The positive inotropic effect of digitalis is due to inhibition of Na+/K+ ATPase pump in cardiac muscle cell membrane leading to:
Decreased activity of Na+/Ca++ exchanger causing decreased influx of sodium and decreased efflux of Ca++ in the sarcolemma
Decreased efflux of Na+ leading to less negative resting membrane potential and opening of voltage gated Ca++ channels on the T tubules
Increased intracellular Na+ causing increased efflux of Na+ and increased influx of Ca++  through Na+/Ca++ exchanger in the sarcolemma
Increased intracellular Na+ decreasing the activity of Ca++ pump in the sarcoplasmic reticulum