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Acid base answer
28 May2009
Making rounds yesterday in the ICU, we found this electrolyte panel
| Electrolyte panel | |||||
|---|---|---|---|---|---|
| Na | 138 | Cl | 109 | BUN | 58 |
| K | 4.2 | HCO3 | 17 | creat | 1.8 |
| Blood Sugar | 206 |
Clinical context – 50 year old woman with known cirrhosis and gram negative sepsis. Her creatinine and BUN are improving with volume expansion.
I asked readers to explain her decreased HCO3
Let me add that she was intubated and on a respirator.
I thank the readers for falling for my trap. I have posted cases like this in the past, but this may be a better example.
You cannot make an acid-base diagnosis without an arterial blood gas.
On FiO2 30%
| ABG | |
|---|---|
| pH | 7.42 |
| pCO2 | 25 |
| pO2 | 112 |
The decreased bicarbonate is appropriate for the degree of hyperventilation. The patient had a respiratory alkalosis with appropriate metabolic compensation.
Our strategy was to decrease the ventilator rate.
Major teaching point here – pCO2 is a pure measure of ventilation. As a 4th year medical student, one of our favorite teachers – Dr. Orhan Muren – said this 100 times if he said it once. By definition the decreased pCO2 equates with hyperventilation.
Related posts:
- March 8, 2010 – an acid base challenge
- A new acid-base problem
- My answer to intubated
- An acidosis question
- My thoughts on March 8 acid-base
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1 Response to Acid base answer
cory
May 28th, 2009 at 6:55 am
Good for your teacher on the blood gas point. And the CO2/ventilation comment is spot-on. People often use hyperventilate as a lay term (when it’s precise meaning is low CO2) when they mean tachypnea (rapid respiratory rate). Acid-base problems can’t be divined precisely without blood gases and blood gases are becoming a lost art.
Couple of points to mention- the natural acid base status of advanced stable cirrhosis is a mild respiratory alkalosis- reasons may include – stimuli to breathe including ascites, chronic anemia, mild hypoxemia from AV shunting, and increased progesterone levels.
I never used less than 40% FIO2 on the ventilator (with the possible exception of a CO2 retainer who we were looking to wean and we were going with slightly lower pO2’s). Yes, I know the pO2 of 112 is more than adequate but if anything goes wrong with a patient, barotrauma, pulmonary complication, cardiac problem providing an FIO2 of 40%, which is nontoxic and has no obvious downsides (except possibly in the occasional CO2 retaining COPD patient) provides an extra margin of safety. The risk-benefit of FIO2 of 40% at all times as opposed to lower FIO2s seems indisputable and no one has ever explained to me why it might not be right. I used to ask this of every visiting professor in the ICU and never got a satisfactory answer why a minimum FIO2 of 40% shouldn’t be the default.