压力跨度及其压力时间变化率3个指标来反映患者的吸气努力程度,并建立数学公式,可直观呈现吸气触发相关因素间的逻辑关系,便于临床分析。

ObjectiveTo find out the circuit pressure and flow at the trigger point by observing the characteristics of the inspiratory trigger waveform of the ventilator,confirm the intra-alveolar pressure as the index to reflect the effort of the trigger according to the working principle of the ventilator combined with the laws of respiratory mechanics,establish the related mathematical formula,and analyze its influencing factors and logical relationship.MethodsA test-lung was connected to the circuit in a PB840 ventilator and a SV600 ventilator set in pressure-support mode.The positive end-expiratory pressure(PEEP)was set at 5 cmH2O(1 cmH2O≈0.098 kPa),and the wall of test-lung was pulled outwards till an inspiratory was effectively triggered separately in slow,medium,fast power,and separately in flow-trigger mode(sensitivity VTrig 3 L/min,5 L/min)and pressure-trigger mode(sensitivity PTrig 2 cmH2O,4 cmH2O).By adjusting the scale of the curve in the ventilator display,the loop pressure and flow corresponding to the trigger point under different triggering conditions were observed.Taking intraalveolar pressure(Pa)as the research object,the Pa(called Pa-T)needed to reach the effective trigger time(TT)was analyzed in the method of respiratory mechanics,and the amplitude of pressure change(ΔP)and the time span(ΔT)of Pa during triggering were also analyzed.Results①Corresponding relationship between pressure and flow rate at TT time:in flow-trigger mode,in slow,medium and fast trigger,the inhalation flow rate was VTrig,and the circuit pressure was separately PEEP,PEEP-Pn,and PEEP-Pn'(Pn,Pn',being the decline range,and Pn'>Pn).In pressure-trigger mode,the inhalation flow rate was 1 L/min(PB840 ventilator)or 2 L/min(SV600 ventilator),and the circuit pressure was PEEP-PTrig.②Calculation of Pa-T:in flow-trigger mode,in slow trigger:Pa-T=PEEP-VTrigR(R represented airway resistance).In medium trigger:Pa-T=PEEP-Pn-VTrigR.In fast trigger:Pa-T=PEEP-Pn'-VTrigR.In pressure-trigger mode:Pa-T=PEEP-PTrig-1R.③Calculation ofΔP:in flow trigger mode,in flow trigger:without intrinsic PEEP(PEEPi),ΔP=V_(Trig)R;with PEEPi,ΔP=PEEPi-PEEP+VTrigR.In medium trigger:without PEEPi,ΔP=Pn+V_(Trig)R;with PEEPi,ΔP=PEEPi-PEEP+Pn+V_(Trig)R.In fast trigger:without PEEPi,ΔP=Pn'+V_(Trig)R;with PEEPi,ΔP=PEEPi-PEEP+Pn'+V_(Trig)R.In pressure-trigger mode,without PEEPi,ΔP=P_(Trig)+1R;with PEEPi,ΔP=PEEPi-PEEP+PTrig+1R.④Pressure time change rate of Pa(FP):FP=ΔP/ΔT.In the sameΔP,the shorter theΔT,the greater the triggering ability.Similarly,in the sameΔT,the bigger theΔP,the greater the triggering ability.The FP could better reflect the patient's triggering ability.ConclusionsThe patient's inspiratory effort is reflected by three indicators:the minimum intrapulmonary pressure required for triggering,the pressure span of intrapulmonary pressure,and the pressure time change rate of intrapulmonary pressure,and formula is established,which can intuitively present the logical relationship between inspiratory trigger related factors and facilitate clinical analysis.