Blood Gas Analysis Flashcards
Compare venous and arterial sampling
Arterial blood gases are preferred when assessing respiratory status, but venous samples also yield useful information regarding metabolic status.
Venous blood tends to have a slightly lower pH and higher partial pressure of carbon dioxide (pCO2) than arterial blood
Peripheral venous samples will not be representative in low flow states and arterial samples may not always be representative of changes occurring in peripheral tissues.
What is the most commonly used sample site for blood gas analysis?
Dorsal pedal
How quickly should you analyse a sample?
Ideally immediately, but definitely within 12 mins as then there starts to be changes
What are the main causes of hypoxaemia?
Hypoventilation Venous admixture -Shunt (eg, right to left shunt) -Ventilation/perfusion (V/Q) mismatch -Diffusion impairment (of little significance small animals) Low inspired oxygen concentration
Outline V/Q mismatch
most common cause of hypoxaemia.
Impairment of normal oxygen and carbon dioxide transfer results from a mismatch in ventilation and blood flow in various regions of the lung.
Pulmonary parenchymal diseases such as pulmonary oedema, pneumonia, pulmonary contusion and pulmonary neoplasia will all lead to hypoxaemia secondary to V/Q mismatch
How does hypoventilation cause hypoxamia?
occurs secondary to either a decreased sensitivity to raising carbon dioxide concentrations in the blood (eg, during anaesthesia or with cerebral impairment) or secondary to a physical/mechanical obstruction to airflow.
this obstruction may result from laryngeal paralysis, brachycephalic obstructive airway syndrome, collapsing trachea, or a foreign body or mass associated with the upper airway
How can you determine the effect of hypoventilation on a patient’s hypoxaemia?
an alveolar-arterial gradient (A-a gradient) can be calculated
This is obtained by subtracting the arterial PaO2 from the so-called ‘ideal’ alveolar PaO2 that the lung would have if there were no V/Q mismatch (a healthy lung).
How does a shunt lead to deoxygenated blood?
results in deoxygenated venous blood bypassing the lungs and mixing with oxygenated arterial blood.
This results in an arterial sample that has a lower oxygen content than normal arterial blood and can occur secondary to multiple congenital cardiac anomalies including a right to left shunting patent ductus arteriosus (most start as left to right) and tetralogy of Fallot
What are the major cations and anions in ECF?
The major cations within the extracellular fluid are potassium (K+), sodium (Na+), calcium and magnesium.
The major anions are bicarbonate (HCO3−), chloride (Cl−), plasma proteins, organic acid anions, phosphate and sulfate
What is the anion gap?
the difference between unmeasured cations and unmeasured anions. In reality, electroneutrality is maintained and there is no gap, just unmeasured anions
An anion gap exists because not all electrolytes are routinely measured
Anion gap = (Na + K) - (Cl + HCO3)
What do you look at to assess the acid base status of a patient
Assess the pH and the PCO2
Check HCO3- and BE
How do you assess and interpret the pH for blood gases?
Is it acidaemic (pH less than 7.35)?
If yes, there must be a metabolic acidosis, a respiratory acidosis, or both
Is it alkalaemic (pH greater than 7.45)?
If yes, there must be a metabolic alkalosis, a respiratory alkalosis, or both
Is the pH within normal range (7.35–7.45)?
There is no acid-base disorder
There is an acid-base disorder with complete compensation
There are two opposing acid-base disorders (mixed)
How do you assess PCO2?
Is PCO2 increased (greater than 45 mmHg)?
Primary respiratory acidosis or
Compensatory response to a metabolic alkalosis
Is PCO2 decreased (less than 30 mmHg)?
Primary respiratory alkalosis
Compensatory response to a metabolic acidosis
How do you examine BE and HCO3-?
Is HCO3− increased (greater than 24 mmol/l) or BE positive?
There is a primary metabolic alkalosis
Is HCO3− decreased (less than 18 mmol/l) or BE negative?
There is a primary metabolic acidosis
How do you examine the oxygenation status of a patient (arterial samples only)
Examine the PaO2
Calculate the A-a gradient
How do you examine the PaO2?
Should be five times the inspired oxygen concentration (FiO2).
For example, on room air (21 per cent O2) PaO2 would be expected to be around 100 mmHg
If PaO2 greater than 90 mmHg then this is normal
If PaO2 = 75–90 mmHg, mild hypoxia
If PaO2 less than 75 mmHg, severe hypoxia
What is the alveolar arterial oxygen gradient?
The alveolar arterial oxygen gradient gives an estimate of the effectiveness of gas transfer and should generally only be interpreted on room air. It reflects the difference between the amount of oxygen in the alveoli and arterial blood.
Reference ranges: less than 20 mmHg on room air
less than 100 mmHg on 100 per cent oxygen
Outline the assessment of the anion gap
The normal range for the anion gap is between 12 to 24 mmol/l.
Increases in the anion gap are much more common than decreases and can be used to help the clinician identify the cause of a metabolic acidosis.
In healthy animals, albumin is the major unmeasured anion, but in disease states a limited number of endogenous and exogenous anions can contribute to an increase in the anion gap.
Endogenous anions include uraemic acids (such as phosphates and sulfates), ketoacids and lactic acid. Exogenous anions are produced in ethylene glycol toxicity (glycolic acid) and aspirin toxicity (salicylate). An elevated anion gap therefore helps the clinician create a relatively small list of differentials to explain the changes seen on blood gas analysis. A decreased anion gap is uncommon but can occur in hypoalbuminaemic patients.
