Arterial blood gases Flashcards
Describe the importance of oxygen “off-loading” from hemoglobin
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.
How does decreased pH affect oxygn dissociation
Rightward shift in oxygen dissociation curve: O2 binds less tightly to hemoglobin. This is the Bohr effect
How does increased temperature affect oxygn dissociation
rightward shift in oxygen dissociation curve: O2 dissociates from hemoglobin easier
How does increased 2,3-BPG affect oxygn dissociation
rightward shift in O2 dissociation curve: happens during chronic hypoxia at altitude
Calculate oxygen delivery to tissues
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)
Calculate CaO2 (concentration of arterial O2)
CaO2 = (SaO2 × [Hb] X 1.39) + (0.003 × PaO2) (in ml O2 / 100 ml blood)
How is O2 delivery decreased in diseases that cause hypoxemia
Reduced saturation of hemoglobin or reduced free O2 content
Calculate oxygen consumption
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
What factors determine amount of O2 in inspired air
Barometric pressure and FIO2.
What factors determine amount of O2 in the alveoli
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)
What factors determine amount of O2 in the arterial blood
venous admixture drops the tension: shunted venous blood and VQ mismatch
What factors determine amount of O2 in the capillary/ cell
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.
Pasteur point
The point at which oxidative phosphorylation drops significantly due to low PO2
Which ABG component is the best reflection of oxygen content?
hemoglobin oxygen saturation- nearly all oxygen in blood exists as oxy-hemoglobin
Hypoxemia vs desaturation
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
Define hypoxia and hypoxemia.
Hypoxemia: PaO2 <1-2 Torr)whereas hypoxemia is low arterial oxygen supply
Causes of hypoxemia
(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.
Causes of hypoxia
low Q (blood flow), low SaO2 due to low PaO2 (Hypoxemia), and deliver problems such as anemia or carbon monoxide
Describe how different causes of hypoxia/hypoxemia can be determined from alveolar arterial gradient
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.
Describe CO2 and O2 in mild/moderate and severe resistance/compliance problems
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
What does CO poisoning do to oxygen-hemoglobin dissociation curve, PaO2, PaCO2 and SaO2 and A-a gradient
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
Triple threat of CO poisoning
Reduces arterial oxygen content, decreases oxygen off loading from hemoglobin and poisons the electron transport chain resulting in cellular anaerobic metabolism
How does low hemoglobin affect ABG
PaO2, SaO2, PaCO2 and A- a gradient are all normal
Describe the ways in which CO2 is carried in blood.
Dissolved gas (more soluble than O2), as bicarbonate ion, and as carbamino compounds which are bound to proteins then bound to hemoglobin.
Haldane effect
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.
High altitude ABG
Low PIO2: Low PaO2, low SaO2, and low PaCO2 (hyperventilation), with normal A-a gradient
Severe COPD ABG
Low PAO2: low PaO2, low SaO2, high PaCO2, normal A-a gradient
Interstitial disease ABG
Diffusion problem: low PaO2, low SaO2, normal PaCO2, elevated A-a gradient. Order a CO single breath test
Moderate COPD ABG
VQ mismatch problem: low PaO2, low SaO2, normal PaCO2, elevated A-a gradient. Use 100% O2 to distinguish btw moderate COPD and pneumonia
Pneumonia ABG
Shunt problem: low PaO2, low SaO2, normal PaCO2, elevated A-a gradient. Use 100% O2 to differentiate btw moderate COPD ad pneumonia
How does 100% O2 differentiate btw V/Q mismatch and shunt
Will coreect arterial oxygenation in V/Q mismatch but not shunt
