Principles of Acid-Base Physiology

Explain the principles underlying acid-base chemistry

There have been several different theories of acid-base chemistry. The one most relevant for the primary exam is the Brønsted–Lowry definition, which defines:

  • An acid as a proton donor
  • A base as a proton acceptor

pH

  • Stands for the power of hydrogen
  • Is a measure of hydrogen ion activity in a solution
    • Activity can be approximated by concentration
    • Therefore, pH can be expressed as a function of hydrogen ion concentration:
      Using pH rather than concentration makes it easier to compare different solutions.

pKa

  • Strong acids (and bases) dissociate completely in solution
  • Weak acids (and bases) only partially dissociate
    They have a dissociated state (A-) and an undissociate state (HA)
  • The ratio of concentrations on each side can be used to calculate the acid dissociation constant, Ka

    This equation describes the strength of an acid by indicating how readily the acid gives up its hydrogen.
  • Similar to pH, this value is often log transformed to pKa produce an index, which allows easy comparison of different substances:
  • pKa has several useful properties:
    • An acid of base will be 50% ionised when the pH of its solution equals its pKa
    • Acids are more ionised above their pKa
    • Bases are more ionised below their pKa
    • An increase in pH of 1 above the pKa will result in that substance being either 90% (for an acid) or 10 % (for a base) ionised

Systemic Effects of Acid-Base Disorders

pH disturbance affects many organ systems:

  • Respiratory
    • Increased
      Peripheral and central chemoreceptors increase ventilation in response to a fall in pH.
    • Oxyhaemoglobin-Dissociation Curve
      Right-shifted by a fall in pH.
    • Bronchoconstriction
      Hypercapnoea causes parasympathetically-mediated bronchoconstriction.
  • Cardiovascular
    • Inotropy
      Inotropy falls in acidosis due to a direct myocardial depressant effect. May be offset by increased SNS tone in low-grade acidosis. Alkalosis may increase inotropy by increasing responsiveness to circulating catecholamines.
    • Decreased response to catecholamines
      When pH < 7.2.
    • Arrhythmias
      Secondary to altered SNS tone and electrolytes.
    • Vasodilation
      Directly due to hypercapnoea.
  • CNS
    H+ ions cannot cross the BBB, however CO2 can.
  • Fluid and Electrolyte
    • Plasma K+ increases by 0.6mmol.L-1 for every 0.1 unit fall in pH
      This is due to impairment of the Na+/K+-ATPase
    • H+ ions bind to the same site on albumin as calcium, so ionised calcium will increase
  • MSK
    • Bones
      Chronic metabolic acidosis consumes bone phosphate to buffer H+ ions, causing osteoporosis.
  • Cellular
    • Enzyme function
      Denaturation and functional impairment.
    • Molecular ionisation
      Change in ionisation may change a molecules ability to cross cell membranes (e.g. reducing dose of thiopentone in acidosis), or affect their function
    • Resting membrane potential
      Change in ion permeability will alter RMP, and therefore how easy it is to generate an action potential.

Change with Temperature

pH is temperature dependant:

  • pH increases by 0.015 for every 1°C fall in temperature
    Due to decreased ionic dissociation of water.
  • Gas solubility almost always increases when temperature falls
    Dissolving is typically (not always) an exothermic reaction. As the kinetic energy content of a molecule falls, its ability to dissociate from solution decreases.
    • As CO2 dissolves, PaCO2 falls
  • As blood gas machines operate at 37°C, a measurement error will occur if a patient is not close to 37°C
    • A hypothermic patient will have a higher pH and CO2 than measured

There are two common methods for managing pH of significantly hypothermic patients (e.g., those on CPB): pH-stat and alpha-stat.

pH-stat

  • CO2 is added to the circuit so that pH and PaCO2 are normal when corrected for temperature
  • This theoretically improves oxygen delivery by preventing the left-shift in the oxyhaemoglobin dissociation curve
  • The increased CO2 also causes cerebral vasodilation, which:
    • Increases speed and uniformity of cerebral cooling
    • Increases risk of cerebral embolic events

alpha-stat

  • pH and CO2 values are maintained at 'normal for 37°C'
    • Measured values will be different, as:
      • pH will be increased
      • CO2 will be decreased
  • Cellular autoregulation is preserved
  • Unlike pH-stat, this does not cause cerebral vasodilation

Referenecs

  1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
  2. ANZCA July/August 1999
  3. Chemlab. Solubility. Florida State University.
Last updated 2017-10-04

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