Acid base disturbances Flashcards
Where in the body are acids produced?
Cellular respiration produces CO2, which reacts with water to give carbonic acid
Metabolic processes give rise to non-volatile acids such as ketones, lactate
How is acid base homeostasis maintained?
Lungs- removes CO2 to stop acidity, limited by HCO3- reserves
Kidneys- excrete H+ and regenerate HCO3-
Buffers- proteins such as Hb and bicarbonate HCO3-
What is a buffer?
A buffer is a chemical system that prevents a radical change in fluid pH by dampening the change in hydrogen ion concentrations in the case of excess acid or base. Most commonly, the substance that absorbs the ions is either a weak acid, which takes up hydroxyl ions, or a weak base, which takes up hydrogen ions
What is the Henderson-Hasselbalch equasion?
What can cause respiratory acidosis and alkalosis?
Hypo-ventilation, retaining CO2= Respiratory acidosis
Hyper-ventilation, removing large amounts of CO2= alkalosis
What can cause metabolic acidosis and alkalosis?
ACIDOSIS= overproduction of H+ e.g. in lactoacidosis, impaired excretion of H+ or unusual losses of HCO3-
ALKALOSIS= Unusual losses of H+ e.g. vomiting or diahorrea or unusual ingenstion of HCO3-
What is the compensatory mechanism for respiratory acidosis?
Renal; slow increased uptake of HCO3-
What is the compensatory mechanism for respiratory alkalosis
Decreased uptake of bicarbonate, usually very marginal as respiratory alkalosis is usually acute
What are the compensatory mechanisms for metabolic acidosis?
Increased respiratory rate to breathe off CO2 via Kussmal breathing
Increased renal uptake of HCO3-, assuming renal impairment is not the underlying cause
What is the compensatory mechanism for metabolic alkalosis?
Usually marginal;
Decreased respiratory rate and decreased renal uptake
What should be measured in arterial blood gases?
pO2, pCO2, H+, HCO3-
What is important to remember when taking blood gases?
Is an arterial sample necessary, would a venous sample do (painful)
Consider using local anaesthetic
Choose an appropriate site; radial or femoral for unwell patients
Analyse ASAP
What questions are important to analyse when looking at blood gas data?
What is base excess?
Amount of H+ per L of blood required to return H+ to reference range
What does a blood gas interpretation plot look like?
Mixed disorders will appear outside the shaded area
What is the disorder of this patient?
Metabolic acidosis with partial respiratory compensation
What are the causes of metabolic acidosis?
Increased acid formation- due to ketoacidosis, lactic acid
Reduced excretion- renal failure or renal tubular acidosis
Loss of HCO3- renally or GI
What is the anion gap and why is it usful?
The difference between the most abundant cations and anions
Is elevated in certain types of metabolic acidosis but normal in others
When is the anion gap normal and raised in metabolic acidosis?
What are the effects of metabolic acidosis on the body?
CV- heart beats less efficenctly
Nervous system- impaired consciousness
Hypokalaemia
Decalcification of bone
What is wrong with this patient?
Respiratory acidosis with full metabolic compensation
What are the causes of respiratory acidosis?
ACUTE= airway obstruction, pneumonia, guillian barre syndrome, myasthenia gravis, COPD exacerbation
CHRONIC= COPD, obesity (obstruction), pulmonary fibrosis, MND
What are the effects of hypercapnia?
Shortness of breath
Anxiety, coma, headache
Systemic vasodilation
What does this patient have?
Acute respiratory alkalosis with no compensation
