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Finals - Respiratory > Blood Gas Interpretation > Flashcards

Blood Gas Interpretation Flashcards

1
Q

What are the normal values for blood gasses?

A 
pH - 7.35-7.45 
PCO2 - 4.7-6kPa/35-45mmHg 
HCO3 - 22-26mEq/L
PO2 - 11-13kPa/80-100mmHg 
O2 sats - 96-100% 
Base excess (BE) -  -2-> +2 
Anion gap - 10-15mEq/L
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2
Q

What are the steps to determining a basic BG result? (no compensation)

A
  1. How is the patient?
    - Will provide clues to help with the interpretation of the gas
    - Can also try to predict the effect of the gasses on the pathological process
  2. Assess oxygenation
    - Hypoxaemic?
    - PaO2 should be >10kPa (75mmHg) on air, and about 10kPa less than the % inspired concentration (at equilibrium - but acutely unwell patients may not be)
  3. Determine the pH or H+ concentration
    - Acidaemia = pH <7.35 or H+ >45nmol/L
    - Alkamaemia = pH >7.45 or H+ <35nmol/L
  4. Determine the respiratory component
    - PaCO2 >6.0kPa (45mmHg) = respiratory acidosis (or resp compensation for metabolic alkalosis)
    - PaCO2 <4.7kPa (35mmHg) - respiratory alkalosis (or resp compensation for metabolic acidosis)
  5. Determine the metabolic component
    - HCO3 <22mmol/L = metabolic acidosis (or renal compensation for resp alkalosis)
    - HCO3 >26mmol/L = metabolic alkalosis (or renal compensation for a respiratory acidosis)

5a. Some clinicians prefer base excess/deficit instead of HCO3- as changes in these values usually mirror each other and largely doesnt make a different to interpretation of the clinical condition
_____

  1. Which of the PCO2 or HCO3 matches the same direction of change as the pH? = primary imbalance of patient
    6a. both can also match the direction of change = ‘mixed picture’ = a very high or very low pH overall
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3
Q

What are some examples basic BG results?

A

Metabolic acidosis:
Low pH + low HCO3

Metabolic alkalosis:
High pH + high HCO3

Respiratory acidosis:
Low pH + high PCO2

Respiratory alkalosis:
High pH + low PCO2

ROME = respiratory opposite + metabolic equal (in terms of relationship between pH and other value)

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4
Q

What is compensation?

A

Body wants to maintain a pH of 7.4 all the time

If lung function means CO2 drops - kidneys have to kick in as a compensatory mechanism to stabilise the pH

Respiratory compensation = fast e.g. minutes as just need to change resp rate

Renal compensation = much slower c.8-24hrs to respond to pH levels either by retaining or excreting more bicarb

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5
Q

How do you assess compensation?

A
  1. does the PCO2 or HCO3 change in the opposite direction of the pH? if it does = compensation
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6
Q

What are some examples compensated BG results?

A

Respiratory acidosis with metabolic compensation:
Low pH + high CO2 + high HCO3

Respiratory alkalosis with metabolic compensation:
High pH + low CO2 + low HCO3

Metabolic acidosis with respiratory compensation:
Low pH + low HCO3 + low CO2

Metabolic alkalosis with respiratory compensation:
High pH + high HCO3 + high CO2

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7
Q

How do you interpret oxygen on a blood gas?

A

PO2 - partial pressure of oxygen in the blood

Oxygen saturation - how saturated the Hb on RBCs is with O2

If either one of these is low = hypoxaemia (doesnt have to be both)

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8
Q

What is base excess?

A

Tells us about the metabolic component of an acidosis/alkalosis

Measures total amount of bases, that buffer the acids in the blood:
Hb
Cl-, PO4, SO4
Albumin

-2 -> +2

Too high/+ve a BE = metabolic alkalosis (as too many buffers) (or compensated respiratory acidosis)

Too low/-ve a BE = metabolic acidosis (as too few buffers) (or compensated respiratory alkalosis)

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9
Q

What is the anion gap?

A

Helps determine the SOURCE of a metabolic acidosis

Ratio of major cations to anions:
Na + K - (Cl + HCO3)

10-15mEq/L

High ratio = > 15mEq/L = more cations

Low ratio = <10mEq/L = more

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10
Q

What are some common causes of respiratory acidosis?

A

Low pH + high CO2:

Respiratory depression - e.g. opiates

COPD

Asthma

Guillian-Barre - muscle paralysis -> poor ventilation

Poorly managed mechanical ventilation

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11
Q

What are some common causes of respiratory alkalosis?

A

High pH + low CO2:

Anxiety/panic attack

Pain - causing an increased resp rate

Hypoxia - leading to increased ventilation to compensate

Pulmonary embolism

Pneumothorax

Excessive mechanical ventilation

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12
Q

What are some common causes of metabolic acidosis?

A

Low pH + low HCO3 + low BE:

Increased anion gap (increased acid production/ingestion) -
DKA
Lactic acidosis
Asprin overdose (acidic)

Decreased anion gap (decreased acid excretion or HCO3 loss) -
Diarrhoea, ileostomy, proximal colostomy
Renal tubular necrosis
Addisons disease

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13
Q

What are some common causes of metabolic alkalosis?

A

High pH + high HCO3 + high BE:

Vomiting
Diarrhoea 
Loop and thiazide diuretics 
Heart failure 
Nephrotic syndrome 
Cirrhosis 
Conn's syndrome
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14
Q

What are some common causes of a mixed acidosis?

A

Low pH + high CO2 + low HCO3:

Cardiac arrest
Multi organ failure

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15
Q

What are some common causes of mixed alkalosis?

A

High pH + low CO2 + high HCO3:

Liver cirrhosis with diuretic use
Hyperemesis gravidarum
Excessive ventilation in COPD

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16
Q

What are type one and type two respiratory failure?

A

Type 1 respiratory failure: hypoxaemia (PaO2 <8 kPa) WITHOUT hypercapnia (PaCO2 <6.0 kPa).

Type 2 respiratory failure: hypoxaemia (PaO2 <8 kPa) WITH hypercapnia (PaCO2 >6.0 kPa).

17
Q

Mr Pina - 72yrs old - comes in with a cadiopulmonary arrest:

pH - 6.98 (low)
PCO2 - 62 (high)
HCO3 - 14 (low)
PO2 - 80 (normal)
O2 sats - 89% (low)
BE - -11 (low)
AG - 18 (high)

What is this state and why?

A

Severe metabolic and respiratory acidosis with hpoxaemia

Very low pH due to low HCO3 + high PCO2

Not compensating for metabolic acidosis by blowing off PCO2 - uncompensated

BE - very low, so a large part of the acidosis is metabolic

The high AG is likely to reflect the low levels of HCO3

PO2 low = hypoxaemia

18
Q

What does a blood gas picture look like for someone with chronic respiratory acidosis?

A

Raised HCO3 in a type 2 respiratory failure ?!

19
Q

What is the Henderson equation?

A

H+ = proportional to CO2/HCO3-

  • CO2 = resp component in lungs
  • HCO3 = metabolic component from kidneys
  • [H+] = K(24) x [CO2 in mmHg]/[HCO3 in mmol/L]

But in the UK we use kPa for partial pressures of gas, or ‘concentrations’ =
H+ nmol/L = 180 x pCO2 in kPa / HCO3 mmol/L

Important to be aware of as can use this equation to check the validity of the blood gas machine (as sometimes does not work as well as expected…)

Finals - Respiratory (13 decks)

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