Test 1- Blood Gas Analysis

Question 1

Henderson-Hasselbalch equation

Answer 1

Question 2

Measured variables:

Answer 2

o pH

o PaCO2

o PaO2

Question 3

Calculated variables:

Answer 3

Calculated variables: o HCO3-

o BE
o Oxygen content (CaO2)

Question 5

-emia

Answer 5

-emia applies to changes in blood

Question 6

-osis

Answer 6

-osis applies to physiological processes

Question 7

respiratory component

Answer 7

Assess PaCO2 for respiratory component

Question 8

metabolic component

Answer 8

Assess BE (or HCO3-) for metabolic component

Question 9

Normal pH:

Answer 9

Normal pH: 7.35 – 7.45

Question 10

Acidosis:

Answer 10

Acidosis: a physiological process, that occurring alone, tends to cause acidemia

Question 11

Alkalosis:

Answer 11

Alkalosis: a physiological process, that occurring alone, tends to cause alkalemia

Question 12

Mixed acid-base disorders:

Answer 12

Mixed acid-base disorders: different kinds of acidosis and/or alkalosis occurring together

Question 13

Primary acid-base disorder:

Answer 13

Primary acid-base disorder: defined by the initial change in HCO3- or PaCO2

Question 14

Compensation:

Answer 14

Compensation: change in HCO3- or PaCO2 in opposite direction to those of the primary disorder. Not classified in terms of acidosis or alkalosis.

Question 15

Normal values

Answer 15

! pH: 7.35 – 7.45

!HCO3- : 24 ± 4 mEq/L

! PaCO2: 35 - 45 mmHg

Question 16

Base Excess (BE)

Answer 16

! 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

Question 17

Anion Gap (AG)

Answer 17

! 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 (lab. dependent)

Question 18

Elevated AG

Answer 18

! If there is a metabolic acidosis, calculate AG

! If there is an elevated AG, then unmeasured anions may explain the cause of the acidosis:

o Lactic acidosis
o Ketoacidosis
o Uremia
o Drug poisoning: aspirin, ethylene glycol, methanol etc.

Question 19

Normal AG

Answer 19

! If AG is normal then the cause of metabolic acidosis may be Cl- retention or HCO3- excretion

! Typical examples o Diarrhea

o Renal diseases

Question 20

Total CO2 (TCO2)

Answer 20

! CO2 exist in blood as HCO3- and dissolved CO2 ! Dissolved CO2 is a small amount (~1.2 mEq/L) ! TCO2 is almost the same as HCO3-

! You can use them interchangeably if HCO3- was not available

Question 21

PaCO2 equation

Answer 21

! PaCO2: partial pressure (mmHg) of CO2 in the arterial blood

! PaCO2 is directly related to CO2 production and inversely to CO2 elimination (alveolar ventilation)

Question 22

>45

Answer 22

Hypercapnia, Hypoventilation

Question 23

35-45

Answer 23

Eucapnia, Normal ventilation

Question 24

<35

Answer 24

Hypocapnia, Hyperventilation

Question 25

PaO2

Answer 25

! PaO2: partial pressure (mmHg) of O2 in the arterial blood

! Does not reflect O2 content

! PaO2 is interpreted in light of the o FiO2

o Ambient pressure

o PaCO2

Question 26

FiO2 and PAO2

Answer 26

! FiO2 = Fraction of inspired O2

o For 100% O2: FiO2 = 1
o For air: FiO2 = 0.21

! PAO2: partial pressure (mmHg) of O2 in the alveolar space

Question 27

A-a PO2 difference

Answer 27

! 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 A-a PO2 difference

! The term “A-a gradient” is often used but it is not ideal since it is not a “gradient”

Question 28

Reasons for V/Q mismatch

Answer 28

! Atelectasis (common under anesthesia)

! Lung diseases such as o Asthma

o Pulmonary edema
o Adult Respiratory Distress Syndrome (ARDS) o Pneumonia
o etc

Question 29

PaO2 / FiO2 ratio

Answer 29

! Serves the same purpose as the A-a PO2 difference but easier to interpret

! Normal: > 500 mmHg

! Should only be used as a rule of thumb when o PaCO2 is normal
o Ambient pressure is at see level

Question 30

Hypoxemia

Answer 30

! Insufficient oxygenation of arterial blood is termed hypoxemia (e.g. low PaO2)

! Not the same as increased A-a PO2 difference

! Hypoxemia is defined as o SpO2 < 90%
o PaO2 < 60 mmHg

Question 31

5 causes for hypoxemia (Low PaO2)

Answer 31

1. Low FiO2 (more exactly PiO2)
2. Hypoventilation
3. Diffusion impairment
4. V/Q mismatch
5. Right to left Shunt
   A-a PO2 difference would be high in these cases

Question 32

Nevertheless, under anesthesia while breathing 100% O2, we use A-a PO2 difference

Answer 32

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

Question 33

additional causes of hypoxemia are:

Answer 33

o Anemia (low Hb content in blood)

o Presence of pathological Hb specdies □ Methemoglobinemia
□ Carboxihemoglobinemia

! Think of it! PaO2 may be normal in these situations

Question 34

Hypoxia

Answer 34

! Insufficient oxygenation of the tissues

! Causes
o Hypoxemia

o Insufficient tissue perfusion by blood

o Insufficient uptake of O2 at cellular level □ Cyanide poisoning
□ Left shift of the O2/Hb dissociation curve