Circulation takes precedence over airway and breathing in a cardiac arrest situation. In this scenario, as previously noted, chest compressions should begin prior to the initial breaths. Subse-quent actions to assess circulation may then vary depending on whether the responder is a lay person or health care provider. Although lay rescuers should assume that an unresponsive patient is in cardiac arrest and need not check pulse; health care provid-ers should assess for presence or absence of a pulse.
After successful delivery of two initial breaths (each 2 s in duration), circulation is rapidly assessed. If the patient has an adequate pulse (carotid artery in an adult or child, brachial or femoral artery in an infant) or blood pressure, breathing is continued at 10–12 breaths/min for an adult or a child older than 8 years, and 20 breaths/min for an infant or a child younger than 8 years of age (Table 55–2). If the patient is pulseless or severely hypotensive, the circulatory system must be supported by a combi-nation of external chest compressions, intravenous drug administration, and defibrillation when appro-priate. Initiation of chest compressions is mandated by the inadequacy of peripheral perfusion, and drug choices and defibrillation energy levels often depend on electrocardiographic diagnosis of arrhythmias.
Chest compressions force blood to f low either by increasing intrathoracic pressure (thoracic pump) or by directly compressing the heart (cardiac pump). During CPR of short duration, the blood flow is cre-ated more by the cardiac pump mechanism; as CPR continues, the heart becomes less compliant and the thoracic pump mechanism becomes more important. As important as the rate and force of compression are for maintaining blood flow, eff ective perfusion of the heart and brain is best achieved when chest compression consumes 50% of the duty cycle, with the remaining 50% devoted to the relaxation phase (allowing blood return into the chest and heart).
To perform chest compressions in the unre-sponsive or pulseless patient, the xiphoid process is located and the heel of the rescuer’s hand is placed over the lower half of the sternum. The other hand is placed over the hand on the sternum with the fingers either interlaced or extended, but off the chest. The rescuer’s shoulders should be positioned directly over the hands with the elbows locked into position and arms extended, so that the weight of the upper body is used for compressions. With a straight downward thrust, the sternum is depressed 1½–2 in. (4–5 cm) in adults, 1–1½ in. (2–4 cm) in children, and then allowed to return to its normal position. For an infant, compressions ½–1 in. (1½–2½ cm) in depth are made with the middle and ring fingers on the sternum one finger-breadth below the nipple line. Compression and release times should be equal.
Whether adult resuscitation is performed by a single rescuer or by two rescuers, two breathsare administered every 30 compressions (30:2), allowing 3–4 s for the two breaths. The cardiac com-pression rate should be 100/min regardless of the number of rescuers. A slightly higher compression rate of more than 100/min is suggested for infants, with two breaths delivered every 30 compressions.
Cardiac output can be estimated by monitoring end-tidal CO 2 (Petco2>10 mm Hg, Scvo2>30%) or arterial pulsations (with an arterial diastolic relax-ation pressure >20 mm Hg). Arterial pulsations dur-ing resuscitation are not a good measure of adequate chest compression; however, spontaneous arterial pulsations are an indicator of ROSC. There is new emphasis in the 2010 guidelines on physiological parameters, such as Petco2, Scvo2, and diastolic arterial pressure, to assess the adequacy of chest compressions.
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