Question

Observing the effect of placing the patient on a ventilator. Mechanical Ventilation with PEEP=0.0.

What accounts for the different direction of the pleural pressure during inspiration while spontaneously breathing compared to during mechanical ventilation?

Under what circumstance is alveolar pressure equal to airway pressure?

Answer 1

A. Two important rules to understand while talking about mechanics of breathing: 1. Gases diffuse from area of high pressure to area of low pressure, 2. According to Boyle's law, volume and pressure are inversely proportional. When volume increases, pressure decreases, and when volume decreases, pressure increases. Inspiration mechanism is best explained with a picture:

Depending on the cause, a patient is on mechanical ventilation is not breathing spontaneously. The ventilator is connected to a endotracheal tube and pushes air and oxygen into the lungs. It holds a constant amount of pressure called positive end expiratory pressure to keep the alveoli from collapsing.  

B. As seen in the image above, normal intrapleural pressure is less compared to the alveoli and atmospheric pressure. In pneumothorax, disruption of the pleura causes air to the enter the pleural cavity but the air can't escape. The trapped air causes equal pressures in the pleural and alveoli. Due to excess pressure, lungs collapse causing pleuritic chest pain, dyspnea, absent breath sounds on the affected side.