Effects of pressure support ventilation and continuous positive airway pressure on diaphragm performance

Many patients who are on mechanical ventilation are on ventilator modes cal led pressure support ventilation (PSV) and continuous positive airway press ure (CPAP) particularly when they are being weaned. As the diaphragm is res ponsible for approximately 75% of breathing, it is important to promote dia phragm shortening to optimize weaning from mechanical ventilation. The purp ose of our 1998 quasi-experimental study was to explore the effects of PSV and CPVP on diaphragm shortening. An animal model was utilized using four S prague-Dawley rats from the same litter purchased from Sasco (Kansas City, USA). Also measured in this study were intrathoracic pressure (Delta ITP), positive inspiratory pressure, respiratory rate, tidal volume, end-tidal ca rbon dioxide, central venous pressure (CVP) and mean arterial pressure (MAP ). Pressure support was increased in increments of 5 cm H2O at CPAP levels of 0, 2 and 4 cm H2O. A direct assessment of diaphragm shortening was achie ved through the adherence of a miniaturized ultrasonic sensor to the inferi or surface of the middle costal surface of the right hemidiaphragm of four Sprague-Dawley rats. Limitations of this study included a small sample size , anaesthetized rats and abdominal dissection for insertion of the ultrason ic sensor. As PSV was increased, there was a decrease in MAP, CVP, respirat ory rate and end-tidal CO2. When increasing levels of CPAP were added to PS V, a decrease in diaphragm shortening was observed. These results support t hat higher levels CPAP may hinder diaphragmatic function thus prolong mecha nical ventilation. The purpose of this pilot study was to explore the effec ts of PSV and CPAP on diaphragm shortening. Also measured were Delta ITP, p ositive inspiratory pressure, respiratory rate, tidal volume, end-tidal car bon dioxide, CVP and MAP. Pressure support was increased in increments of 5 cm H2O at CPAP levels of 0, 2 and 4 cm H2O. A direct assessment of diaphra gm shortening was achieved through the adherence of a miniaturized ultrason ic sensor to the inferior surface of the middle costal surface of the right hemidiaphragm of four Sprague-Dawley rats. Limitations of this study inclu ded a small sample size, anaesthetized rats and abdominal dissection for in sertion of the ultrasonic sensor. As PSV was increased, there was a decreas e in MAP, CVP, respiratory rate and end-tidal CO2. When increasing levels o f CPAP were added to PSV, a decrease in diaphragm shortening was observed.