An essential criterion of respiratory resuscitation is that the method be immediately available and effective. Mouth to mouth breathing bears these characteristics. A sufficient amount of oxygen is present in expired air to ensure oxygenation of a victim's blood. Almost all of the disadvantages of manual thoracic pressure techniques are overcome by positive pressure applied to the airway with expired air via mouth to mouth or mouth to nose breathing. With either, the resuscitator supports the head and mandible, the adequacy of gas exchange is judged by rise and fall of the chest, and tidal volume is thus estimated while accumulation of secretions or vomit and obstruction of the airway can be detected. Little expenditure of energy is required to ventilate the lungs in this manner.
External chest compression is employed in situations where positive pressure cannot be applied to the airway, severe maxillofacial injury, inaccessibility of the head, or attempted resuscitation in unusual locations, such as treatment of an electrocuted lineman on a telephone pole. The prone pressure technique is least effective in moving air to clear the area and consequently is no longer advocated by the experts. The only advantage is the prone position, which offers a better chance of a clear airway and escape of vomits from the mouth. This is more effective than the prone pressure technique and this is more recommended.
Posterior or anterior chest compression may be combined with displacement of the patient's hips or shoulders to expand the thoracic cage. With the patient prone, this is accomplished by elevation of the hips or by lifting the flexed arms over the head. Either maneuver increases chest volume by relieving pressure on the ribs, allowing the abdominal cavity to expand, thus pulling the diaphragm downward. When the patient is supine, lung volume is increased by hyperextension of the arms, then by compression of the thorax. This maneuver increases the anterior-posterior diameter of the chest through traction on the shoulder girdle. A nearly adequate volume of air can be moved with these methods if the trachea is intubated. In the usual emergency, however, the resuscitator cannot be certain of a patent airway and it is difficult to gauge the proper degree of chest compression. Furthermore, these methods are ineffectual when the patient is lying on a mattress that yields to pressure.
Cardiac flow ceases when the heart stops beating or when the ventricles fibrillate. At normal body temperature, the well oxygenated brain can tolerate ischemia for less than four to five minutes. Beyond this, even though cerebral blood flow is restored, irreversible damage to the brain usually will have occurred. Should circulatory arrest occur, therefore, the diagnosis must be made at once and a mehtod of moving oxygenated blood to the brain immediately applied. Several factor, singly or in combination, may precipitate cardiac arrest or ventricular fibrillation: complete heart block, myocardial depression, heightened ventricular irritability and inadequate coronary blood flow.