Respiratory - Assessment of Oxygenation in CRITICALLY ILL

What things need to occur for tissue demands to be met by the delivery of O2?

• adequate ventilation/perfusion

• transfer of O2 across alveolar-capillary membrane

• presence of hemoglobin to carry O2

• adequate cardiac output to deliver O2 to the tissue bed

• release of O2 from the hemoglobin molecule

• ability of cells to utilize O2

why is oxygen at the cellular level SOOO important?

• sufficient oxygen is needed for production of ATP (which is needed for cell energy/life)

• without sufficient oxygen, lactic acid is produced (LACTIC ACIDOSIS), showing evidence of anaerobic metabolism, organ failure, and eventual death

why is NOT sufficient enough to look at only PaO2 and SaO2 in a septic patient?

• a patient in septic shock may have a normal PaO2, SaO2, hemoglobin, clear lungs, and adequate ventilation/oxygen delivery, but have a lactate of 10, meaning lactic acidosis

  • OXYGEN UTILIZATION IS AFFECTED BY SEPSIS, RESULTING IN ANAEROBIC METABOLISM AT THE CELLULAR LEVEL

Indicators of oxygenation - PaO2 (normal range? relevance?)

• normal range - 80-100mmHg on room air

• clinical relevance - reflects how well oxygen is moving from the alveoli into the bloodstream, meaning conditions such as PNA, pulmonary edema, ARDS, or a PE can significantly drop PaO2

Indicators of oxygenation - SaO2 (normal range? relevance?)

• normal range - 95-99% on room air

• clinical relevance - measures the percentage of hemoglobin sites occupied by oxygen in arterial blood (REMEMBER - a drop in hgb does not always mean in a drop in SaO2)

Indicators of oxygenation - mixed venous oxygen saturation (SvO2) (normal range? relevance?)

• normal range - 60-75%

• clinical relevance - most sensitive indicator of oxygenation at the cellular level; REFLECTS THE BALANCE BETWEEN OXYGEN DELIVERY (DO2) and OXYGEN CONSUMPTION (VO2)

  • in low cardiac output states, less oxygenated blood reaches tissues, leading to tissues extracting more oxygen from blood, leading to a decreased SvO2

  • in SEPTIC SHOCK, our tissues’ ability to consume oxygen is impaired (VO2 is low), leading to a HIGH SvO2

Indicators of oxygenation - CaO2 (oxygen content) (normal range? clinical relevance?)

• normal range - 15-20mL / 100 mL blood

• clinical relevance - HELPS DETERMINE OXYGEN DELIVERY

  • conditions that affect CaO2:

    • severe anemia - example (hgb 5, SaO2 100%, PaO2 95 → normal oxygenation, markedly reduced CaO2; WHY? → there isn’t enough hemoglobin to carry oxygen)

    • hypoxemia - example (hgb 15, SaO2 80% → CaO2 decreases because less hgb is saturated)

Indicators of oxygenation - oxygen delivery (DO2) (normal range? clinical relevance?)

• normal range - 900-1,100 mL/min

• clinical relevance - the total amount of oxygen delivered to tissues each minute; relies on CO and CaO2 (oxygen content in blood); low CO, severe anemia, and hypoxemia result in reduced DO2

Indicators of oxygenation - oxygen consumption (VO2) (normal range? clinical relevance?)

• normal range - 250-350 mL/min

• clinical relevance - it is LOW with septic shock, due to the tissues’ inability to utilize oxygen even when delivery is adequate

Indicators of oxygenation - alveolar-arterial gradient (normal range? clinical relevance?)

• normal range - < 10 mmHg

• clinical relevance - calculates difference between alveolar oxygen and arterial oxygen; indicates whether gas transfer is normal or how bad the V/Q mismatch/shunt is

oxyhemoglobin dissociation curve - what is it?

• describes the relationship between PaO2 (oxygen dissolved in arterial blood) and SaO2 (% of hemoglobin saturated with oxygen); it shows how READILY hemoglobin binds oxygen in the lungs and releases oxygen to the tissues

oxyhemoglobin dissociation curve - difference between LEFT shift and RIGHT shift?

• Left shift - certain conditions make hemoglobin “HOLD ON” to oxygen molecules

  • such as alkalosis (low H+), low PaCO2, hypothermia, low 2,3-DPG; BAD FOR TISSUES, as SaO2 is high, but the O2 is stuck to the hgb

• Right shift - certain conditions allow hemoglobin to “RELEASE” oxygen more easily to the tissues

  • such as acidosis (high H+), high PaCO2, fever, high 2,3-DPG; GOOD FOR TISSUES, as SaO2 is low but O2 is easily release to the tissues

2,3-diphosphoglycerate (2,3-DPG) - what is it?

• an organic phosphate found in RBCs that has the ability to alter the affinity of hgb for oxygen

  • DECREASED? - results in hgb holding on to O2

  • INCREASED? - results in hgb more readily releasing O2

2,3-diphosphoglycerate (2,3-DPG) - conditions that result in DECREASED 2,3-DPG

• multiple blood transfusions of banked blood

• hypophosphatemia

• hypothyroidism

  • RESULTING IN LESS O2 AVAILABLE TO TISSUES

2,3-diphosphoglycerate (2,3-DPG) - conditions that result in INCREASED 2,3-DPG

• chronic hypoxemia (such as prolonged time spent at high altitudes or chronic HF)

• anemia

• hyperthyroidism

  • RESULTING IN MORE O2 AVAILABLE TO TISSUES

Carbon Monoxide Poisoning - why is it SOO dangerous?

• carbon monoxide (CO) has a greater affinity for hemoglobin than oxygen (APPROXIMATELY 230 TIMES GREATER), meaning oxygen cannot be carried, leading to tissue hypoxia

Carbon Monoxide Poisoning - why can you NOT use a pulse oximeter to monitor for oxygenation status?

• the pulse oximeter cannot differentiate between CO and O2, therefore an SpO2 of 95% in the presence of CO poising only means that the hemoglobin is saturated with a total of 95% molecules

  • if the CO level of the blood is 40%, the maximum O2 that can be carried by the hemoglobin is 60%

Carbon Monoxide Poisoning - carboxyhemoglobin (COHb) levels / associated clinical presentation

• 0-5% = NORMAL

• 6-15% = often in smokers, truck drivers

• 15-40% - headache, some confusion

• 40-60% - loss of consciousness, Cheyne stokes breathing

• 50-70% - MORTALITY

Carbon Monoxide Poisoning - treatment

• 100% FiO2 until symptoms resolve and carboxyhemoglobin level is <10%

• hyperbaric oxygen chamber if available, generally within 30 minutes

Lung Compliance - general definition

• the degree of elasticity of tissue in the lungs (THEREFORE, a decrease in compliance = INCREASED RESISTANCE/STIFFNESS)

Lung Compliance - static compliance (what is it? equation?)

• measurement of the elastic properties of the LUNGS

• Equation: tidal volume / plateau pressure (minus PEEP)

  • NOTE: an increase in plateau pressure will DECREASE compliance

Lung Compliance - dynamic compliance (what is it? equation?)

• measurement of the elastic properties of the AIRWAYS

• Equation: tidal volume / peak inspiratory pressure (PIP) (minus PEEP)

  • NOTE: an increase in peak inspiratory pressure will DECREASE compliance

Lung compliance - normal range for static/dynamic compliance?

• BOTH is about 45-50 mL/cm H2O

Lung compliance - dynamic vs. static compliance

• patients with pulmonary problems that mainly involve the airways (such as asthma) have a DECREASE in dynamic compliance, but their static compliance remains normal

• patients with pulmonary problems that mainly involve the lungs (such as PNA, ARDS) have a DECREASE in static compliance, but their dynamic compliance may also decrease as the lung pressures may transmit up to the airways

Lung compliance - Status asthmaticus, ARDS

• status asthmaticus

  • static compliance (lungs) would be normal

  • dynamic compliance (airways) would be LOW

• ARDS

  • static compliance (lungs) would be LOW

  • dynamic compliance (airways) would be LOW, due to lung pressures transmitting up to the airways