A Narrative Review of Clinical Applications of Systolic Time Intervals
Authors: Seetharam Smitha Pernaje , Shankar M S Vinutha , Reddy Niranjan 
Published: August 2022

Systolic time interval (STI) estimation is an established noninvasive method for the quantifiable assessment of left ventricular (LV) performance in well-being and disease states; it stays valuable for clinical application and forms no burden to the subjects. This manuscript reviews the potential clinical applications and prognostic value of STI for the assessment of LV systolic function in cardiovascular disease (CVD). STIs could be obtained by several noninvasive imaging modalities such as transthoracic echocardiography, tissue Doppler imaging M-mode echocardiography, conventional echocardiography, and so on. In view of that, a literature review for studies reporting the clinical applications of STI in assessing LV systolic function among CVD patients was carried out using PubMed search. Accordingly, the current review describes how STI can be measured; reliability of cardiac time interval measurement in patients with CVD and its role in a clinical setting. With the advent of modern techniques, STI could be easily measured in a clinical setting. Likewise, STI parameter, particularly preejection period and LV ejection time ratio (PEP/LVET), has got the highest degree of correlation with LV ejection fraction (LVEF) in assessing LV performance. Furthermore, reproducibility of systolic ejection time (SET) achieved by the TDI M-mode method is outstanding and better when compared with the reproducibility of SET obtained by the conventional pulsed Doppler method. Furthermore, prolonged SET is independently related with enhanced outcomes among heart failure with reduced EF (HFrEF, i.e., EF ≤40%) but not HF with preserved EF (HFpEF, i.e., EF >40%) patients, indicating that stabilizing SET would be helpful in the case of systolic dysfunction. Clinically, tissue Doppler-derived time intervals could be beneficial to analyze abnormal cases in comparison with other invasive and noninvasive methods of ventricular function examination. Furthermore, phonoelectrocardiography-derived STI parameters, particularly electromechanical activation time-to-LVET ratio, may have a significant role in the diagnostic approach of heart failure (HF) in patients with undifferentiated dyspnea. In addition, in HF patients, PEP/LVET of >0.43 helps to detect LVEF <35% by pulsed Doppler echocardiography. Moreover, LVET continues to be an independent predictor of incident HF and provides incremental prognostic value on the future HF risk and death but not myocardial infarction. In conclusion, STI measurement could be useful, particularly in identifying LVEF <35% in the case of refractory HF patients. This could be beneficial in the selection of patients requiring cardiac resynchronization, specifically when accurate LVEF evaluation by echocardiography proves challenging in atrial fibrillation or if the evaluation is done by a trainee echocardiographer. Furthermore, the cardiac time intervals including SET can be acquired irrespective of rhythm. Good image quality is required for the assessment of LVEF. In contrast, evaluation of SET could be useful in the case of echocardiograms with poor quality images. As a final point, the present review suggests using an echocardiographic parameter like STIs to procure additional information regarding the risk of mortality in patients with HFrEF along with LVEF measurement.