Steve Wood, Ph.D.

Department of Physiology

Ross University School of Medicine

   Case Study 1:

A 50 year old man with type 1 diabetes and diabetic nephropathy has the following labs:  sodium 130 mEq/l, potassium 6.5 mEq/l, creatinine 2.16 mg/dl, chloride 109 mEq/l, pH 7.29, Paco2 27 mm Hg, Pao2 107 mm Hg, bicarbonate 12 mEq/l.

 

What is the acid-base status?

Is the disturbance respiratory or metabolic?

Is there any compensation?

What is the oxygenation status?

What is his anion gap?

Case Study 2

A 60 year old man was admitted to an ER in Denver (inspired PO2 = 126 mm Hg) with an exacerbation of chronic obstructive pulmonary disease. His arterial blood gases on air showed pH 7.34, Paco2 52 mm Hg, Pao2 42 mm Hg, and bicarbonate 28 mEq/l.

What is the acid-base disturbance? 

Is it respiratory or metabolic?  

Is there any compensation?  

What is the oxygenation status? (assume R = 0.8)

 

 

After several days of bronchodilators, diuretics, and prednisone, he feels much better and an ABG is repeated:

pH = 7.46; PCO2 = 64; PO2 = 34; HCO3 = 40.

Now what is the acid base status? 

Is it respiratory or metabolic? 

Is there any compensation?

What is the oxygenation status?

How might the acid base status be related to the use of diuretics?

 

             

 

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© S.C. Wood 2006

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answers

Case 1

What is the acid-base disturbance?  Acidosis,

Is it respiratory or metabolic?   low bicarb indicates metabolic acidosis

Is there any compensation? Yes, bicarb is reduced by 12 mEq/l, should result in pH of 7.2 but actual pH is close to 7.3, therefore there must be respiratory compensation (proof is low PCO2) 

What is the oxygenation status? (assume R = 0.8)  normal  Alveolar PO2 is high due to hyperventilation and arterial PO2 is also high, so gas exchange is fine

What is his anion gap?  (130+6.5) – (109+12) = 15.5 mEq/l

 

Case 2

 

What is the acid-base disturbance?  acidosis

Is it respiratory or metabolic?   Respiratory, PCO2 is high

Is there any compensation?   yes, pH is predicted to be 7.3 but actual is 7.34 indicating some renal compensation (proof is higher bicarb).  Base excess not calculated but would be about +2.

What is the oxygenation status? (assume R = 0.8)  arterial PO2 is low and this is mainly due to hypoventilation.  Alveolar arterial PO2 difference is elevated somewhat due to V/Q inequality.   Alveolar PO2 is calculated as inspired – PaCO2/R = 126 – 52/.8 = 126 – 65 = 61.  arterial PO2 = 42.  A-a PO2 = 61-42 = 19 mm Hg

 

 

Now what is the acid base status?  Alkalosis    

Is it respiratory or metabolic?  Metabolic, bicarb is high

Is there any compensation? yes, the pH is predicted to be much higher than 7.46 with a bicarb of 40 (proof is high PCO2 from alkalosis inhibition of ventilation)

What is the oxygenation status?  Hypoxic.  Due to lower alveolar PO2 (now PAO2 =  126 – 64/.8 = 126 – 80 =  46 mm Hg.

How might the acid base status be related to the use of diuretics?  Diuretics create potassium loss in urine.  Hypokalemia produces shift of H into cells = metabolic alkalosis.