Resuscitator. Many types of respiratory resuscitatorsare available, and each has its own characteristic prin-ciples of operation. The resuscitator shown in Figure 42–9A consists of a tank supply of oxygen or air; a mechanism for applying intermittent positive pressure and, with some machines, negative pressure as well; and a mask that fits over the face of the patient or a connector for joining the equipment to an endotra-cheal tube. This apparatus forces air through the mask or endotracheal tube into the lungs of the patient during the positive-pressure cycle of the resuscitator and then usually allows the air to flow passively out of the lungs during the remainder of the cycle.
Earlier resuscitators often caused damage to the lungs because of excessive positive pressure. Their usage was at one time greatly decried. However, resus-citators now have adjustable positive-pressure limits that are commonly set at 12 to 15 cm H2O pressure for normal lungs (but sometimes much higher for non-compliant lungs).
Tank Respirator (the “Iron-Lung”). Figure 42–9Bshows thetank respirator with a patient’s body inside the tank and the head protruding through a flexible but airtight collar. At the end of the tank opposite the patient’s head a motor-driven leather diaphragm moves back and forth with sufficient excursion to raise and lower the pressure inside the tank. As the leather diaphragm moves inward, positive pressure develops around the body and causes expiration; as the diaphragm moves outward, negative pressure causes inspiration. Check valves on the respirator control the positive and negative pressures. Ordinarily these pressures are adjusted so that the negative pressure that causes inspiration falls to -10 to -20 cm H2O and the positive pressure rises to 0 to +5 cm H2O.
Effect of the Resuscitator and the Tank Respirator on Venous Return. When air is forced into the lungs under posi-tive pressure by a resuscitator, or when the pressure around the patient’s body is reduced by the tank respirator, the pressure inside the lungs becomes greater than pressure everywhere else in the body. Flow of blood into the chest and heart from the peripheral veins becomes impeded. As a result, use of excessive pressures with either the resuscitator or the tank respirator can reduce the cardiac output— sometimes to lethal levels. For instance, continuous exposure for more than a few minutes to greater than 30 mm Hg positive pressure in the lungs can cause death because of inadequate venous return to the heart.