Acid–base disorder

Acid–base imbalance
A Davenport diagram illustrates acid–base imbalance graphically.
Specialty	Internal medicine

Acid–base imbalance is an abnormality of the human body's normal balance of acids and bases that causes the plasma pH to deviate out of the normal range (7.35 to 7.45). In the fetus, the normal range differs based on which umbilical vessel is sampled (umbilical vein pH is normally 7.25 to 7.45; umbilical artery pH is normally 7.18 to 7.38). It can exist in varying levels of severity, some life-threatening.

Classification

Blood gas, acid-base, and gas exchange terms

PaO2	Arterial oxygen tension, or partial pressure
PAO2	Alveolar oxygen tension, or partial pressure
PaCO2	Arterial carbon dioxide tension, or partial pressure
PACO2	Alveolar carbon dioxide tension, or partial pressure
PvO2	Oxygen tension of mixed venous blood
P(A-a)O2	Alveolar-arterial oxygen tension difference. The term formerly used (A-a DO2) is discouraged.
P(a/A)O2	Alveolar-arterial tension ratio; PaO2:PAO2 The term oxygen exchange index describes this ratio.
C(a-v)O2	Arteriovenous oxygen content difference
SaO2	Oxygen saturation of the hemoglobin of arterial blood
SpO2	Oxygen saturation as measured by pulse oximetry
CaO2	Oxygen content of arterial blood
pH	Symbol relating the hydrogen ion concentration or activity of a solution to that of a standard solution; approximately equal to the negative logarithm of the hydrogen ion concentration. pH is an indicator of the relative acidity or alkalinity of a solution

An excess of acid is called acidosis or acidemia, while an excess in bases is called alkalosis or alkalemia. The process that causes the imbalance is classified based on the cause of the disturbance (respiratory or metabolic) and the direction of change in pH (acidosis or alkalosis). This yields the following four basic processes:

process	pH	CO2	compensation
metabolic acidosis			respiratory
respiratory acidosis			renal
metabolic alkalosis			respiratory
respiratory alkalosis			renal

Mixed disorders

The presence of only one of the above derangements is called a simple acid–base disorder. In a mixed disorder, more than one is occurring at the same time. Mixed disorders may feature an acidosis and alkosis at the same time that partially counteract each other, or there can be two different conditions affecting the pH in the same direction. The phrase "mixed acidosis", for example, refers to metabolic acidosis in conjunction with respiratory acidosis. Any combination is possible, as metabolic acidosis and alkalosis can co exist together.

Calculation of imbalance

The traditional approach to the study of acid–base physiology has been the empirical approach. The main variants are the base excess approach and the bicarbonate approach. The quantitative approach introduced by Peter A Stewart in 1978 is newer.

Causes

There are numerous reasons that each of the four processes can occur (detailed in each article). Generally speaking, sources of acid gain include:

1. Retention of carbon dioxide
2. Production of nonvolatile acids from the metabolism of proteins and other organic molecules
3. Loss of bicarbonate in feces or urine
4. Intake of acids or acid precursors

Sources of acid loss include:

1. Use of hydrogen ions in the metabolism of various organic anions
2. Loss of acid in the vomitus or urine
3. Gastric aspiration in hospital
4. Severe diarrhea
5. Carbon dioxide loss through hyperventilation

Compensation

The body's acid–base balance is tightly regulated. Several buffering agents exist which reversibly bind hydrogen ions and impede any change in pH. Extracellular buffers include bicarbonate and ammonia, while proteins and phosphate act as intracellular buffers. The bicarbonate buffering system is especially key, as carbon dioxide (CO₂) can be shifted through carbonic acid (H₂CO₃) to hydrogen ions and bicarbonate (HCO₃⁻) as shown below.

${\ce {HCO_3^- + H+ <=> H2CO3 <=> CO2 + H2O}}$

Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation. This alters the concentration of carbon dioxide in the blood, shifting the above reaction according to Le Chatelier's principle, which in turn alters the pH. For instance, if the blood pH drops too low (acidemia), the body will compensate by increasing breathing, expelling CO₂, and shifting the reaction above to the right such that fewer hydrogen ions are free–thus the pH will rise back to normal. For alkalemia, the opposite occurs.

The kidneys are slower to compensate, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. In responses to acidosis, tubular cells reabsorb more bicarbonate from the tubular fluid, collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to increased formation of the NH₃ buffer. In responses to alkalosis, the kidney may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine metabolism and ammonia excretion.

External links

• On-line text at AnaesthesiaMCQ.com
• Nosek, Thomas M. "Section 7/7ch12/7ch12lin". Essentials of Human Physiology. Archived from the original on 2016-03-24.
• Overview at kumc.edu
• Overview at mcgill.ca
• Stewart's original text at acidbase.org
• Overview at med.utah.edu
• Overview at anaesthetist.com
• Overview at anst.uu.se
• Tutorial at acid-base.com
• Online acid–base physiology text
• Diagnoses at lakesidepress.com
• Interpretation at nda.ox.ac.uk
• Acid Base Tutorial