Study selection and quality assessment
Two reviewers (MA and AK) screened independently and blindly the potential eligibility of each of the abstracts and titles that result from executing the search strategy using a systematic review software (Covidence, London, UK). The reviewers evaluated the abstracts for inclusion and exclusion criteria, in case that all are not disclosed in abstract methods, the study was classified as potentially eligible and the complete article was examined to determine inclusion. Subsequently, disagreements were solved by consensus. Then, both reviewers evaluated the full text versions of all potentially eligible studies. Inclusion criteria are: (1) studies contains data of echocardiographic parameters in two groups; group with sleep apnea disorder and a control group (2) studies with data of echocardiographic parameters in different groups with varying sleep apnea severity.
Echocardiographic parameters needed for inclusion were prespecified according to the update published in 2016 by the American Society of Echocardiography (ASE) and the European Association of Cardiovascular Imaging (EACVI) for the evaluation of Left Ventricular Diastolic Function by Echocardiography(6). Any study with at least one parameter was included. The parameters commonly found were mitral valve deceleration time, isovolumic relaxation time, the ratio of early mitral inflow velocity to mitral annular early diastolic velocity E/e’, left atrial volume index (> 34 mL/m2), the ratio of early to late mitral inflow velocities E/A ( > 14), and left ventricular mass index.
Diagnosis of sleep apnea was considered according to the guidelines of American Academy of Sleep Medicine Clinical Practice Guideline (7). The sleep disorder group consisted of symptomatic patients who had an in-laboratory polysomnography (PSG) or home sleep apnea testing (HSAT) confirmation of sleep apnea with an apnea hypopnea index (AHI) greater than 5.