assessment of acid-based balance part 2 W3 Flashcards
practical aspects of blood gas samples?
usually collect from radial artery
painful, risk of damage (can compromise blood sup to hand)
collect from femoral artery in arrest situation
interpretation of ABGs
is the patient adequately oxygenated?
what is their pH/H+?
is there pCO2 or bicarbonate disturbance
base excess?
concentration of hydrogen ions required to return pH to reference range if if pCO2 were in reference range (~5.3kPa)
when should base excess be deranged, and when is it positive/negative?
should only be deranged in a metabolic disorder
negative in metabolic acidosis
positive in metabolic alkalosis
what is standardised bicarbonate?
what we would expect the bicarbonate to be if pCO2 were in reference range (~5.3kPa)
standardised bicarbonate values in resp/met disorder?
in ref range in purely resp disorder.
should be same as actual bicarbonate concentration in metabolic disorder.
inboth respirartory and metabolic, quite different from actual bicarbonate
case: 19 yr-old female. confused, restless, tachypnoeic.
high H+, slightly high O2, low bicarbonate.
not hypoxic
acidaemia
respiratory alkalosis - compensation from body (partial)
metabolic acidosis (low bicarb, high H+) - primary disorder
metabolic acidosis - causes of increased acid formation? (different to last slide in last lecture idk why)
ketoacidosis (diabetic, starvation, alcoholic)
lactic acidosis (tissue hypoxia)
poisoning (salicylate, methanol)
inherited metabolic disorders (early infancy)
metabolic acidosis - causes of reduced excretion? (also different?)
renal failure (AKI, CKD, AKI ON CKD)
renal tubular acidosis (types 1 and 4)
metabolic acidosis - causes of loss of bicarbonate?
gastrointestinal (diarrhoea, high-output small bowel fistula)
renal (renal tubular acidosis type 2)
what is the anion gap used for
narrowing differential of metabolic acidosis
what is the anion gap?
theoretical: difference between most abundant cations and anions
which causes of metabolic acidosis have a normal anion gap?
severe diarrhoea
high intestinal fistula output
renal tubular acidosis
which causes of metabolic acidosis have an elevated anion gap?
DKA
lactic acidosis
salicylate (aspirin) overdose
ethylene glycol / methanol poisoning
renal failure
what type of acidosis is found with a normal anion gap? what does this mean?
hyperchloremic acidosis - bicarbonate ions are replaced with chloride ions
what is occurring in acidosis with elevated anion gap?
bicarbonate ions are replaced with anions corresponding to lactate, keto-acids etc
clinical effects of metabolic acidosis?
cardiovascular - negative inotropic effect (if severe)
O2 delivery - acutely, H+ causes R-shift of oxyHb dissociation curve (facilitates O2 delivery). after several hrs, H+ reduces 2,3-DPG causing L-shift of curve (impairs O2 delivery)
nervous system - impaired consciousness (little correlation with H+)
potassium homeostasis - leakage from cells causing high plasma K+, may also be lost renally
bone - choric acidosis then buffering by bone, leads to decalcification
case - male, 55, cough, wheeze, breathless
low O2 and high CO2
primary disorder - resp acidosis
compensation - metabolic alkalosis. know its chronic as kidneys take a while to compensate.
not vice versa because of resp symptoms.
causes of acute respiratory acidosis?
airway obstruction, cardioresp arrest
infective exacerbation of COPD
neuro - opiate toxicity, GB syndrome, MA
causes of chronic respiratory acidosis?
COPD
obstructive - obesity
restrictive - pulmonary fibrosis
neuro - MND, myopathy
effects of respiratory acidosis?
hypercapnia (high CO2):
-shortness of breath
-raised respiratory rate
-neuro effects (anxiety, coma, headache, extensor plantares, myoclonus)
-systemic vasodilatation
