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Blood gas analysis Flashcards

1
Q

Acid-base homeostasis (basics)

A
  • [H+] has high biological relevance
  • H+ is constantly being produced by metabolism
  • Acid-base homeostasis = maintaining normal [H+]
    • This involves integrated functions of lungs, kidneys, liver, and GI tract
  • pH = -log10[H+]
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2
Q

What are the different buffering systems of the body?

A
  • Chemical buffers (most basic buffering system)
    • Extracellular = HCO3–acts w/in seconds
    • Intracellular = phosphate, proteins–act w/in hours
  • Respiration–acts w/in minutes
    • Respiratory compensation
  • Renal system (excreting H+)–acts w/in hours-days
    • Metabolic compensation
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3
Q

Henderson-Hasselbach equation (super basic)

A
  • pH ≈ (HCO3- / PaCO2)
  • This is a simple way to understand how changes in HCO3- and PaCO2 would influence the pH
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4
Q

Blood gas analysis: what are the measured variables? Calculated variables?

A
  • Measured
    • pH
    • PaCO2
    • PaO2
  • Calculated
    • HCO3-
    • BE
    • Oxygen content (CaO2)
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5
Q

What does the suffix ‘-emia’ apply to? What does the suffix ‘-osis’ apply to?

A
  • -emia = changes in blood
  • -osis = physiological processes
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6
Q

What do you assess for the respiratory component? Metabolic component?

A
  • PaCO2 = respiratory
  • BE (base excess), (HCO3) = metabolic
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7
Q

What is the normal pH range?

A
  • Normal = 7.35 - 7.45
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8
Q

Define the following:

Acidosis

Alkalosis

Mixed acid-base disorders

Primary acid-base disorder

Compensation

A
  • Acidoses = a physiological process, that occurring alone, tends to cause acidemia
  • Alkalosis = a physiological process, that occurring alone, tends to cause alkalemia
  • Mixed = different kinds of acidosis and/or alkalosis occurring together
  • Primary = defined by the initial change in HCO3 or PaCO2
  • Compensation
    • When a guy buys a big truck to compensate for a tiny d**k
    • Change in HCO3 or PaCO2 in opposite direction to those of the primary disorder. Not classified in terms of acidosis or alkalosis
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9
Q

Here’s a pretty chart for acid base disorders

A
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10
Q

When would you diagnose a mixed acid-base disorder (3)?

A
  • pH value is unexpected from a change in HCO3 or PaCO2
  • Normal pH with abnormal HCO3 or PaCO2
  • HCO3 or PaCO2 are changing in opposite directions
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11
Q

What are the normal values for pH, HCO3 and PaCO2? What about in herbivores? Cats?

A
  • pH = 7.35 - 7.45
  • HCO3 = 24 +/- 4 mEq/L
  • PaCO2 = 35 - 45 mmHg
  • Herbivores produce more bicarb
    • Normal values for HCO3 are higher
  • Cats are special
    • HCO3 = ~20 mEq/L
    • PaCO2 = ~30 mmHg
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12
Q

Base excess (BE)–all the things

A
  • Refers to an excess or deficit in the amount of base present in the blood
  • Defines the metabolic component of acid-base disturbances
  • Positive BE = metabolic alkalosis
  • Negative BE (base deficit) = metabolic acidosis
  • Refers to the difference of HCO3 from normal value if PaCO2 and body temperature were normalized
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13
Q

Anion gap (AG)–all ze thingzz

A
  • The amount of positive and negative ions should be equal in the blood
  • Main positive ions: Na+ and K+
  • Main negative ions: Cl- and HCO3-
  • The difference between them can be used to estimate the amount of unmeasured anions (= AG)
  • AG = (Na + K) - (Cl - HCO3)
  • Normal AG is 16 +/- 4 mEq/L
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14
Q

Elevated AG

A
  • If there is a metabolic acidosis, calculate AG
  • If AG is elevated, then unmeasured anions may explain the cause of the acidosis:
    • Ketoacidosis
    • Lactic acidosis
    • Uremia
    • Drug poisoning: aspirin, ethylene glycol, methanol, etc.
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15
Q

Normal AG (with metabolic acidosis)

A
  • Cause may be Cl- retention or HCO3- excretion
  • Typical examples
    • Diarrhea
    • Renal diseases
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16
Q

Total CO2

A
  • CO2 exists in blood as HCO3 and dissolved CO2
  • Dissolved CO2 is a small amount (~1.2 mEq/L)
  • TCO2 is almost the same as HCO3
    • Can use interchangeably if bicarb is not available
17
Q

What are some other methods to diagnose acid-base disorders?

A
  • Strong ion difference (SID)
  • Total concentration of non-volatile weak acids (Atot)
18
Q

PaCO2 equation

A
  • PaCO2: partial pressure (mmHg) of CO2 in the arterial blood
  • PaCO2 is directly related to CO2 production and inversely to CO2 elimination (alveolar ventilation)
  • PaCO2 ≈ (CO2 production) / (alveolar ventilation)
19
Q

Normal PaCO2 values? What should these definitions not be used for?

A
  • Definitions should not be used to describe breathing pattern in the patient
  • Any combination of RR, tidal volume, or breathing effort can reflect any PaCO2 value
20
Q

Increased PaCO2 will do what 4 things?

A
  • Lower the PAO2
  • Lower the pH
  • Reflect respiratory acidosis
  • Reflect respiratory compensation for metabolic acidosis
21
Q

PaO2

A
  • Partial pressure (mmHg) of O2 in the arterial blood
  • Does not reflect O2 content
  • Interpreted in light of
    • FiO2
    • Ambient pressure
    • PaCO2
22
Q

FiO2 and PAO2

A
  • FiO2 = fraction of inspired O2
    • For 100% O2: FiO2 = 1
    • For air: FiO2 = 0.21
  • PAO2 = partial pressure (mmHg) of O2 in the alveolar space
23
Q

Alveolar gas equation

A
  • PAO2 is a calculated value
24
Q

A-a PO2 difference

A
  • The upper limit of PaO2 is the PAO2
  • If gas exchange in the lungs would be ideal, these values would be the same
  • In real life, PaO2 is always lower
  • The difference between them is termed the A-a PO2 difference
  • The term “A-a gradient” is also used but isn’t ideal since it is not a ‘gradient’
25
Q

What does a higher than normal A-a PO2 difference mean? What are some reasons for high values?

A
  • Means that the PaO2 is lower than expected
  • Reasons
    • Ventilation/perfusion inequality (V/Q mismatch)
      • More specifically: low V/Q will drive PaO2 down
    • Right to left shunt
26
Q

What are some reasons for V/Q mismatch?

A
  • Atelectasis (common under anesthesia)
  • Lung diseases such as
    • Asthma
    • Pulmonary edema
    • Adult respiratory distress syndrome (ARDS)
    • Pneumonia, etc.
27
Q

PaO2/FiO2 ratio

A
  • Serves the same purpose as the A-a PO2 difference but is easier to interpret
  • Normal = > 500 mmHg
  • Should only be used as a rule of thumb when
    • PaCO2 is normal
    • Ambient pressure is at sea level
28
Q

Hypoxemia

A
  • Insufficient oxygenation of arterial blood
  • Defined as:
    • ​SpO2 < 90%
    • PaO2 < 60 mmHg
29
Q

What are 5 causes of hypoxemia (low PaO2)

A
  1. Low FiO2 (more exactly PiO2)
  2. Hypoventilation
  3. Diffusion impairment
  4. V/Q mismatch
  5. Right to left shunt
30
Q

T/F: Under anesthesia, while breathing 100% O2, we use A-a PO2 difference to aid the diagnosis of V/Q mismatch resulting from pulmonary atelectasis

31
Q

What are some additional causes of hypoxemia?

A
  • Anemia
  • Presence of pathological Hb species
    • Methemoglobinemia
    • Carboxihemoglobinemia
  • PaO2 may be normal in these situations
32
Q

What is hypoxia? What are some causes?

A
  • Insufficient oxygenation of tissues
  • Causes
    • Hypoxemia
    • Insufficient tissue perfusion by blood
    • Insufficient uptake of O2 at celluar level
      • Cyanide poisoning
      • Left shift of the O2/Hb dissociation curve
33
Q

How much O2 is in the blood?

A
  • CaO2: O2 content of the arterial blood
  • CaO2 = Hb bound + dissolved in plasma
  • CaO2 on air = 19.8 ml/dl
  • CaO2 on 100% O2 = 21 ml/dl
34
Q

What are some pitfalls for arterial samples?

A
  • Use small, sharp needles
  • May use any artery
  • Collect sample anaerobically
  • Measure immediately or put on ice and measure w/in 2 hrs
  • Use lithium heparin (not Na)
  • Limiting dilution of blood samples with heparin to < 4%

Anesthesiology (16 decks)

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