week 4 Oxygen Delivery and Cascade

Oxygen Delivery and Therapy

• Oxygen administration changes FiO2 and affects two oxygen delivery variables: SaO2 and PaO2
• The Oxygen Cascade from Atmosphere to Mitochondria:
  • Atmosphere to Alveoli: PaO2 of 159 mmHg, diluted by water vapor and CO2 to 99 mmHg
  • Capillary to Mitochondria: PaO2 of 99 mmHg, diluted by venous admixture to 92 mmHg, and further decreased by diffusion distance

Oxygen Therapy

• Goals of oxygen therapy:
  • Reduce or correct hypoxemia
  • Reduce the body's compensatory mechanisms
  • Reduce tachycardia
• Effectiveness of oxygen therapy depends on the cause of hypoxemia and hypoxia
  • Shunt (VQ): oxygen administration generally more effective as ventilation is ok

Oxygen Delivery Systems

• Low-flow/Variable Performance Systems:
  • Oxygen flow rate is less than the patient's inspiratory flow rate
  • Do not provide all the gas necessary to meet inspiratory demand
  • Room air is entrained to make up the difference, resulting in a variable FiO2
• High-flow/Fixed Performance Systems:
  • Gas flow rate > patient's inspiratory flow rate
  • Gas flow sufficient to meet patient's demand
  • Delivers a fixed FiO2 not affected by the patient's ventilatory pattern

Devices and Their Characteristics

• Nasal Cannula:
  • Inexpensive, comfortable
  • FiO2 will be less in patients who are SOB
  • FiO2 may be reduced in mouth breathers
• Simple Face Mask (Hudson Mask):
  • Provides 100-200ml reservoir
  • Higher concentration of oxygen
  • Some rebreathing of CO2
  • A minimum of 6LPM is required for all face masks to flush expired CO2
• Tracheostomy Mask:
  • Similar to a simple face mask
  • Delivery is dependent on the status of the cuff
• Non-Rebreather Face Mask:
  • Contain a 750ml reservoir bag that stores O2 during expiration
  • On inspiration, the patient draws pure O2 from the reservoir bag
  • Increases O2 concentration
  • Theoretically can deliver up to FiO2 1.0, but in reality, the FiO2 is nearer 0.6-0.8
• Manual Resuscitation Bag:
  • Self-refilling, non-rebreathing resuscitators
  • Normally disposable
  • Can deliver very close to FiO2 of 1.0

Oxygen Toxicity and Side Effects

• Absorption Atelectasis:
  • Develops in patients with high FiO2
  • N2 is the most abundant gas in the alveoli
  • The pressure exerted by N2 splints the alveoli open
  • FiO2 1.0 "washes out" all the N2, leaving only O2 which is rapidly absorbed, causing alveolar collapse
• Oxygen and CO2 Retention:
  • The traditional theory is that oxygen administration to CO2 retainers causes loss of hypoxic drive, resulting in hypoventilation and type 2 respiratory failure. This is a myth.
  • The real explanation involves:
    • Increased V/Q mismatch (most important)
    • The Haldane effect
• How Oxygen Causes CO2 Retention:
  • V/Q mismatch:
    • In COPD, over time patients optimize their gas exchange by hypoxic vasoconstriction
    • Administering a high level of oxygen will increase oxygen tension, reducing hypoxic pulmonary vasoconstriction
    • Results in increased perfusion to areas where ventilation is poor
  • The Haldane effect:
    • Describes the difference in the quantity of carbon dioxide carried in oxygenated and deoxygenated blood
    • CO2 has a higher affinity for deoxygenated Hb than oxygenated Hb
    • Giving a patient high levels of oxygen will decrease the affinity of CO2 for Hb, resulting in a higher PaCO2

Nursing Care and Considerations

• Mouth care/nasal care
• Pressure area care – observe ears and nares
• Ensure treatment/medication order unless it is an emergency
• Oxygen is a medication with positive and negative effects
• Take home messages:
  • Overall goal of oxygen therapy is to avoid tissue hypoxia
  • The devices are different and terms are used interchangeably
  • Faced with a hypoxic patient, ALWAYS think Oxygen first
  • Long-term oxygen therapy is not without side effects
  • Consider different approaches for the chronic CO2 retainer when planning oxygen therapy