During volume ventilation, peak inspiratory pressure is the pressure needed to overcome which two of the following factors? 1. Muscle pressure 2. Airflow resistance 3. Elastic recoil pressure 4. Transthoracic pressure

a. 1 and 2 c. 3 and 4
b. 1 and 3 d. 2 and 3

ANS: A
During volume ventilation, pressure increases as the volume is being delivered. The pressure at the end of inspiration, or peak inspiratory pressure, represents the pressure needed to overcome both airflow resistance and the elastic recoil pressure of the lungs and thorax. This concept is demonstrated in the equation of motion: Muscle pressure + Ventilator pressure = Elastic recoil pressure + Flow resistance pressure. Muscle pressure is the pressure generated by a person's mus-cles of ventilation to overcome elastic recoil pressure and airflow resistance. Transthoracic pres-sure is a pressure gradient between the pleural space and the body surface. During spontaneous breathing, it is the total pressure needed to expand or contract the lungs and chest wall together.