Arterial Blood Gases

Henderson-Hasselbalch equation:

· Increase in HCO₃^- (and other anions) without an increase in pCO₂ so as to increase the 20:1 ratio will increase the pH

· Increase in pCO₂ without an increase in HCO₃^-so as to decrease the 20:1 ratio will decrease the pH

· A pH change due primarily to changes in HCO₃^- (and other anions) is deemed metabolic in origin

· A pH change due primarily to changes in pCO₂ is deemed respiratory in origin

For normal body metabolism the blood pH must remain within a range of 7.35-7.45. Anything outside the range of between 6.8 and 7.8 will result in death
A direct measure

· The pH tells the acidity or alkalinity of a solution; in terms of a solutions hydrogen ion concentration

· p refers to negative logarithm; H refers to hydrogen ions

· a pH of 7 means 10^-7 or 0.0000001 hydrogen ion concentration

BE

· Indirect, calculated value

· The base excess is the quantity of extra base of extra base above normal (principally bicarbonate ions) after the exclusion of the haemoglobin content.

· The deviation of the buffer base content from normal is calculated, and then this value is corrected for the Hb content to give a value called the base excess

· Is a calculation of the amount of acid (HCL) or base (NaOH) that would be need to be added to a litre of blood (in vitro) to titrate the pH back to 7.4 at a pCO₂of 40 mmHg and at 37°C

· If the base excess is positive: then the patient has an excess of metabolic base in the body fluids & therefore has metabolic alkalosis

· If the base excess is negative: then the patient has an deficit of metabolic base in the body fluids & therefore has metabolic acidosis

Bicarbonate [HCO₃^-]

· The bicarbonate ion concentration is normally between 22 to 26 mmoles per litre.

· Indirect, calculated value

· Bicarbonate plays an extremely important role as a buffer in minimising the pH changing effects that occurs when there is an excess of hydrogen ions or hydroxyl ions added. However the addition of hydrogen ions will effectively tie up the available bicarbonate ions thereby depleting them

Regulated by kidney

Partial pressure Carbon dioxide [pCO₂]

The normal partial pressure of carbon dioxide ranges between 35 and 45 mmHg.
Direct measure; partial pressure of dissolved CO₂ in plasma water
Indicator of ventilatory effectiveness especially alveolar ventilation (or oxygenator)
Since most of metabolically generated CO2 is eliminated via the lungs, an elevation in CO2 indicates respiratory abnormalities
Carbon dioxide is considered an acid substance because when it combines with water, carbonic acid is formed; this weak acid dissociates into hydrogen & bicarbonate ions

Carbon dioxide is a waste product of metabolism. It is transported from the tissues in the blood and excreted by the lungs. It is this elimination of carbon dioxide by the lungs that plays the important short-term role in controlling hydrogen ion concentration in the blood.

· The lungs regulate the partial pressure of carbon dioxide and subsequent blood pH by altering respiration rate and depth: increased respirations eliminating carbon dioxide and increasing pH; decreasing respirations retaining carbon dioxide and decreasing pH.

Partial pressure of Oxygen [pO₂]

Normal 80 - 104 mmHg
Direct measure; partial pressure of oxygen in blood
Reflects amount of oxygen dissolved in plasma
Gives an indication of adequacy of respiratory ventilation & diffusion especially reflecting the quantity of oxygen diffusing from the alveoli into the pulmonary capillaries
Note that a normal pO₂ may not necessarily mean adequate O₂ delivery to the cells if the Hb is reduced or defective
Depressions in O2 are commonly associated with degeneration of the respiratory membranes or ventilation-perfusion imbalances
Lower in newborn (40-60); elderly (80 - 1 for every year over 60)

Arterial oxygen saturation [SaO₂]

Normally >94% saturated
A measure of the percentage of O₂ combined with Hb to the total amount of O₂ with which Hb could combine
Plotted on oxygen haemoglobin dissociation curve or measured directly
Is the extent to which haemoglobin is saturated with oxygen
Note that 97% of oxygen transport is with Hb
Influenced by temperature, pH, pCO₂