Background The influences of intrathoracic pressure (ITP) to hemodynamic and respiratory parameters during cardiopulmonary resuscitation (CPR) are confusing.In this research,we investigated the phasic changes of ITP during CPR and reveal the relationships among the hemodynamics,respiratory parameters,and ITP.Methods After 8 minutes of untreated ventricular fibrillation,which was induced in twenty intubated male domestic pigs,12 minutes of 30:2 CPR was performed.Continuous respiratory variables,hemodynamics,ITP and blood gas analysis were measured during CPR.After that,defibrillation was done and prognostic indicators after CPR was recorded.Results Average ITP at baseline was -(14.1±1.6) mmHg (1 mmHg=0.133 kPa).When gasping inspirations were going on,it decreased sharply to near -50 mmHg.ITP fluctuated up and down quickly from near -20 mmHg to 20 mmHg when compressions were performed.These phasic changes became mild as the CPR was performed,the contrast of high and low ITP decreased to (12.95±2.91) mmHg at the end of 12 minutes of CPR.Total alveolus minute volume decreased too,because of the decrease of compression and gasp related ventilations.Curve correlation was found between the tidal volume of compression and ITP:ITP=607.33/(1+3134×e-0.58×TV),(e:natural constant,R2=0.895).Negative correlations were found between the right atrial diastolic pressure and ITP (r=-0.753,P ＜0.01); and positive correlations were found between the coronary perfusion pressure and ITP (r=-0.626,P＜0.01 ).Conclusions ITP is one of the key factors which can influence the prognosis of CPR.Correlations were found between the changes of ITP and the tidal volumes of compressions,right atrial diastolic pressure and coronary perfusion pressure during CPR.More positive ITP during compression and more negative during decompression were good to ventilation and perfusion.