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Y4 Respiratory > Acid-base balance > Flashcards

Acid-base balance Flashcards

1
Q

By measuring ABG’s (the partial pressure of gases dissolved in arterial blood) what information does it provide us with ?

A

Both ventilation status and acid base balance.

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2
Q

What patients need an ABG?

A
  • Resp disease - Asthma/COPD
  • CVS derangement/ shock/ bleeding
  • DKA
  • Renal failure
  • Collapsed/ unconcious
  • Cardiac arrest
  • ICU patients
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3
Q

List some of the instances where ABG measurements can be useful

A
  • Exacerbations of COPD
  • Severe asthma,
  • DKA
  • severe sepsis
  • Acute LVF and pulmonary oedema
  • Post-resuscitation
  • Tri-cyclic antidepressant overdose etc
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4
Q

What should be done prior to ABG sampling ?

A

Allens test

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5
Q

Describe what allens test is

A

A test use to confirm the patency of the Ulnar Artery.

  1. Occlude both radial and ulna arteries with direct digital pressure.
  2. The hand will become pale.
  3. Release pressure over the ulnar artery.
  4. If colour returns to the hand, this confirms ulnar artery patency
  5. Therefore sampling from the radial artery may proceed
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6
Q

State the key equation you need to know to understand acid-base stuff ?

A
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7
Q

What is bicarbonate and what is its levels controlled by ?

A

It is a base (alkali) that is excreted or resorbed by the kidneys

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8
Q

Appreciate this

A
  • CO2 is a waste product of respiration of all cells in the body
  • Its retention/excretion is controlled by the lungs
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9
Q

What is meant by the abbreviation PaO2 and what is its normal range ?

A
  • PaO2 sometimes called PO2 is the partial pressure of oxygen in arterial blood i.e. the effect of the dissolved oxygen in plasma
  • Normal range = 12-15
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10
Q

What is the normal range of PaCO2 & what is it a measure of ?

A
  • Normal range = 4.5-6 kPa
  • It is the partial pressure of carbon dioxide gas dissolved in arterial blood plasma & is a measure of aveolar minute volume
  • pCO2 falls if minute volume increases pCO2 rises if minute volume falls
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11
Q

What is alveolar minute volume and what does it reflect?

A
  • Minute volume = RR x alveolar tidal volume
  • This reflects ventilatory adequacy
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12
Q

What is base excess and its normal range

A
  • It is a measure of how much acid (H+ ions) has to be removed or added to the system to correct the pH (assuming a normal CO2)
  • It is therefore a measure of metabolic acidosis or alkalosis
  • The normal range is -2 to +2
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13
Q

What does a base excess of < -2 indicate ?

A
  • < -2 = sick patient
  • < -5 = very sick patient
  • < -10 = exstremly sick patient, consider ITU
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14
Q

What does a negative or a positive base excess indicate ?

A
  • A negative BE indicates a metabolic acidosis (-ve i.e. need to remove acid)
  • A positive BE indicates a metabolic alkalosis (+ve i.e. need to add acid)
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15
Q

Why is urine profoundly acidic ?

A

Because Hydrogen ion concentration ([H+], pH) is primarily excreted through the urine

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16
Q

What is the normal range of H+ ion concentration in the body ?

A
  • [H+] is 35-45 nanomol/L
  • or pH 7.35-45
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17
Q

What is the normal range of HCO3-?

A

24-30mmol/l

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18
Q

What is bicarbonate (HCO3-) in terms of acid-base balance ?

A

This is the main buffering agent (alkali or base) used to compensate for excess carbonic, lactic or any other acid dissolved in plasma

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19
Q

What is the only limit to the following equation ?

A

The initial concentration of HCO3-

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20
Q

Define what is meant by Acidosis and Alkalosis

A
  • Acidosis = increased­ [H+], or a process tending to cause increased­ [H+]
  • Alkalosis = decreased [H+], or a process tending to cause decreased [H+]
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21
Q

In terms of the acid-base equation, what is considered the respiratory and conversley the metabolic component ?

A
  • pCO2 is the respiratory component
  • HCO3- is the metabolic component - excretion/reabsorption is renal
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22
Q

Define what respiratory acidosis is

A

Increased [H+] due to increased­ pCO2

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23
Q

Define what respiratory alkalosis is

A

Decreased [H+] due to decreased pCO2

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24
Q

Define what metabolic acidosis is

A

Increased [H+] due to decreased HCO3-

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25
Q

Define what metabolic alkalosis is

A

Decreased [H+] due to increased­ HCO3-

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26
Q

How does the body try to compensate for increased H+ ion concentration (acidosis)?

A

The lungs blow off CO2 (hyperventilation)

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27
Q

When you’ve got too much CO2, how does the body compensate for this?

A

The kidneys get rid of H+ (and in the process reclaim HCO3-)

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28
Q

How does the body compensate for metabolic acidosis ?

A

PRIMARY problem is too much H+ ==> COMPENSATE by blowing off CO2

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29
Q

How does the body compensate for respiratory acidosis ?

A

PRIMARY problem is too much CO2 ==> COMPENSATE by excreting H+ (and simultaneously regenerating HCO3-)

30
Q

Go over the steps for evaluating blood gases

A
31
Q

List some of the clinical causes of respiratory acidosis

A
  • Acute COPD
  • Decompensation in other respiratory conditions e.g. life-threatening asthma / pulmonary oedema
  • CNS (depression) - opiate overdose, stroke, neuropathy, myopathy

The underlying cause is alveolar hypoventilation

32
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Uncompensated respiratory acidosis

33
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Compensated respiratory acidosis

34
Q

List some of the causes of respiratory alkalosis

A
  • Early acute asthma with over ventilation
  • Pulmonary embolus, pneumonia, pulmonary oedema
  • Anxiety
  • CNS disorders (raised intra-cranial pressure): stroke, subarachnoid haemorrhage, encephalitis
  • Altitude
  • Pregnancy

The underlying cause is alveolar hyperventilation

35
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Respiratory alkalosis uncompensated

36
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Respiratory alkalosis compensated

37
Q

List some of the potential causes of metabolic acidosis

A
  • Lactate: shock (septic, cardiogenic, hypovolaemic), hypoxia
  • ketones: diabetic ketoacidosis, alcohol
  • Urate: renal failure
  • Acid poisoning: salicylates, methanol
  • gastrointestinal bicarbonate loss: diarrhoea, ureterosigmoidostomy, fistula
  • Renal tubular acidosis
  • Drugs: e.g. acetazolamide
  • Ammonium chloride injection
  • Addison’s disease

Underlying cause is excess production of H+ ions or sometimes loss of HCO3- ions

38
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Uncompensated metabolic acidosis

39
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Metabolic acidosis with respiratory compensation

40
Q

List some of the potential causes of metabolic alkalosis

A
  • Loss of gastric fluid - Vomiting / aspiration (e.g. peptic ulcer leading to pyloric stenos, nasogastric suction)
  • Diuretics causing K+ loss & excess renal HCO3- reabsorption
  • Post hypercapnic mechanical ventilation
  • Liquorice, carbenoxolone
  • Hypokalaemia
  • Primary hyperaldosteronism
  • Cushing’s syndrome
  • Bartter’s syndrome
  • Congenital adrenal hyperplasia

The underlying cause is loss of hydrogen ions or a gain of bicarbonate. It is due mainly to problems of the kidney or gastrointestinal tract

41
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Uncompensated metabolic alkalosis

42
Q

What type of acid-base problem is displayed by these biochemistry results ?

A

Compensated metbaolic alkalosis

43
Q

What are the blood gases which suggest type 1 and type 2 respiratory failure ?

A
44
Q

Define what is meant by a hypoxic respiratory drive

A

The hypoxic drive is a form of respiratory drive in which the body uses oxygen chemoreceptors instead of carbon dioxide receptors to regulate the respiratory cycle. … And that in this case, when there is an increase in oxygen levels the body will decrease the rate of respiration.

45
Q

What is the normal respiratory response to hypercapnia or hypoxia ?

A

Mammals respond to hypercapnia and hypoxia by increasing minute ventilation (Ve)

46
Q

In acute asthma what is the respiratory response, expected blood gases and the oxygen therapy you should give

A
  • Expected blood gases - low PaO2 & low PaCO2
  • Respiratory response - Alveolar hyperventilation due to increased respiratory drive
  • Oxygen therapy = give high flow, high concentration of O2 (60%)
47
Q

In late asthma what is the respiratory response, expected blood gases and the oxygen therapy you should give

A
  • Expected blood gases - low PaO2 & high PaCO2 (due to the hypoventilation)
  • Respiratory response - Alveolar hypoventilation due to decreased respiratory drive
  • Oxygen therapy = give high flow, high concentration of O2 (60%)
48
Q

What may happen in COPD which means when there is an acute exacerbation you may need to give different O2 therapy?

A
  • Many patients who are know to have COPD function with an abnormally low PaO2 in the long term, and with an abnormally raised PaCO2. These patients lose their normal increased respiratory drive in the presence of hypercarbia, so hypoxia is the only stimulus that keeps them breathing. This is why the target O2 saturation levels by pulse oximetry in patients with known COPD are 88 - 92% (as opposed to 94 - 98% in patients without COPD).
  • Giving supplemental oxygen to a patient with COPD risks removing their hypoxic drive. This results in reduced ventilation, with increasing PaCO2, drowsiness and acute respiratory failure
  • ==> need to Initially use low concentration of O2 (24-28%) to avoid suppressing possible hypoxic drive in acute exacerbations

Note - This initial management of an acutely unwell patient is high flow oxygen at 15l/min using a mask with a reservoir bag. (NO MATTER WHAT)

49
Q

Not all COPD patients retain CO2 i.e. are in type II resp failure, how do you determine a COPD patient is a chronic CO2 retainer & therefore is at risk of resp despression from high flow oxygen, compared to simply showing acute CO2 retention on ABG?

A

Acute CO2 retention will not have any metabolic compensation from the kidneys, whereas chronic CO2 retention will usually result in a raised HCO3- to compensate

50
Q

What are the steps for interpreting ABG results ?

A
  1. Look at the pO2
  2. Look at the pH
  3. Look at the pCO2 - A raised pCO2 associated with acidosis, or a low pCO2 associated with alkalosis indicates a primary respiratory problem. A low pCO2 associated with acidosis indicates that the primary problem is metabolic
  4. Look at the bicarbonate - If this is raised in association with an alkalosis, or decreased in the presence of an acidosis, this indicates a primary metabolic problem. An increased bicarbonate level associated with an acidosis indicates that the metabolic response to a respiratory acidosis
  5. Look at the base excess - this should correlate with the bicarbonate level, and gives an indication of the severity of the acid-base imbalance. If the BE is raised in the presence of an alkalosis, this indicates a primary metabolic problem. If there is an abnormally negative base excess (base deficit) associated with an acidosis, this again indicates a primary metabolic problem.
51
Q

What is the anion gap and what is the use of it ?

A
  • An anion gap (AG) is calculated by using on electrolyte levels, and is of assistance in determining the cause of a metabolic acidosis, including any poisons that may have been ingested.
  • Anion gap = (Na + K) – (Cl + HCO3)
  • Normal range = 12 +/- 4
52
Q

What are the causes of a reduced anion gap?

A
  • A reduced anion gap is less common.
  • It may be simply the result of laboratory or sampling error, or may be seen associated with myeloma or hypoalbuminaemia.
53
Q

What are the causes of raised anion gap metabolic acidosis and explain the underlying reason why they cause a raised anion gap?

A

Think MUDPILES

  • Methanol ( formic acid)
  • Uraemia – DKA/ alcoholic ketoacidosis
  • Paraldehyde
  • Isoniazid
  • Lactic acidosis
  • Ethylene glycol
  • Salicyl

They cause a raised anion gap because essentially there is some osmole i.e. MUDPILES causes, which is not accounted for in the anion gap equation resulting in a raised gap.

54
Q

What are the causes of metabolic acidosis with a normal anion gap ?

A

Think ABCD:

  • Addison’s
  • Bicarb loss e.g. diarrhoea, fistula
  • Chloride
  • Drugs
55
Q

Ans the following question

A

Partially compensated metabolic acidosis

56
Q

Ans the following question

A
57
Q

Ans the following question

A

Partially compensated respiratory acidosis

58
Q

Ans the following question

A
59
Q

Interpret the ABG’s and provide a working diagnosis

A
  • Hypoxia, Respiratory acidosis with no compensation
  • Working diagnosis = pnuemonia
60
Q
A

He is hypoxic

61
Q
A

Metabolic acidosis (the BE is outwidth normal and is very negative indicating you need to remove lots of acid and that its a Metabolic acidosis)

62
Q
A

No compensation

63
Q

What is the most likely diagnosis in this patient ?

A

DKA

64
Q

Interpret the ABG’s and state the likely diagnosis

A
  • Non hypoxic, respiratory alkalosis
  • No compensation
  • Likely diagnosis = anxiety related hyperventilation
65
Q

Interpret the following ABG’s and state the likely diagnosis

A

Likely diagnosis = meningococcal septecaemia

66
Q

Interpret the ABG’s and state the likely diagnosis

A

Likely diagnosis = exacerbation of COPD

67
Q

Interpret the ABG’s and state the likely diagnosis

A

Perforated dudodenal ulcer (diclofenac use is big hint)

68
Q

Interpret the ABG’s and state the likely diagnosis

A

Likely diagnosis = hyperemesis

69
Q

36 y/o admitted after taking her fathers tablets all washed down with a bottle of vodka

Now drowsy and has been vomiting

A

Salicylate e.g. aspirin

70
Q

75 y/o had a VF cardiac arrest, paramedics restored spontaneous circulation after 3 shocks, CPR was not initiated prior to this

On arrival:

  • Comatose (GCS 3)
  • Ventilated with 50% O2 via tracheal tube
  • HR 120
  • BP 150/95
A
71
Q

75 y/o admitted following VF cardiac arrest, spontaneous circulation restored after 2 shocks by paramedics

At present:

  • Comtaose
  • Apnoeic
  • Lungs ventilated on automatic ventilator
A
72
Q

What can acidosis do to cardiac contractility ?

A

It decreases cardiac contractility; the tension generated by isolated myofibrils at a fixed calcium concentration is reduced at low pH.

Y4 Respiratory (27 decks)

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