Aging Begins at 30

When You Can't Breathe for Yourself

Ian Maclean Smith, M.D.
Emeritus Professor
Department of Internal Medicine
University of Iowa Hospitals and Clinics
Creation Date: February 2002
Last Revision Date: February 2002
Peer Review Status: Internally Peer Reviewed

Dr. Martin Tobin of Loyola University Veterans Hospital in Chicago, who is an expert in respirator use, has reviewed the recent literature in the New England Journal of Medicine on the June 28^th, 2001.

Patients are admitted to the ICU primarily to receive breathing support. A worldwide review shows the indications for breathing support are 66% for breathing failure (acute respiratory disease, heart failure, septic shock etc.), 15% for coma, 13% for smokers' lung (emphysema or COPD) and muscle nerve paralysis problems in 5%.

There are three main ways to help breathing: assisted-control ventilation, intermittent mandatory ventilation and pressure-supported ventilation. In the first the machine delivers a standard breath (called tidal volume) with each respiratory attempt of the patient. With intermittent mandatory ventilation, standard breaths are delivered at standard times but the patient can still initiate a voluntary breath. In the third method, pressure-supported ventilation, the physician sets the oxygen pressure rather than the volume of every breath. In all cases the object is to give maximum muscle rest to chest and diaphragmatic muscles. Minor variations are covered by a jargon of letters known only to anesthesiologists. The variants aim at muscle rest, maintaining the patient optimally, preventing lung damage, promoting faster healing and in general enhancing the marriage of the machine to the patient's needs. If you breathe too much for the patient the breathing muscles will atrophy.

Each air sac is surrounded by elastic fibers that help the lung recoil after inspiratory stretching. Tobacco destroys these valuable elastic fibers. To get good recoil the pressure at the end of expiration (called PEEP) must be just right. It seems logical that the higher the pressure the better the recoil, but this is not so. If the inflow increases from 30 liters per minute to 90, respiratory efforts are increased 40%.

The next problem is to get more oxygen into the blood without damaging the delicate membranes that line the air sacs, especially if these sacs contain abnormal amounts of fluid. Without special techniques one third of the lung is without air, one third has less air than it should and one third is normally aerated. Too much oxygen leads to bleeding in the sacs and subsequent air blocking membrane is formed. Better air distribution can be obtained by lowering the tidal volume. Doing this lowered the death rate 60% in some diseases and 22% in others. Blood oxygen can be raised by as much as 50% by incorporating sighs (single large mechanically induced breaths) such as increased pressure three times a minute for one hour. More PEEP can do it too but may over dilate the part of the lung that is normally perfused and cause damage. Something as simple as nursing the patient face down rather than on their back can improve the oxygen reaching the blood by better air distribution. In summary, pushing in too much air at too high a pressure is bad.

How much waste gas (carbon dioxide) should be allowed to accumulate in the blood? One way is to decrease the "dead space" by minimizing the tube length between the patient and the machine. When carbon dioxide is allowed to accumulate there is an increase of the pressure in the brain, depression of the muscular work of the heart, an increase in blood pressure inside the lungs and reduced blood flow to the kidneys.

One big problem is that after the patient has been tided over their emergency need for oxygen how do you wean them away from the machine. About a quarter can't make with the accumulation of carbon dioxide stressing the heart badly. The ratio of breathing frequency to breath volume (tidal volume) tells the doctor best if the patient can be weaned below 100. Below 100 the success rate is 95%, over 100 it is less than 5%. Testing patients at weaning takes a lot of judgment to conserve nursing time. When the ratio is right, once-a-day trials for half an hour of independent breathing is best.

If we look at a group of patients aged 80 and older in a separate study, comprising about 300 out of 2,000 ventilated patients, we find that 31% survived not so different from the 45% in the younger group. If you omit the 20% of the elderly with significant kidney, liver, generalized disease or malnutrition then survival is 38% of the old old versus 49% of those under age 80. So aggressive support in the sick oldest old is not futile. What's more, 40% of these survivors were still alive 2 years later.

You never thought there were so many variables, or that results compared well with ventilation in younger patients, did you?

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See related Patient Topics Critical Care, Emphysema, Lungs and Breathing or Procedures and Therapies.

See related Provider Topics Critical Care, Emphysema, Lungs and Breathing or Procedures and Therapies.

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