A machine that temporarily takes over the function of the heart and lungs to facilitate operations such as open heart surgery, heart transplants, and heart–lung transplants.
A heart–lung machine consists of a pump (to replace the heart’s function) and an oxygenator (to replace the lung’s function). It bypasses the heart and lungs, and the heart can be stopped.
Use of a heart–lung machine tends to damage red blood cells and to cause blood clotting. These problems can be minimized, however, by the administration of heparin, an anticoagulant drug, beforehand.heart–lung transplant A procedure in which the heart and lungs of a patient are removed, and replaced with donor organs. This surgery is used to treat diseases in which the lung damage has affected the heart, or vice versa. Such diseases include cystic fibrosis, fibrosing alveolitis, and some severe congenital heart defects (see heart disease, congenital). A heart–lung machine is used to take over the function of the patient’s heart and lungs during the operation, which is no more dangerous than a heart transplant.
heart-rate The rate at which the heart contracts to pump blood around the body. Most people have a heart-rate of between 60 and 100 beats per minute at rest. This rate tends to be faster in childhood and to slow slightly with age. Very fit people may have a resting rate below 60 beats per minute.
The heart muscle responds automatically to any increase in the amount of blood returned to it from active muscles by increasing its output. During extreme exercise, heart-rate may increase to 200 contractions per minute and the output to almost 250 ml per beat.
The heart-rate is also regulated by the autonomic nervous system. The parts of this system concerned with heart action are a nucleus of nerve cells, called the cardiac centre, in the brainstem, and 2 sets of nerves (the parasympathetic and sympathetic).
At rest, the parasympathetic nerves – particularly the vagus nerve – act on the sinoatrial node to maintain a slow heart-rate. During or in anticipation of muscular activity, this inhibition lessens and the heart-rate speeds up.
Sympathetic nerves release noradrenaline, which further increases the heart-rate and force of contraction. Sympathetic activity can be triggered by fear or anger, low blood pressure, or a reduction of oxygen in the blood.
Release of adrenaline and noradrenaline by the adrenal glands also acts to increase heart-rate.
The rate and rhythm of the heart can be measured by feeling the pulse or by listening with a stethoscope; a more accurate record is provided by an ECG.
A resting heart-rate above 100 beats per minute is termed a tachycardia, and a rate below 60 beats per minute a bradycardia. (See also arrhythmia, cardiac.)... heart–lung machine
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