The Effects Of Different Tidal Volume Levels And Mechanical Ventilatory Modes On The Hemodynamics

Objective:to compare the effect of different tidal volume levels on the hemodynamics during mechanical ventilation,and the effect of Bi-phasic positive airwaypressure ventilation mode(BIPAP) and Synchronized intermittent mandatory ventilation mode(SIMV) on the mechanics.Methods:â‘ Animal experiment: Tracheotomy and then ventilation were performed in 5 sedated and anaesthetized sheep .The sheep were kept sedated for lack of auto-respiratory during the study. Under the BIPAP mode,the inspiratory pressure was adjusted to deliver tidal volume at 6,10,15 and 20ml/kg levels and PEEP setting of 5cmH₂O. And then the ventilation mode was switched for SIMV with tidal volume of 5,10,15 and 20ml/kg levels.CVP and hemodynamic data were collected on all above conditions. All the ventilation mode were continued randomly.â‘¡Clinical study: 24 patients who performed invasive mechanical ventilation and PiCCO monitor for several reasons were divided into normal cardiac function group(CI≥2.2L/min·m²) and heart failure group(CI<2.2L/min·m²).Under the BIPAP mode,the inspiratory pressure was adjusted to deliver tidal volume at 6,10,15 ml/kg levels. And then the ventilation mode was switched for SIMV with tidal volume of 5,10,15ml/kg level.Respiratory parameters and hemodynamic data were collected 20 minutes after all above conditions. All the ventilation conditions were continued randomly. To stop the experiment if HR,MAP decreased significantly and malignant arrhythmias were observed. PEEP was set on 5cmH₂O at all the groups to exclude the effect of PEEP.Results:â‘ In the animal experiment, CI and ITBVI were decreased ,Pmean and SVRI increased while HR,MAP didn't significantly changed as tidal volume increased.CI was a little higher under BIPAP mode than SIMV when tidal volume≤15ml/kg,but there was no statistical difference.â‘¡In all the patients,CI,ITBVI decreased as tidal volume increased,Pmean and PEEPi changed significantly among three tidal volume levels. MAP had not changed significantly in the normal heart function group,but decreased significantly at 15ml/kg tidal volume level in heart failure group. HR had not changed significantly in all the groups.â‘¢CI,ITBVI,Pmean and PEEPi all have no significant differences under two ventilation modes in normal cardiac function group, and SVRI is higher inBiPAP group than in SIMV group. In heart failure group,CI was significantly higher under BIPAP at 10ml/kg level ,and CI seemed to be higher under BIPAP mode ,but had no significant difference(P>0.05). All Pmean were higher under SIMV at 6,10,15ml/kg levels than under BIPAP mode. SVRI,HR and PEEPi had no significant differences between all the groups.Conclusions:â‘ Animal experiment:CI was decreased as tidal volume increased even under normalcardiac function. We speculate that ITP,which increased as Pmean increased affected venous return and then heart preload decreased. The decrease of CI was also due to the constriction of ventricular diastole and filling as lung hyperinflation. When tidal volume≤15ml/kg,the effect on the hemodynamics under BIPAP was ralatively smaller.â‘¢In all the patients, increase of ITP affects venous return and reduces ITBVI duo to the gradual increase of Pmean and PEEPi as tidal volume increases, and meanwhile the constriction of ventricular diastole and filling as lung hyperinflation decreased CI. Decrease of MAP was later than the decrease of CI obviously. Tidal volume should be set at about 6ml/kg during mechanical ventilation.â‘¢In the normal cardiac function patients,the two ventilation modes had no significantly different effects on the hemodynamics due to the compensation of normal heart. In the heart failure group, BiPAP mode has the smaller effect on hemodynamics because of the ralative lower mean airway pressure at the same tidal volume.