You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Investigation of the effects of chest physiotherapy in different positions on the heart and the respiratory system after coronary artery bypass surgery. This trial was conducted to investigate the effect of chest physiotherapy in different positions on the heart and the respiratory system after coronary artery bypass surgery. Patients are divided into two groups of 30 patients each in the study. To the patients in the first group (30 patients), percussion-vibration was performed in the 45° supine position, while slightly laterally lying and endotracheal aspiration was performed in the supine position. To the patients in the second group (30 patients), percussion-vibration was performed in the 0° supine position, while slightly laterally lying and endotracheal aspiration was performed in the supine position. The procedures are repeated two times for all patients and their means were taken. The pre- and postapplication values of patients were measured from central venous and arterial catheters and the values of patient monitors were recorded. Comparison of the two groups in terms of respiratory values did not reveal a significant difference, but chest physiotherapy with the head of the bed at 0° was determined to improve cardiac functions. Evaluation of the groups in terms of pre- and postphysiotherapy applications showed a significant increase in mixed venous oxygen saturation in both groups. Chest physiotherapy with the head of the bed elevated to 45° may be recommended in patients who carry a risk of pulmonary complications and who are candidates for chest physiotherapy at an early stage.
{"Chemical": ["oxygen"]}