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Editor-In-Chief: C. Michael Gibson, M.S., M.D.  Carlos A Lopez, M.D. 
- For acidosis referring to acidity of the urine, see renal tubular acidosis.
Acidosis is an acid-base imbalance that is an increased acidity (i.e. an increased hydrogen ion concentration). If not further qualified, it refers to acidity of the blood plasma.
Generally, acidosis is said to occur when arterial pH falls below 7.35, while its counterpart (alkalosis) occurs at a pH over 7.45. Arterial blood gas analysis and other tests are required to separate the main causes.
Strictly speaking, the term acidemia would be more appropriate to describe the state of low blood pH, reserving acidosis to describe the processes leading to these states. Nevertheless, most physicians use the terms interchangeably. The distinction may be relevant where a patient has factors causing both acidosis and alkalosis, where the relative severity of both determines whether the result is a high or a low pH.
The rate of cellular metabolic activity affects and, at the same time, is affected by the pH of the body fluids. In mammals, the normal pH of arterial blood lies between 7.35 and 7.50 depending on the species (e.g. healthy human-arterial blood pH varies between 7.35 and 7.45). Blood pH values compatible with life in mammals are limited to a pH range between 6.8 and 7.8. Changes in the pH of arterial blood (and therefore the extracellular fluid) outside this range result in irreversible cell damage (Needham, 2004).
- Acidosis can either be metabolic or respiratory.
- Both are caused by low arterial pH.
- Metabolic acidosis is due to an increased accumulation of acid equivalents through impairment of the regulatory ability of the liver, kidneys, or metabolism.
- Respiratory acidosis is caused by a retention of carbon dioxide due to inadequate hypoventilation or pulmonary ventilation.
Respiratory acidosis results from a build-up of carbon dioxide in the blood (hypercapnia) due to hypoventilation. It is most often caused by pulmonary problems, although head injuries, drugs (especially anaesthetics and sedatives), and brain tumors can also bring it on. Emphysema, chronic bronchitis, asthma, severe pneumonia, and aspiration are among the most frequent causes. It can also occur as a response to chronic metabolic alkalosis.
Blood gases show pH below 7.35 as above mentioned, and PaCO2 will be high (>45 mmHg / 6 kPa).
The key to distinguish between respiratory and metabolic acidosis is that in respiratory acidosis, the CO2 is increased while the bicarbonate is either normal (uncompensated) or increased (compensated). Compensation occurs if respiratory acidosis persists for days or longer and a chronic phase is entered with partial buffering of the acidosis through renal bicarbonate retention.
Metabolic acidosis may result from disturbances in the ability to excrete acid via the kidneys. Renal acidosis is associated with an accumulation of urea and creatinine as well as metabolic acid residues of protein catabolism.
An increase in the production of metabolic acids may also produce metabolic acidosis. For example, lactic acidosis may occur from 1) severe (PaO2 <36mm Hg) hypoxemia causing a fall in the rate of oxygen diffusion from arterial blood to tissues, or 2) hypoperfusion (e.g. hypovolemic shock) causing an inadequate blood delivery of oxygen to tissues. A rise in lactate out of proportion to the level of pyruvate, e.g. in mixed venous blood, is termed "excess lactate" and is the best indicator of an inadequate flow of oxygen into the body's mitochondria from either cause. Oxygen debt (and muscle excess lactate) is also seen in strenuous exercise. Once oxygenation is restored, the acidosis clears quickly. Another example of increased production of acids occurs in starvation and diabetic acidosis. It is due to the accumulation of ketoacids (ketosis) and reflects a severe shift from glycolysis to lipolysis for fuel needs.
Acidic poisons, iron etc., and decreased production of bicarbonate may also produce metabolic acidosis.
Metabolic acidosis can result in stimulation of chemoreceptors and so increase alveolar ventilation, leading to respiratory compensation, otherwise known as Kussmaul breathing, which is a specific type of hyperventilation. Should this situation persist the patient is at risk for exhaustion leading to respiratory failure.
Mutations to the V-ATPase 'a4' or 'B1' isoforms result in distal renal tubular acidosis—a condition that leads to metabolic acidosis—in some cases with sensorineural deafness.
In blood gas tests, it is characterised by a low pH, low blood HCO3, and normal or low PaCO2. In addition to arterial blood gas one can use the anion gap to differentiate between possible causes.
The Henderson-Hasselbalch equation is useful for calculating blood pH, because blood is a buffer solution. The amount of metabolic acid accumulating can also be quantitated by using buffer base deviation, a derivative estimate of the metabolic as opposed to the respiratory component. In hypovolemic shock for example, approximately 50% of the metabolic acid accumulation is lactic acid, which disappears as blood flow and oxygen debt are corrected.
Common causes of acidosis
- Traumatic shock
- Severe hypoxemia
- Severe diarrhoea
- Renal failure
- Multiple organ dysfunction syndrome
- Nausea and Vomiting
- Mesenteric insufficiency
- Ischemic colitis
- Ethanol ingestion
- Cardiogenic shock
- Acetylsalicylic acid
Causes by Organ System
Causes in Alphabetical Order  
- 3-alpha-hydroxyisobutyryl-CoA hydrolase deficiency
- 3-Hydroxyacyl-CoA dehydrogenase II Deficiency
- 3-methylglutaconic aciduria
- Acetylsalicylic acid
- Acute disseminated encephalomyelitis
- acute glomerulonephritis
- Acute liver failure
- Acute renal failure
- Acute respiratory distress syndrome
- Adrenal cortex insufficiency
- Beriberi heart disease
- Biliary fistula
- Biotinidase deficiency
- Carbon monoxide poisoning
- Carbonic anhydrase inhibitors
- Cardiac arrest
- Cardiogenic shock
- Cataract and cardiomyopathy
- Cerebral edema
- Cerebral hypoxia
- Cerebral malaria
- Chronic interstitial nephritis
- Chronic kidney disease
- Chronic mesenteric insufficiency
- Chronic obstructive lung disease
- Clostridium difficile
- CNS depression
- Coenzyme Q cytochrome c reductase deficiency
- Complex 1 mitochondrial respiratory chain deficiency
- Congestive heart failure
- Copperhead snake poisoning
- Crotalidae snake poisoning
- Cyanide poisoning
- Decreased renal acid excretion
- Diabetic ketoacidosis
- Diethylene glycol
- Disseminated intravascular coagulation
- Distal renal tubular acidosis
- Ethylene glycol
- Fanconi syndrome
- Fructose-16-bisphosphatase deficiency
- Glutaric acidemia type 2
- Glutaric aciduria type 1
- Glycogen storage diseases
- GRACILE syndrome
- Grand mal seizure
- Hemorrhagic shock
- Hepatic failure
- Hydroxyacyl-coa dehydrogenase deficiency type 2
- Hydroxysteroid dehydrogenase deficiency
- Hyperchloremic acidosis
- Hyperosmolar non-ketotic diabetic coma
- Hypokalemic distal renal tubular acidosis
- Hypothermic shivering
- Inborn urea cycle disorder
- Infant respiratory distress syndrome
- Ischemic colitis
- Isopropyl alcohol
- Kidney disorders
- Lipoamide dehydrogenase deficiency
- Liver disease
- Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency
- Malignant hypertension
- Medium-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency
- Metabolic disorders
- Microvillus inclusion disease
- Mitochondrial DNA depletion syndrome
- Mitochondrial encephalomyopathy aminoacidopathy
- Mitochondrial myopathy
- Mitochondrial neurogastrointestinal encephalopathy syndrome
- Multiple carboxylase deficiency
- Myocardial Infarction
- NADH CoQ reductase deficiency
- Necrotizing enterocolitis
- Ondine's curse
- Pancreatic fistula
- Permanent neonatal diabetes mellitus
- Propionic acidemia
- Propylene glycol
- Proximal renal tubular acidosis
- Pulmonary edema
- Pulmonary embolism
- Pulseless electrical activity
- Pyruvate carboxylase deficiency
- Pyruvate dehydrogenase phosphatase deficiency
- Renal circulatory insufficiency
- Renal failure
- Renal tubular acidosis
- SCHAD deficiency
- Severe liver disease
- Short bowel syndrome
- Sideroblastic anemia
- Small intestine fistula
- ST elevation myocardial infarction
- Status asthmaticus
- Succinic acidemia
- Torsade de pointes
- Total parenteral nutrition
- Type I glycogen storage disease
- Urinary diversion
- Ventricular arrhythmias
- Ventricular fibrillation
- Von Gierke disease
- Wilson's Disease
Treatment of any of the varieties of metabolic acidosis is focused upon correction of the underlying problem. However, neutralizing the acidosis with infusions of bases like sodium bicarbonate may be temporarily helpful in some critical emergencies.
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