Acidosis
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Introduction
Acid-base homeostasis is very important for metabolic functioning, especially affecting cardiovascular, respiratory, and neurological tissues. Protein conformation and activity is profoundly influenced by pH, which is accordingly tightly regulated.
Normal bicarbonate (HCO3) serum concentration is 24 mEq/L, while the normal pCO2 is 40 mmHg.
The Case of...
a simple case introducing clincial presentation and calling for a differential diagnosis to get students thinking.
Causes of Acidosis
Acidosis can be metabolic or respiratory. Each disorder has an approprate compensation.
- metabolic
- respiratory
Metabolic Acidosis
Metabolic acidosis occurs when metabolic or dietary acid production exceeds acid secretion, and is characterized by a decrease in serum bicarbonate concentration.
Non-anion gap acidosis results from: HCl gain (intake)
HCO3 loss
decreased HCO3 production
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Important causes of wide anion gap acidosis include:
acronym: MUDPILE CATS |
Respiratory Acidosis
Respiratory acidosis occurs due to alveolar hypoventilation. This can occur in respiratory depression due to drugs, alcohol, increased airway resistance due to asthma, impaired gas exchange in fibrosis or pneumonia, or, most commonly, with COPD.
The kidney excretes increased H+ and reabsorbs increased bicarbonate in attempts to compensate.
Pathophysiology
Signs and Symptoms
- history
- physical exam
History
Physical Exam
Respiratory compensation of metabolic acidosis can lead to hyperventilation.
Acidosis can lead to somnolence, confusion, and carbon dioxide narcosis. Asterixis may be present.
Investigations
- lab investigations
- approach to blood gases
Lab Investigations
blood
- CBCD
- chloride: normal in anion gap metabolic acidosis
- BUN and creatinine
- glucose
- lactate
- ketones
- serum alcohol/methanol
- serum osmolality; can indicate ingestion of methanol, ethylene glycol, ethanol?
urine
- urinalysis
- electrolytes
- oxalate crystals (ethylene glycol)
- urine anion gap (Na+ + K+- Cl-) : positive charge suggests lack of NH4+, seen in type I renal tubular acidosis
arterial blood gases
- as described in next section
Approach to Blood Gases
Acidemia requires arterial blood gases for diagnosis.
A low HCO3 suggests metabolic acidosis, while a high pCO2 points toward respiratory acidosis.
Compensation
In metabolic acidosis, each drop in HCO3 should equal drop in pCO2.
In respiratory acidosis, each increase of 10 pCO2 should equal an increase in HCO3
Anion Gap
The anion gap equals Na - (HCO3 + Cl). It is normally 10-14 and represents proteins, ammonium, and other anions.
Part of the AG is mediated by albumin. For each drop in serum albumin of 10g/L, lower the baseline AG by 3.
If the AG is elevated, compare with the decrease of HCO3.
- If the increase in AG < decrease in HCO3, there is a co-existing non-AG metabolic acidosis
- If the increase in AG > decrease in HCO3, there is a co-existing metabolic alkalosis
Osmolar gap
The osmolar gap is measured - calculated osmolality, and is calculated as 2Na + BUN + glucose (all units in mmol/L).
A normal gap is <10. If not consider causes of additional osmoles, such as
- methanol
- ethylene glycol
- ESRD
- alcoholic ketoacidosis
- formaldehyde
Differential Diagnosis
Treatments
ABCs
oxygen, IV, intubation
NaHCO3 1-2 amp bolus
Monitor and treat hyperkalemia
consider NaHCO3 if:
- [HCO3] < 8 mmol/L, especially with very low pH
- no metabolizable anion: salicylate, formate, oxalate, sulfate
be cautious of causing hypokalemia, of volume overload, or of overshoot alkalosis.
Treat underlying cause.
- fluid and insulin for DKA
- increased tissue perfusion in lactic acidosis
- EtOH and dialysis for methanol overdose
- alkaline diuresis and possible dialysis for ASA
Consequences and Course
Resources and References
Topic Development
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