A 12-year-old girl presented at our institution with three episodes of near syncope. The patient's family history was negative for syncopes as well as for other inheritable disorders and she was not on any medication. She could perform moderate aerobic sports (cycling, hiking, swimming) without any problems. All three spells occurred in the morning and were interpreted as orthostatic intolerance. The physical examination revealed a systolic murmur; the 12-lead-ECG was normal. By echocardiography a discrete prolapse of the mitral valve with trivial mitral regurgitation was diagnosed.
Head-up-tilt table testing was performed in a quiet room at 9 am after a light breakfast with enough fluid intake in order to avoid dehydration as a cause for positive testing. The room temperature was 18 degrees centigrade. The patient was positioned supine on the tilt table without a venous line for 20 minutes. As the patient relaxed and power spectral analysis of heart rate variability showed an increasing vagal tonus, the tilt table was set in the upright position of 60 degrees within 10 seconds according to the guidelines. The test was to be continued until symptoms would arise or to a maximum length of 45 minutes of upright position.
The monitoring device ("Task Force Monitor", CNS Systems, Graz, Austria) consisted of a central computer driving several monitoring systems working independently from each other.
Beat-to-beat blood pressure was measured by a finger cuff measuring online beat-to-beat blood pressure on the 2nd and 3rd finger of the left hand and using the so called vascular unloading technique. The pulse signal with its variables is measured for every heartbeat and transformed into a pulse waveform similar to that obtained by invasive arterial blood pressure monitoring. The waveform is displayed on the master screen giving the relative values of beat-to-beat blood pressure.
Calibration of this system occurs through a conventional non-invasive blood pressure cuff positioned on the right upper arm and is performed every five minutes.
We obtained a standard 6-lead-ECG from the patient's chest giving its signal to the central monitoring device, which in turn measures beat-to-beat heart rate and consecutive R-R-intervals. Every R-R-interval is displayed in milliseconds and correlated online with the changes in beat-to-beat blood pressure given in millimetres of mercury. At this point, baroreceptor sensitivity can be calculated according to the sequence method.
By analysing online the heart rate variability it was possible to gain a power spectral analysis delineating the low-frequency band (0,1 Hz, LF) and high frequency band (0.3 Hz, HF). Thus, sympathetic (LF) and parasympathetic (HF) dominance of the autonomous nervous system can be shown during the test and a LF/HF ratio of greater 2 was used for defining a prominent sympathotonus.
Stroke volume measurement is possible through impedance cardiography. Three electrodes were positioned on the right and left lateral chest wall immediately adjacent to the xiphoid as well as in the neck, respectively. A fourth, reference electrode, is set on the left foot. The relative changes in chest impedance measured by these electrodes are transmitted to the central device and transformed into an online waveform signal shown on the monitor giving the beat-to-beat stroke volume and its changes over time and within the cardiac cycle. By multiplying stroke volume and heart rate we obtain the cardiac output in absolute values and standardized for body surface area (cardiac index). From the two variables of cardiac output and mean arterial blood pressure it is possible to calculate total peripheral vascular resistance and its changes during the tilt test.