Scott C Sherman MD Associate Professor of Emergency Medicine Rush Medical College Assistant Program Director Cook County Emergency Medicine Residency Chicago Illinois Acid Base Made Easy The differential diagnoses for acid base problems can be reduced to a workable few by using a minimal amount of laboratory data Following well established principles and formulas the presenter will help you resolve common acidbase problem cases Identify etiologies of anion gap and non anion gap acidosis Explain the principle of osmolar gap Differentiate the causes of acid base disturbance and discuss appropriate management Identify potential life threatening disorders by working through real ED cases WE 248 Wednesday October 7 2009 5 00 PM 5 50 PM Boston Convention Exhibition Center No significant financial relationships to disclose Acid Base Made Easy Scott C Sherman MD Assistant Residency Director Department of Emergency Medicine Cook County Hospital Stroger Assistant Professor of Emergency Medicine Rush Medical College Acid base analysis strikes fear into the minds of both seasoned clinicians and their junior counterparts Multiple formulas and rules exist to help guide us through the forest of diagnoses and complex problems This lecture is set up to provide a simple systematic approach to interpreting arterial blood gas ABG samples All that is needed is a little clinical information obtained from a history and physical examination a few readily available laboratory tests and the knowledge of five simple steps Getting in the routine of performing these steps on each patient in which an ABG and electrolytes are performed will help decrease the rate of missed complex acid base disturbances and hopefully improve patient care Five Steps of Acid Base Analysis1 5 Step 1 Acidemia pH 7 38 or alkalemia pH 7 42 Step 2 Primary respiratory or metabolic disturbance Look at PCO2 on ABG or HCO3 on metabolic panel Step 3 Is there appropriate compensation for the primary disorder Metabolic acidosis PCO2 1 5 x serum HCO3 8 2 Metabolic alkalosis PCO2 0 6 x HCO3 2 Respiratory acidosis PCO2 10 HCO3 by 1 acute or 4 chronic Respiratory alkalosis PCO2 10 HCO3 by 2 acute or 5 chronic Step 4 Is there an anion gap metabolic acidosis AGMA AG Na HCO3 Cl If 12 an AGMA is present Step 5 If metabolic acidosis is there another concomitant metabolic disturbance If AGMA then calculate Gap AG HCO3 AG 12 24 HCO3 If the Gap is 6 there is a combined AGMA and metabolic alkalosis If the Gap is 6 there is a combined AGMA and NAGMA If NAGMA for every 1 mEq L Cl there should be a 1 mEq L HCO3 5 If HCO3 decrease is less than predicted then NAGMA and metabolic alkalosis 1 Explanation of the Five Steps Step 1 This step is straightforward Look at the pH Is the blood acidemic or alkalemic This is the primary disorder Any compensation for a metabolic disturbance by the lungs or vice versa will not bring the pH back to normal Step 2 Determine whether the primary disorder is respiratory or metabolic This is accomplished by looking at the bicarbonate on the chemistry or the pCO2 on the ABG In acidemia low bicarbonate 24 and low pCO2 40 suggests a metabolic acidosis Alternatively a high bicarbonate 24 and high pCO2 40 suggests that the primary disorder is a respiratory one The opposite is true for alkalemia A patient with an elevated bicarbonate 24 and pCO2 40 supports a metabolic alkalosis while low bicarbonate 24 and low pCO2 40 supports a respiratory alkalosis Step 3 The next question you would like to answer is whether or not the other body system kidneys in a primary respiratory disorder or lungs in a primary metabolic disorder are compensating appropriately Metabolic acidosis Whether compensation is adequate or not is easiest to answer when the primary disorder is a metabolic acidosis In this case Winter s formula is used Winter s formula states that the patient s pCO2 should be equal to the serum bicarbonate multiplied by 1 5 plus eight6 When this number is within two of the pCO2 the respiratory system is compensating appropriately PCO2 1 5 HCO3 8 2 If the patient s pCO2 is higher than expected a respiratory acidosis is present in addition to the primary metabolic acidosis If the patient s pCO2 is less than expected then there is a respiratory alkalosis in addition to the primary metabolic acidosis Metabolic alkalosis The respiratory system compensates for a metabolic alkalosis by increasing the pCO2 level However unlike a metabolic acidosis the normal respiratory compensation to a metabolic alkalosis is difficult to predict and the pCO2 level rarely rises above 50 mmHg However the increase in the pCO2 is approximately equal to the increase in HCO3 multiplied by 0 61 Respiratory acidosis alkalosis Renal compensation for respiratory acid base abnormalities improves with time Acute changes 48 72 hours in respiratory function occur due to titration of bicarbonate by available buffer systems In chronic situations 72 hours the kidney is able to alter production and resorption of bicarbonate and ultimately affect a larger change In acute respiratory acidosis for every pCO2 increase of 10 mmHg bicarbonate increases by 1 mEq L In chronic respiratory acidosis a similar change in pCO2 will result in a bicarbonate change of 4 mEq L In patients with acute respiratory alkalosis a pCO2 decrease of 10 mmHg produces a drop in bicarbonate of 2 mEq L and when chronic the bicarbonate drops 5 mEq L These formulas will give the clinician a rough estimate of whether the respiratory acid base disorder is being compensated for by the kidneys appropriately and whether the patient is suffering from an acute chronic or acute on chronic respiratory ailment 2 Step 4 Calculate the anion gap This step must be carried out on all patients not just those with a primary metabolic acidosis The presence of an anion gap with rare exceptions means that an anion gap metabolic acidosis is present7 The anion gap is calculated by subtracting the sum of the chloride and bicarbonate from the sodium level Na Cl HCO3 For the purposes of this lecture and the sample calculations within it the normal anion gap will be assumed to be 12 In reality it is most likely slightly lower than this approximately 6 10 but it varies from individual to individual and the most accurate determination would be a baseline anion gap before the patient became ill Step 5 This step is useful to detect a previously undetected metabolic disorder not discovered on the previous four steps The premise is that for