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Related to intrathoracic pressure: intrapleural pressure
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Elevated PPV and SVV cut-off values for discriminating fluid responsiveness might be expected under conditions of pneumoperitoneum combined with steep Trendelenburg position, because elevated intra-abdominal pressure shifts the diaphragm cephalad, leading to increased intrathoracic pressure and stiffened abdominal part of the chest wall (27).
With spontaneous ventilation, there is a reduction in intrathoracic pressure during inspiration and a subsequent decrease in pulmonary vascular pressure (Swan, 1991).
Some cases after EBP report that the epidural needle was correctly located within the epidural space (15,16), so it has been speculated that while the operator is collecting venous blood, the dural rent from an earlier unintentional dural puncture allows air to pass to the subarachnoid space, because of negative intrathoracic pressure during inspiration, which reduces epidural pressure to subatmospheric.
In effect, in spontaneously breathing subjects, intrathoracic pressure decreases during inspiration, thereby increasing venous return and inducing collapse of the IVC.
This provides active decompression of the chest, promotes optimal chest wall recoil and creates a negative intrathoracic pressure (vacuum) that helps return blood to the heart.
It stands to reason that the central blood vessels would be better filled during spontaneous ventilation, motivated by the lower intrathoracic pressure, more favorable conditions for venous return, and higher vascular distending pressures.
Indirect monitoring of intrathoracic pressure by means of pulse transit time can give quite good separation of obstructive from central apnoeas (38,39).
Mechanically ventilated patients have a higher mean CVP, which is more indicative of mean intrathoracic pressure than of cardiac filling or intravascular fluid status.
With breathing, there is approximately a 3 to 4 mmHg difference in systolic pressure between inspiration and expiration due to changes in intrathoracic pressure (Darovic, 2002).
With mediastinal emphysema, further transmission of air up fascial planes can be reduced by eliminating straining and increasing intrathoracic pressure with cough suppressants, stool softeners, analgesics, and bed rest.
Unfortunately, we could not rule out the upper airway resistance syndrome as we could not measure intrathoracic pressure.
PEEP is commonly used and is effective in the management of hypoxia secondary to alveolar capillary leak and ventilation perfusion mismatch by increasing intrathoracic pressure and improving arterial oxygenation.