Objective To explore the mechanism of intrathoracic pressure(ITP) influncing cardiac function and facilitate noninvasive determination of ventricular pressure theoretically.Methods With Valsalva and Mueller maneuver,two-dimension images of standard long axis views and the cross-sectional views were recorded in 20 volunteers,aged from 18 to 45 years,at the specific ITP levels(including-20 mmHg、-10 mmHg、0 mmHg、+ 10 mmHg and + 20 mmHg).The subjects were instructed to mantain for at least 10 s,and three successive measurements were recorded and averaged.The stroke volume(SV) and radius of curvature(R) were obtained from further off-line analysis.Results With the ITP maintaining at -20 mmHg,-10 mmHg,0 mmHg(end expiration),+ 10 mmHg and + 20 mmHg respectively,the corresponding radiuses of curvature were (2.35 ±0.24)cm,(2.25 ± 0.23)cm,(2.14 ± 0.21)cm,(2.02 ± 0.21) cm,(1.93 ± 0.19) cm,there were statistically significances between two groups (P =0.006,0.031,0.005 and ＜0.001,respectively].When the ITP were at 0 mmHg(end expiration),+ 10 mmHg and + 20 mmHg,the stroke volume were (71.54±8.81)ml,(73.20±9.52)ml and (78.81± 14.61)ml (P =0.674,0.135).When the intrathoracic pressure decreased from 0 mmHg to-20 mmHg,the stroke volume were (78.81±14.61)ml,(68.28 ±9.28)ml and (59.69±7.52)ml(P =0.029,0.037).Conclusions The ITP has different effects on the two ventricles,and subsequently generates a pressure gradient across the IVS which can alter its shape and position at end-diastole.With the IVS shifting,the preload and filling function of left ventricle gets changed acorrding to the Frank Starling principle.