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Publisher Summary This chapter discusses work, energy, and power, with special reference to ventilation and cardiac output. Mechanical work is a form of energy, and other forms of energy include heat energy and chemical energy. Work is done or energy is expended whenever the point of application of a force moves in the direction of the force. A constant-pressure generator type of ventilator presents a good example of the interrelationship between the force and distance moved and the pressure and volume changes. During expiration, the pressure at the ventilator falls rapidly to atmospheric and remains at zero while the air is expired. During inspiration, about half the mechanical energy used is stored as potential energy in the elastic tissues of the lung and the chest wall. This energy is subsequently used for the work of expiration. The remaining half of the mechanical energy of inspiration is used in overcoming airway resistance and in moving the air and tissues. As power is the product of pressure and flow, the work of the heart is directly proportional to the mean blood pressure and to the cardiac output. Hypertension results in additional energy requirements even if the cardiac output is unchanged, and hypotension lowers the energy demands on the heart, provided that cardiac output is not simultaneously raised. A high cardiac output also increases the work load of the heart. An increase in the energy requirements of the heart, if maintained, may lead to heart failure. |
