05: Gas Exchange & Transport

Question 1

Hypoxemia

Answer 1

• PaO2 < 90mmHg
• May cause tissue hypoxia, dyspnea

Question 2

Determinants of CO2 transfer (VCO2)

Answer 2

• CO2 is highly soluble in tissue
  
  • 20x more than O2
• Even in presence of lung dz, VCO2 preserved (except severe lung dz)

Question 3

V/Q ratio

Answer 3

• Lung dz causes change in V and/or Q within diseased alveoli
• V/Q ratio determines alveolar PO2 within a single alveolus, thus alveolar PO2 is a meausre of alveolar health

Question 4

Alveolar health

Answer 4

• Healthy: V/Q = 0.8 to 1, alveolar PO2 = 100mmHg
• Sick: V/Q < 0.8 or > 1.0, alveolar PO2 =/= 100mmHg

Question 5

Causes of impaired gas exchange

Answer 5

• Impaired diffusion
• Abnormal V/Q ratio
  
  • Areas of low V/Q
  • R-to-L shunt
• Clinically identified by AaDO2 > 10 mmHg

Question 6

The Alveolar Gas Equation

Answer 6

P_AO₂ = P_iO₂ - (P_ACO₂/R)

Assumes “perfect” alveoli (pulmonary capillary PO2 equilibriates w/ alveolar PO2, arterial PO2 = alveolar PO2, alveolar PO2 determined by PiO2 and alveolar ventilation)

Question 7

Hypoxic Pulmonary Vasoconstriction (HPV)

Answer 7

• Low alveolar PO2 (low V/Q) –> local arteriolar vasoconstriction
• Directs blood flow away from under-ventilated lung units (match Q to low V)
• May cause pulmonary vascular resistance

Question 8

Hypocapneic bronchoconstriction

Answer 8

• Low alveolar PCO2 (high V/Q due to low Q) –> local bronchoconstriction
• Directs ventilation away from underperfused lung units (decreases V to match low Q)

Question 9

Right-to-left shunt

Answer 9

• Two types
  
  • Intrapulmonary: perfused lung units aren’t ventilated (V/Q = 0)
  • Extrapulmonary: ASD, VSD, PDA
• Shunting de-O2 blood directly to pulmonary veins or arterial circulation
• PCO2 = PO2 of mixed venous (pulmonary arterial) blood = 40mmHg
• Leads to hypoxemia that is not responsive to high PiO2 (supplemental O2)

Question 10

Diffusing Capacity of Carbon Monoxide

Answer 10

D_LCO = rate of CO transfer / partial pressure CO in alveolar space

• Low DLCO + abnormal PFTs
  
  • Disease affecting alveoli
    
    • Emphysema
    • Interstitial lung disease
    • Pneumonia
    • Pulmonary edema
  • Exception = pure airway dz (e.g., asthma)
• Low DLCO + normal PFTs
  
  • Anemia (low [Hb])
  • Carboxyhemoglobinemia (HbCO)
  • Pulmonary vascular disease
  • Early lung disease (DLCO decreases before spirometry)
• High DLCO
  
  • Increased pulmonary blood volume
    
    • Obesity
    • Supine position
    • L-R shunt
    • Very mild heart failure
    • Alveolar hemorrhage

Question 11

Arterial Oxyhemoglobin Saturation

Answer 11

Normal 97-100%

Abbreviated as S_aO2 (ABG) or S_pO2 (pulse oximeter)

Hb-bound O2 content = Hb x SO2 x binding capacity

(in arterial blood, CaO₂= 15 x 1 x 1.34 = 20.1 mL O2 per dL blood)

Question 12

CO₂ content in blood

Answer 12

• 3% bound to Hb (carbaminohemoglobin)
• 5% dissolved in plasma
• 92% bicarbonate

Question 13

Oxygen delivery to tissues

Answer 13

D.O₂ = CO x C_aO₂

Question 14

Management of hypoxemia

Answer 14

• Nasal cannula: up to 6L/min, for mild to moderate hypoxemia, “high-flow” cannula can deliver up to 40L/min
• Simple Face Mask: up to 10L/min
• Non-Rebreather Face Mask: up to 15L/min
• Venturi Mask: up to 50% FiO2

Question 15

Long-term O2 therapy

Answer 15

• Indicated for chronic hypoxemia:
  
  • P_aO2 < 55mmHg (or SpO2 = 88%)
  • PaO2 56-59 mmHg (or SpO2 = 89%) plus:
    
    • Hematocrit > 55%
    • Cor pulmonale (abn. enlargement of R. heart)
    • Dependent edema
• Risks:
  
  • Retinopathy of prematurity
  • Oxidative lung injury: FiO2 > 60% for more than a few days
  • Fire
  • Worsening hypercapnia

Question 16

Supplemental oxygen-induced hypercapnia

Answer 16

• Only a concern if patient already hypercapnic (e.g., COPD)
• ↑PAO2 leads to:
  
  • Relief of hypoxic pulmonary vasoconstriction
  • Worsening of V/Q matching
  • VCO2 reduced, PaCO2 rises
• Sudden rise in PaCO2 can be fatal
• Therefore, in these patients, aim to keep SpO2 89-92%, then slowly titrate FiO2 up

Question 17

Acute hypoxemic respiratory failure

Answer 17

• PaO2 < 60mmHg or SpO2 < 90% despite at least 10-15 L/min O2 via NRB
• Treatment
  
  • Underlying cause
  • Mechanical ventilation
  • Non-invasive ventilation
  • “High flow” nasal cannula

Question 18

Increasing VO2 using a mechanical ventilator

Answer 18

1. Set FiO2 as high as 100%
2. Apply Positive End-Expiratory Pressure** (PEEP)​**
   
   • ​​Prevent alveolar collapse
   • ↓shunt fraction
   • Improve LV function
   • ↓auto-PEEP
   • Decreased cerebral perfusion?