All pressures were measured by P23 ID Statham transducers
(Ohmeda Co) and recorded on a Grass Model 7D
Polygraph recorder (Grass Medical Instruments). Blood gases
were immediately analyzed at each experimental period by
an AVL 990 Automatic Blood Gas system (AVL Scientific
Corporation). Cardiac output was determined by the thermodilution
method (Cardiac Output Computer 78231C,
Hewlett-Packard).
CPR was achieved by use of the Thumper Mechanical
CPR system (Michigan Instruments) to provide consistent,
optimal CPR support for cardiac-arrested pigs. The Thumper
is designed to perform in accordance with the guidelines
for CPR and emergency cardiac care of the American Heart
Association (AHA), and these functions are continuouslyperformed without the fatigue associated with manual CPR.
The Thumper is used in a variety of emergency services,
including rescue vehicles, to provide uninterrupted CPR
support. The Thumper uses a single compressed-oxygen
source to power a piston and ventilate the lungs. Output
was set at 37 L/minute of oxygen and 80 compressions per
minute (as recommended in the AHA guidelines), with ventilation
occurring during a 1.625-second delay after each
fifth compression. The compression force was set at 50 psi
input. The ventilation pressure range was set at 50 cm H2O.
Oxygen content of ventilating gases was 80% to 100%.