The conductance catheter technique could be improved by determining instantaneous parallel conductance (G_P) which is known to be time-varying, and by including a time-varying calibration factor in Baan's equation ((t)). We have recently proposed solutions to the problems of both time-varying G_P and time-varying , which we term "admittance" and "Wei's equation", respectively. We validate both our solutions in mice, compared to the currently accepted methods, hypertonic saline (HS) to determine G_P and Baan's equation calibrated with both stroke volume (SV) and cuvette. We performed simultaneous echocardiography in closed chest mice (n=8) as a reference for left ventricular (LV) volume, and demonstrate that an off-center position for the miniaturized Pressure-Volume (PV) catheter in the LV generates end-systolic and diastolic volumes calculated by admittance with less error (p<0.03) (-2.49±15.33µl error) compared to those same parameters calculated by SV calibrated conductance (35.89±73.22µl error), and by cuvette calibrated conductance (7.53±16.23 µl ES and -29.10 ± 31.53µl ED error). To utilize the admittance approach, myocardial permittivity (_m) and conductivity (_m) were calculated in additional mice (n=7) and those results are used in this calculation. In aortic banded mice (n=4) increased myocardial permittivity was measured (6364±5777 control, 14511±7458 banded, p<0.05), demonstrating that muscle properties vary with disease state. Increased inotropy in response to iv dobutamine was detected with greater sensitivity with the admittance technique compared to traditional conductance (4.9±1.4 to 12.5±6.6mmHg^.µl^-1 Wei's equation (p<0.05), 3.3±1.2 to 8.8±5.1mmHg^.µl^-1 using Baan's equation (p=NS). New theory and method for instantaneous G_P removal, as well as application of Wei's equation, are presented and validated in vivo in mice. We conclude that for closed-chest mice, admittance (dynamic G_P) and Wei's equation (dynamic) provide more accurate volumes than traditional conductance, are more sensitive to inotropic changes, eliminate the need for hypertonic saline, and can be accurately extended to aortic banded mice.