ABGs, control of respiration and respiratory adaption Flashcards
what is arterial blood gas testing
Main reason for it is to assess acid base balance and ventilatory status
how is CO2 a volatile acid
Reacts with water to produce carbonic acid
>90% (in terms of moles) of acid in body is C02
Retention of CO2 is respiratory acidosis
Retained CO2 will drive equation to the right (higher proton and lower pH)
What are fixed acids
Fixed or non volatile acids are products from oxidation of dietary substrates
Have to be physically eliminated from the body via kidneys or liver (lactate converted to glucose)
what do buffers do in the body
Buffers prevent pH dropping too low when we make acids everyday (blood pH is higher than pure water)
what buffers are in the body
Bicarbonate
Blood proteins
Phosphates
how do we detect abnormal accumulation of fixed acids
Electroneutrality, same number of cations and anions
Cations = Na and K, some uncounted ions
Anions= Cl and bicarbonate, some uncounted
what are uncounted ions
Some cations and anions are found in small amounts, are not routinely measured or counted in blood chemistry panel
Na+K=144mEq/l
Cl+Bicarb=128mEq/l
what is an anion gap
More uncounted anions than cations (uncounted anions – cations= anion gap)
AG=(Na+K)-(Cl+bicarb)
Or no K as minimal
Normal gap is 12
what are the main causes of AG acidosis
Glycols (ethylene and propylene) Oxoproline L-lactate Common D-lactate Methanol Aspirin Renal Failure Common Ketoacidosis Common GOLD MARK
What 2 categories of metabolic acidosis
Addition of acid (AG acidosis)
Loss of bicarbonate (non-anion gap acidosis) aka hypercholraemic
how is bicarbonate lost in metabolic acidosis
Loss of bicarb in kidneys or gut
Kidneys unable to remove acid from blood (decreased reabsorption or production)
Pancreas secretes bicarb to balance out gut pH (loss by diahorrea)
what are types/causes of metabolic acidosis
Renal tubular acidosis
Type I-III
All types result in urinary loss of bicarb and hyperchloremic acidosis
Gi losses
Acetazolamide
Excessive chloride administration (intravenous fluids with NaCl)
how are Acid base disorders evaluated via ABG testing
pH PCO2 PO2 HCO3 BEecf base excess sO2* oxygen saturation
how can an ABG be interpreted
Examine PH, PCO2 and HCO3, if abnormal then
Determine the primary process, acidaemia or alkalaemia based on pH
If metabolic acidosis calculate anion gap
Identify the compensatory process
Determine if a mixed acid-base disorder is present
Determine the cause
what is acidosis and alkalosis
acidosis- increase in acid (CO2 or fixed)
alkalosis- low volatile acid or an increase in bicarb
what is acidaemia and alkalaemia
acidaemia - low blood pH
alkalaemia - high blood pH
how can the primary process be determined
If acidosis present respiratory acidosis - high PCO2 Metabolic acidosis - low HCO3 If alkalosis present Respiratory alkalosis low PCO2 metabolic alkalosis high HCO3
How can the compensatory process identified
Primary disturbance – compensatory response
Resp acidosis – comp metabolic alkalosis (ie retain HCO3)
Resp alkalosis – comp metabolic acidosis (ie eliminate HCO3)
Metabolic acidosis – comp resp alkalosis (ie eliminate more CO2)
MetaboLic alkalosis – compensatory resp acidosis (ie retain more CO2)
how can compensation vary
Metabolic – kidneys slow (hours to days)
Lungs- respiratory fast (seconds)
Neither case is compensation 100%
Full compensation reaching 7.35-7.45
how can a mixed disorder be evaluated
Anion gap similar in value to the reduction in bicarb
Anion gap is present but pH is alkalaemic
Incomplete compensation for any primary process
what is a mixed disorder
Mixed disorder- two or more primary acid-base disturbances
how is the cause determined
Metabolic acidosis (anion gap GOLD MARK, non AG – renal tubular acidosis, GI loss) Metabolic alkalosis (vomiting, increased aldosterone (some medications – contraction alkalosis)) Respiratory acidosis (increased dead space (emphysema, weakness, depression of respiratory centre) Respiratory alkalosis (hyperventilation due to pain or anxiety, pregnancy)
what are pons
Part of the brainstem (with 2 nuclei) not essential for respiration but exert fine control over medullary neurons
what makes up the respiratory centre
Dorsal respiratory group control quiet breathing, trigger inspiratory muscles
Ventral respiratory group trigger and inspiratory and expiratory impulses during exercise or other times of active exhalation
what are the inspiratory muscles and their innervations
Efferent nerves -> inspiratory muscles Diaphragm(s): phrenic nerves, C3-5 External intercostal muscles: thoracic nerves T1-T11 Sternocleidomastoid: XI cranial nerve Scalene muscles: C3-8
what are the exhalatory muscles and their innervations
Efferent nerves-> muscles of exhalation
Abdominal Wall: T5-T12
Internal intercostal muscles: T1-T12
how do inputs alter the outputs of the RC
can modify or modulate this rhythm (emotional inputs from cerebral cortex, lung receptors, chemosensors – central and peripheral)
what does the rhythm generator do
Rhythm generator in the medulla controls basic autonomic pattern of breathing via a group of neurons concentrated in the Pre-Botzinger complex
what are the mechano and irritant receptors in the lungs
1 C-fiber nociceptors: sensitive to a variety of inhaled or locally produced chemical mediators
2 mechanically sensitive receptors (aka cough receptors) cause a cough due to aspiration of foreign particles
3 Lung stretch receptors help terminate inspiration and initiate exhalation when lungs adequately inflated
The neuronal projections from cells with these receptors travel along vagal nerve afferent fibres to RC
what are chemosensors
Detect H+ in the CSF, very sensitive (increase ventilation is too high)
Reflect blood H+, PO2 and CSF CO2 but not directly sensed by central chemoreceptors
what are peripheral chemosensors
Carotid body: bundle of cells outside the bifurcation of the carotid arteries
Aortic Arch: bundle of cells in aortic arch
Back up each other, normally carotid body does bulk of sensing
Both respond to PaO2 (hypoxaemia) and PaCO2, carotid bodies also detect pH
how does PCO2 affect ventilatory response
Body (via inc output from RC) increases ventilation if PaC02 builds up in blood
Body sensitive to small changes in PaCO2 and VE can increase a great deal
what do opioids do to ventilatory
Opioids blunt sensitivity, opioid ingestion one of most common causes of acute hypercarbic respiratory failure
what does P02 do to ventilatory response
Normally an increase in ventilation occurs (only) when PaO2 drops significantly
Sensitivity to PaO2 is altered by PaCO2, more sensitive to hypoxaemia in hypercarbia (body less sensitive to PaO2 changes)
Due to role of CO2 as a by product of respiration and as an acid, body highly engineered to keep blood pH constant
what causes respiratory depression
Opioids/ narcotics Alcohol Anasthesia and other sedatives Cerebral diseases eg stroke Leads to Hypoxaemic hypoxia Hypercarbia Acute respiratory acidosis
how does loss of robust respiratory drive affect ventilatory response
Poor ventilation, lots of dead spaces eg COPD
RC becomes less sensitive to chronic elevations in PaCO2 and resp response is blunted
Results in
Chronic respiratory acidosis
Metabolic compensation
Hypoxaemia due to hypoventilation
how are blood gases done in clinical practice
same as AGB interpretation +Consider PaO2
Results written pH/PaCO2/PaO2/Bicarb
Interpret compensation/primary disturbance/oxygenation
what is base excess
Another way to measure presence of metabolic disturbance
Dose of acid to return blood to normal pH
