This was a retrospective study. The study population consisted of 106 AOP hospitalized in the Neonatal Intensive Care Unit of Shenzhen Children’s Hospital between October 2021 and October 2022, and 79 cases had complete records of respiratory support parameters. Among them, 39 AOP patients received mechanical ventilation support through trachea (AOP Invasive group), 40 AOP patients inhaled oxygen through face mask, head mask or low flow nasal intubation, (b) nasal continuous positive airway pressure ventilation through nasal continuous positive airway pressure, high flow nasal intubation, and (c) inhaled oxygen autonomously through box oxygen inhalation (AOP Noninvasive group). 40 nonanemic preterm infants matched for age, sex, and gestational age were selected as the control group.

Inclusion criteria for the AOP and control groups were as follows: gestational age < 37 weeks; adherence to the British Committee for Standards in Haematology anemia index for preterm infants; complete clinical records and echocardiographic data. The inclusion criteria for the control group were as follows: gestational age < 37 weeks; not meeting the criteria for anemia; complete clinical records and echocardiographic data. Exclusion criteria for the AOP and control groups were as follows: congenital heart disease [except patent foramen ovale (PFO)]; hemodynamically significant patent ductus arteriosus (hsPDA); receipt of cardiac medications or diuretics; severe pneumonia; severe infection; hypoglycemia; hyperbilirubinemia; ABO hemolytic anemia; history of red blood cell transfusion; neonatal hemorrhagic disorders (e.g., pulmonary hemorrhage, gastrointestinal bleeding, etc.); and prenatal and delivery hemorrhagic disorders.

Clinical baseline data were collected for all participants, including sex, gestational age, corrected gestational age, age, body length, body weight, blood pressure, respiratory support parameters, hemoglobin (HB), and hematocrit (HCT). Written informed consent was obtained from all participants, and the study was approved by the ethics committee of our hospital [Shenzhen Children’s Hospital Medical Ethics Review (Scientific Research) No. 2,022,120].

A complete transthoracic echocardiogram was performed under resting or sleeping conditions using Vivid E95 ultrasound system (General Electric Vingmed Ultrasound, Milwaukee, WI, USA) 6 S-D probe to obtain the best image quality of 70–80 frames per second. All parameters were averaged over three consecutive cardiac measurement cycles. Image acquisition was performed by ultrasound doctors unaware of the study content and clinical data, all in strict accordance with the echocardiography operation standard of American Society of Echocardiography (ASE). Left ventricular end-diastolic diameter (LVEDD), left ventricular end-systolic diameter (LVESD), interventricular septum thickness in end-diastole (IVSD), and posterior wall thickness (PWT) were measured by M-mode echocardiography; left ventricular end-diastolic volume (LVEDV), left ventricular end-systolic volume (LVESV), stroke volume (SV), cardiac output (CO), and heart rate (HR) were obtained by apical 4-chamber view three dimensional echocardiography; early diastolic peak velocity (E) of mitral valve was measured by pulsed wave Doppler imaging; peak early diastolic velocity of mitral annulus (e’) was measured by tissue Doppler imaging.

Brachial blood pressure of the right upper arm was measured using the noninvasive blood pressure cuff mode of the bedside ECG monitor when the heart rate was stable for 20 min and without limb movement for 10 min before echocardiography, which obtained systolic blood pressure (SBP) and diastolic blood pressure (DBP). The respiratory support parameters such as fractional concentration of inspired oxygen and mean airway pressure were recorded, and the mean values of three measurements were taken for statistical analysis.

Dynamic images of three consecutive cardiac cycles from apical three-chamber, apical four-chamber, and apical two-chamber images were applied, and the stored raw data were imported into offline software (EchoPAC PC version 203; GE Healthcare, Horten, Norway) to measure the data. The endocardial and epicardial borders were tracked in the dynamic images using the automatic imaging function, and the area of interest was adjusted by manually correcting the endocardial border or width if necessary, and the global longitudinal strain (GLS) was obtained for 17 segments of the left ventricular myocardium. Input brachial artery blood pressure, and choose mitral and aortic valve opening and closing time points on apical three-chamber dynamic images. The software obtains noninvasive PSL by integrating GLS, blood pressure, and valve opening and closing times. The annular area surrounded by PSL, the product of two-dimensional strain and ventricular pressure, represents the total work done from mitral valve closure to mitral valve opening, represented by global work index (GWI) (Fig. 1). Meanwhile, other parameters of myocardial work were calculated as follows: global constructive work (GCW, mmHg%), which includes the sum of the work done by systolic myocardial shortening and isovolumic diastolic myocardial stretching, that is, the work done by the myocardium contributing to ventricular systolic ejection; global wasted work (GWW, mmHg%), which includes the sum of work done by systolic myocardial stretch and isovolumic diastolic myocardial shortening, work that does not contribute to ventricular systolic ejection; global work efficiency (GWE, %), calculated as GWE=GCW/(GCW + GWW)

Pressure-strain loop diagram

The figure shows three example of the pressure strain loops and the 17 segments of myocardial work, including invasive and noninvasive respiratory support group and control group. On the top left, the area under the loop represents GWI, and the lower shows the 17-segment bull’s-eye view of the GWI; the higher the GWI, the closer the color is to green, and vice versa, the closer it is to blue. AOP Invasive invasive respiratory support group, AOP Noninvasive noninvasive respiratory support group, GWI global work index.

Statistical analysis was performed using SPSS 28.0 (SPSS version 28.0; IBM Corp., Armonk, NY, USA), and graphs were presented using GraphPad Prism (version 7.0.5). Means ± standard deviations (x ± s) and 95% confidence intervals (CI) were used to represent measures that conformed to a normal distribution with equal variance. Medians (upper and lower quartiles) were used to represent measures that did not conform to a normal distribution. The number of cases (proportions) [n (%)] was used to represent count data. Continuous variables were analyzed by T test or Mann-Whitney U test. 15 patients in the AOP group and 15 in the control group were randomly selected for repeated testing. Bland-Altman analysis was used to test the interobserver and intraobserver consistency of myocardial work parameters. Interobserver consistency was assessed by two experienced ultrasound physicians after independent measurements, and intraobserver consistency was assessed by one ultrasound physician after two measurements. Two-sided P < 0.05 was considered statistically significant.