Arterial blood gases

Question 1

Describe the importance of oxygen “off-loading” from hemoglobin

Answer 1

Off loading is the dynamics of O2 unbinding from hemoglobin. Tissues can only use freely-dissolved oxygen, and fast unbinding makes O2 available to the tissues. Without fast unbinding, total oxygen levels might be high, but most oxygen molecules would stay bound to
hemoglobin and be unavailable for use.Off loading is the dynamics of O2 unbinding from hemoglobin. Tissues can only use freely-dissolved oxygen, and fast unbinding makes O2 available to the tissues. Without fast unbinding, total oxygen levels might be high, but most oxygen molecules would stay bound to
hemoglobin and be unavailable for use.

Question 2

How does decreased pH affect oxygn dissociation

Answer 2

Rightward shift in oxygen dissociation curve: O2 binds less tightly to hemoglobin. This is the Bohr effect

Question 3

How does increased temperature affect oxygn dissociation

Answer 3

rightward shift in oxygen dissociation curve: O2 dissociates from hemoglobin easier

Question 4

How does increased 2,3-BPG affect oxygn dissociation

Answer 4

rightward shift in O2 dissociation curve: happens during chronic hypoxia at altitude

Question 5

Calculate oxygen delivery to tissues

Answer 5

Oxygen delivery (DO2) per minute: Q X CaO2. Where Q is cardiac output, CaO2 is total concentration of oxygen in arterial blood (hemoglobin bound and freely dissolved)

Question 6

Calculate CaO2 (concentration of arterial O2)

Answer 6

CaO2 = (SaO2 × [Hb] X 1.39) + (0.003 × PaO2) (in ml O2 / 100 ml blood)

Question 7

How is O2 delivery decreased in diseases that cause hypoxemia

Answer 7

Reduced saturation of hemoglobin or reduced free O2 content

Question 8

Calculate oxygen consumption

Answer 8

Oxygen consumption (VO2)= Q x (SaO2 - SvO2) x [Hb] x 1.39. Where Q is cardiac output, SaO2 is arterial O2 saturation, SvO2 is venous O2 saturation, [Hb] is Hb concentration. Normal VO2 is 240ml O2/minute

Question 9

What factors determine amount of O2 in inspired air

Answer 9

Barometric pressure and FIO2.

Question 10

What factors determine amount of O2 in the alveoli

Answer 10

Water vapor dilutes inspired oxygen then gas exchange with blood further dilutes the oxygen. Oxygen tension in alveoli depends on alveolar ventilation (Va) and oxygen uptake (VO2)

Question 11

What factors determine amount of O2 in the arterial blood

Answer 11

venous admixture drops the tension: shunted venous blood and VQ mismatch

Question 12

What factors determine amount of O2 in the capillary/ cell

Answer 12

Hemoglobin conc., blood flow (Q), oxygen-hemoglobin binding dynamics. There is a further drop btw capillaries and mitochondria in cell b/ of limits of O2 diffusion and utilization of O2 in mitochondria. O2 is lowest in mitochondria.

Question 13

Pasteur point

Answer 13

The point at which oxidative phosphorylation drops significantly due to low PO2

Question 14

Which ABG component is the best reflection of oxygen content?

Answer 14

hemoglobin oxygen saturation- nearly all oxygen in blood exists as oxy-hemoglobin

Question 15

Hypoxemia vs desaturation

Answer 15

NOT the same thing: hypoxemia indicates low PaO2, while desaturation indicates low SaO2. You can have low saturation with normal PaO2 in cases such as CO poisoning where CO competes with O2 for hemoglobin binding

Question 16

Define hypoxia and hypoxemia.

Answer 16

Hypoxemia: PaO2 <1-2 Torr)whereas hypoxemia is low arterial oxygen supply

Question 17

Causes of hypoxemia

Answer 17

(1). Low ambient PO2 in inspired air (altitude).(2) Hypoventilation (reduced VA ). This reduces alveolar oxygen indirectly by increasing alveolar PACO2 (recall the alveolar gas equation). (3) Diffusion limitations between alveoli and pulmonary capillaries. (4) V/Q mismatch. (5) Shunt.

Question 18

Causes of hypoxia

Answer 18

low Q (blood flow), low SaO2 due to low PaO2 (Hypoxemia), and deliver problems such as anemia or carbon monoxide

Question 19

Describe how different causes of hypoxia/hypoxemia can be determined from alveolar arterial gradient

Answer 19

An A-a gradient of < or = 10 Torr is normal. Hypoxemia due to altitude and hypoventilation gives a normal A-a gradient. In these cases, the hypoxemia is due to problems in oxygen delivery to all alveoli. Hypoxemia due to shunt, V/Q mismatch, and diffusion problems gives a widened A-a gradient.

Question 20

Describe CO2 and O2 in mild/moderate and severe resistance/compliance problems

Answer 20

Mild to moderat disease: Normal CO2, Low O2 due to VQ mismatch. Severe disease: high CO2 due to hypoventilation and low O2 due to hypoventilation and VQ mismatch

Question 21

What does CO poisoning do to oxygen-hemoglobin dissociation curve, PaO2, PaCO2 and SaO2 and A-a gradient

Answer 21

Curve shifts to the left (decreases oxygen-hemoglobin dissociation) thus it prevents oxygen from being delivered to tissues, PaO2 stays the same, PaCO2 is normal, SaO2 decreases. A-a gradient is normal

Question 22

Triple threat of CO poisoning

Answer 22

Reduces arterial oxygen content, decreases oxygen off loading from hemoglobin and poisons the electron transport chain resulting in cellular anaerobic metabolism

Question 23

How does low hemoglobin affect ABG

Answer 23

PaO2, SaO2, PaCO2 and A- a gradient are all normal

Question 24

Describe the ways in which CO2 is carried in blood.

Answer 24

Dissolved gas (more soluble than O2), as bicarbonate ion, and as carbamino compounds which are bound to proteins then bound to hemoglobin.

Question 25

Haldane effect

Answer 25

De-oxygenated hemoglobin in venous blood binds to carbamino compounds better than oxygenated, and this contributes to the higher levels of CO2 in venous blood compared to arterial blood.

Question 26

High altitude ABG

Answer 26

Low PIO2: Low PaO2, low SaO2, and low PaCO2 (hyperventilation), with normal A-a gradient

Question 27

Severe COPD ABG

Answer 27

Low PAO2: low PaO2, low SaO2, high PaCO2, normal A-a gradient

Question 28

Interstitial disease ABG

Answer 28

Diffusion problem: low PaO2, low SaO2, normal PaCO2, elevated A-a gradient. Order a CO single breath test

Question 29

Moderate COPD ABG

Answer 29

VQ mismatch problem: low PaO2, low SaO2, normal PaCO2, elevated A-a gradient. Use 100% O2 to distinguish btw moderate COPD and pneumonia

Question 30

Pneumonia ABG

Answer 30

Shunt problem: low PaO2, low SaO2, normal PaCO2, elevated A-a gradient. Use 100% O2 to differentiate btw moderate COPD ad pneumonia

Question 31

How does 100% O2 differentiate btw V/Q mismatch and shunt

Answer 31

Will coreect arterial oxygenation in V/Q mismatch but not shunt