http://www.medrants.com/archives/4987 Contemplating medicine and the health care system Sat, 20 Mar 2010 04:42:37 +0000 http://wordpress.org/?v=2.9.2 hourly 1 http://www.medrants.com/archives/4987/comment-page-1#comment-529600 cory Mon, 16 Nov 2009 14:33:29 +0000 http://www.medrants.com/?p=4987#comment-529600 The disorder the second day techincally is a post hypercapneic metabolic alkalosis. This is common in patients with chronic CO2 retention that are ventilated to a normal level. But in practice this is essentially a relative (not absolute) respiratory alkalosis in the sense these people have reset their CO2s usually around 60. When you provide more ventilation than that to them, you are essentially breathing them into an alkalosis that is not normal for them and that they won't tolerate because they will ultimately reset back to 60.   Attempting to stimulate their ventilation with Diamox is a reasonable approach, but it rarely allows them to reset to a normal pCO2 and a normal pH. The underlying compensatory metabolic alkalosis is a stimulus to hypoventilation that works at cross purposes to the Diamox. I think the best approach in this patient is to stabilize from a cardiac standpoint, investigate any reversible pulmonary component (upper airway obstruction, bronchospasm) and titrate the oxygen appropriately. These patients can be stable for many years with pCO2s between 50 and 60 and relatively normal pHs.  My general rule, one that I am happy to discuss because it is unsettled, is to avoid correcting respiratory acid/base problems with metabolic means and vice -versa. Stabilize the primary problem and let the body fix the ph problem.    The disorder the second day techincally is a post hypercapneic metabolic alkalosis. This is common in patients with chronic CO2 retention that are ventilated to a normal level.
But in practice this is essentially a relative (not absolute) respiratory alkalosis in the sense these people have reset their CO2s usually around 60. When you provide more ventilation than that to them, you are essentially breathing them into an alkalosis that is not normal for them and that they won't tolerate because they will ultimately reset back to 60.  
Attempting to stimulate their ventilation with Diamox is a reasonable approach, but it rarely allows them to reset to a normal pCO2 and a normal pH. The underlying compensatory metabolic alkalosis is a stimulus to hypoventilation that works at cross purposes to the Diamox.
I think the best approach in this patient is to stabilize from a cardiac standpoint, investigate any reversible pulmonary component (upper airway obstruction, bronchospasm) and titrate the oxygen appropriately.
These patients can be stable for many years with pCO2s between 50 and 60 and relatively normal pHs. 
My general rule, one that I am happy to discuss because it is unsettled, is to avoid correcting respiratory acid/base problems with metabolic means and vice -versa. Stabilize the primary problem and let the body fix the ph problem. 
 

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