Impedance cardiography: a role in vasovagal syncope diagnosis?

BACKGROUND: vasovagal syncope is the most common cause of syncope in all age groups, with diagnosis usually based on history, examination and basic investigations to exclude alternative causes of syncope. Where doubt exists, the head-up tilt (HUT) test is used for diagnosis but is time consuming and lacks a gold standard to accurately assess sensitivity and specificity. Alternative methods of diagnosing vasovagal syncope would thus be useful. OBJECTIVE: to investigate the potential for impedance cardiography (ICG)-derived haemodynamic measures to predict HUT test outcome in unexplained syncope. DESIGN: prospective controlled study. SUBJECTS: eighty-six patients with unexplained syncope and 43 non-syncopal controls. METHODS: all subjects underwent continuous heart rate, blood pressure and ICG measurements during 10 min supine rest and during HUT. Vasovagal syncope was diagnosed when patients experienced symptom reproduction with concomitant haemodynamic derangements. RESULTS: during rest prior to HUT, the syncopal group had higher mean heart rate (P = 0.0008) and lower baroreceptor effectiveness index (P < 0.0001) compared to non-syncopal controls. On comparing patients who presented with unexplained syncope who subsequently had a positive HUT (therefore a diagnosis of vasovagal syncope 55 [64%]; mean age 47 years, range 17-85) to those having a negative tilt test (n = 31; mean age 47 years, range 17-88), there were no significant differences found in cardiovascular or autonomic parameters prior to HUT. A predictive ROC curve model at a 85% threshold allowed using cardiac index (CI), end-diastolic index (EDI) and left ventricular work index (LVWI) would identify those who would have a positive HUT from baseline cardiovascular measurements (CI >3.5, EDI > 77, LVWI >4.7) with 93% sensitivity and 17% specificity. CONCLUSION: supine haemodynamic measures derived from transthoracic ICG can simply, non-invasively and sensitively differentiate HUT-positive patients from those with negative tilt tests. Further work is needed, particularly in older patients, before this technique can be used in clinical practice.