Abstract: A pacemaker or other cardiac stimulation device is configured to predict the onset of vasovagal syncope and administers pacing therapy to prevent the syncope from occurring. Prediction of the onset of vasovagal syncope is based upon an analysis of the contractility of the heart muscle. In an example described herein, the contractility of the heart muscle is detected and compared with the average contractility. If the current contractility exceeds the average contractility by a predetermined threshold, a high risk of vasovagal syncope is thereby detected and the heart is paced at a vasovagal syncope prevention rate which may be, for example, 20 to 40 beats per minute faster than a previous heart rate. The contractility of the heart is determined, for example, by measuring the impedance of the heart tissue, by measuring the movement of heart tissue in the wall of the heart, by pressure using an accelerometer, or based on an electrogram signal.