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Respiratory Failure 

The definition of respiratory failure. From a pure blood gas standpoint, it is when the amount of CO2 we make, because of our metabolism, exceeds the amount we can get rid of. It means that your PCO2 has gone up but, for acute respiratory failure, importantly Now, some of those people may normally have a PCO2 of 70 and they get sick and now itís 100 and itís the change that is perhaps the most important. So the key thing here is that not only does their CO2 rise but their pH drops as well.

Thinking about the differential for respiratory drive, largely itís a failure of the signal generator to send out a signal or perhaps the wires. Brainstem suppression is common and itís almost exclusively due to drugs we give and so usually, for example, with brainstem damage due to trauma or disease, you stimulate drive. I can think of two cases where drive was suppressed because of a specific lesion.

The second general category in our differential diagnosis is muscular weakness and it can be neurologic mediated through things like polio, Lou Gehrigís disease, multiple sclerosis, myasthenia gravis or spinal cord injury but perhaps more commonly would be things like malnutrition and electrolyte disorders like hypophosphatemia, for example, in which profound weakness.

The third category would be when your work of breathing is high and I have a picture of an Ambu bag here because I want you to think about if you intubated somebody in the ER and you had to bag them, your right hand is now doing the work. With airway obstruction, Iíve got a picture of an endotracheal tube filled with a lot of secretions in an upper airway. Weíre really talking about secretions, bronchitis, clots in the airway due to bleeding, foreign body. Iíve pulled a dime, a gold tooth, broccoli.

The third category would be when the lungs themselves are stiff, the so-called meat of the lungs or parenchyma are involved. It could be in the form of heart failure. Everyone wants to say, "Oh you mean like pulmonary fibrosis." "Iíll say yeah. That would be 2% of the causes." But good old fashioned heart failure would be by far the most common cause of a stiff lung in most care settings so that could be cardiac in origin but it could also be due to edema, due to leaky capillaries from a lung injury like an aspiration or a sepsis.

The second part of that stiffness would be not the lung itself but the pleura in the chest wall and this could be in the form of chest wall diseases. Some of the most profound CO2 retention Iíve seen are in older people, mostly women, with severe kyphoscoliosis. The last category of work is a little different. I hadnít thought of this a whole lot before I did my fellow ship but essentially youíve got this situation where youíve got one type of alveolus that is well perfused, well ventilated and the blood comes in blue and goes out red as it should. But youíve got other alveoli, perhaps with a pulmonary embolism in which you ventilate that unit.

They present with poor exercise tolerance. "Doc, you know I just canít get up and do the lawn anymore." If youíve ever had a person with a pulmonary embolism, this is what they tell you in terms of whatís different about their day to day. Itís not just PE.  Well, how do we assess these? Well, some pretty simple things. I think to look at drive, you really want to look at the breathing pattern and essentially if the patient is in respiratory distress, their drive is intact. Itís the person who should be in distress whoís not thatís