(A) Respiratory physiological changes after opening the chest
1, open side lung atrophy
Cause: Loss of negative pressure in the pleural cavity and alveolar atrophy on the open side.
Effects: Alveolar ventilation and diffusion area is sharply reduced (about 50% of the normal area), and pulmonary circulatory resistance increases. The absence of endotracheal intubation and artificial respiration can cause ventilation/perfusion (V/Q) ratio imbalance on the open side, which subsequently causes hypoxemia and respiratory acidosis, and then affects circulatory function.
2.Mediastinal movement and oscillation
The reason for mediastinal movement: the chest cavity on the open side changes from negative pressure to positive pressure, pushing the mediastinum to the healthy side.
Inspiratory phase: ① negative pressure of the thoracic cavity on the healthy side increases, which pushes the mediastinum more towards the healthy side. ②The pressure in the lung on the healthy side turns to negative pressure, while the atrophied lung on the open side still maintains positive pressure, and the gas in the lung on the open side flows to the healthy side, further shifting the mediastinum to the healthy side.
Exhalation phase: ①The pressure in the lung on the healthy side changes from negative to positive, and the gas in the lung is pressed toward the open side of the lung. ②The negative pressure value of the pleural cavity on the healthy side decreases, prompting the mediastinum to move to the open side.
Concept: Such a shift of the mediastinum back and forth with the two phases of respiration is called Mediastinal shift. The magnitude of the mediastinal shift is related to the respiratory dynamics and the elasticity and compliance of the lung tissue. Mediastinal shift causes respiratory distress and hypoxia, and the distortion of large blood vessels in the cardiac cavity leads to obstruction of venous return, reduced return blood volume, and lower cardiac output. It can be eliminated by endotracheal intubation with artificially controlled breathing.
3.Paradoxical breathing and oscillatory gas
Concept: Open chest causes mediastinal oscillation, which also produces oscillation of gas in the lungs. When inhaling, part of the gas is “sucked” from the open side lung into the healthy side lung, and when exhaling, part of the gas is “exhaled” from the healthy side lung into the open side lung, which is called Paradoxical Respiration. The gas flowing between the two lungs is called “oscillatory gas”.
The amount of gas flow depends on the resistance in the airway and the intensity of voluntary breathing.
The degree to which the extra-vocal respiratory resistance is greater than the open-chest side bronchial respiratory resistance determines the severity of paradoxical breathing.
4. Abnormal alveolar ventilation to perfusion (V/Q) ratio
The alveolar atrophy on the open side, insufficient ventilation, and failure to reduce pulmonary blood flow due to the weakened or inhibited hypoxic pulmonary vascular (HPV) contraction mechanism under anesthesia, resulting in V/Q less than 0.8 and increased venous shunts, decreased SpO2 and CO2 accumulation, the severity of which depends on the functional status of the healthy side of the lung and proper management during anesthesia.
(B) The effect of open chest on circulatory function
1.Decrease in cardiac output
Reasons: ①The loss of negative pressure in the pleural cavity leads to a decrease in vena cava return and a decrease in right ventricular preload. ②The heart swings with the mediastinum and the entrance of the vena cava is distorted, which hinders the vena cava return. ③Increased resistance of the atrophic pulmonary vascular bed, decreased left heart return, and decreased left ventricular preload. ④V/Q ratio disorders. ⑤ Poor respiratory management resulting in hypoxia and carbon dioxide accumulation affecting pulmonary blood flow. ⑥Surgical operation directly compresses the heart and large blood vessels.
2. Cardiac function and arrhythmia
Causes: ① Decrease in cardiac blood output and blood pressure affects myocardial blood supply. ②Respiratory disturbance causing hypoxic carbon dioxide accumulation. (3) Direct stimulation of the heart or large blood vessels by surgical operation, compression and strain.
Supraventricular tachycardia is common, with ventricular arrhythmias and even cardiac arrest in severe cases.
(C) Other pathophysiological changes after chest opening
1. The change of pleural cavity and intrapulmonary pressure after chest opening and the stimulation of the pulmonary hilum by surgical operation lead to respiratory, circulatory and endocrine dysfunction.
2. Large loss of body heat and body fluid
(iv) The effect of body position on respiration
1.Patients undergoing thoracic surgery are mostly placed in the lateral position, and the intra-abdominal organs push the diaphragm into the chest, causing it to rise about 4 cm and reducing the functional residual air volume (FRC) of each of the two lungs by about 0.8 L.
2.When awake, the ventilation volume (V) of the lower thoracic side was greater than that of the upper thoracic side; its blood flow (Q) was also greater than that of the upper thoracic side under the influence of gravity, so there was no major change in the V/Q ratio of the two lungs, and no major change in oxygenation function.
3, general anesthesia further reduces the FRC of lateral recumbent patients by 0.4 L. The application of muscle relaxants and the effect of gravity make the upper lung well ventilated with insufficient blood flow; while the lower lung further reduces the pulmonary FRC due to the position and mediastinal downward shift and weight compression and the increase of intra-abdominal pressure, ventilation is also reduced, while blood flow is more, forming insufficient ventilation and excessive blood flow.
4, the upper side of the open chest during thoracic surgery for positive pressure, surgical operations, compression, etc. so that the upper lung expansion is incomplete, ventilation is insufficient, therefore, respiratory function depends mainly on the lower side of the lung and the appropriate ventilation method, in order to avoid hypoxia and carbon dioxide accumulation.