Hypoxemia/Hypoxia

Question 1

Mechanisms of Hypoxia

Answer 1

• Ventilation Perfusion (V/Q) mismatch
• Shunt
• Diffusion impairment
• Alveolar hypoventilation
• Decreased inspired oxygen tension (altitude)

*Can have more than one

Question 2

Alveolar Hypoventilation:

Why are these patients hypoxic?

What are some examples?

Answer 2

• Why are these patients hypoxic?
  
  • Alveolar gas equation: PAO₂ = PiO₂ – 1.25 x PaCO₂
  • Hypoventilation = rise in PaCO₂; leads to reduction in PAO2
• What are some examples?
  
  • Drug overdose, Neuromuscular disease, Advanced COPD
• Always results in elevated PCO₂
• A-a gradient remains normal
• Alveolar gas equation: PAO₂ = PiO₂ – 1.25 x PaCO₂
• Improves with supplemental oxygen

Question 3

Shunt

Answer 3

• Extreme version of VQ Mismatch
• Adequate Blood flow, Poor Ventilation
• Intracardiac: Septal defects (VSD, ASD, PFO)
• Intrapulmonary: Arteriovascular malformations, ARDS
• Generally does not correct with 100% oxygen

Question 4

Hypoxemia

Answer 4

PaO2 less than 60mmHg

Question 5

High Altitude Cerebral Edema

Answer 5

• Ataxia, decline in mental function and consciousness
• Elevation above 3000-3500m

Question 6

Diffusion Impairment

Answer 6

• Increased thickness of alveolar capillary membrane
  
  • Some destruction of capillary surface membrane that results in reduction of gas exchange across membrane
• Decreased area for diffusion (like in emphysema)
• Decreased blood transit time
  
  • Seen when people start to exercise; not able to equilibrate as quickly as normal person
• Generally does not cause hypoxia at rest but with exertion

Question 7

In VQ mismatch, poor ventilation can lead to ____ and poor perfusion can lead to _____

Answer 7

Poor ventilation = shunt

Poor perfusion = dead space

Question 8

Alveolar Gas Equation

Answer 8

PAO₂ = 147 - 1.25 x PaCO₂

Question 9

VQ Mismatch

Answer 9

• Unequal matching of either ventilation or perfusion to a single lung unit
• Can result in hypoxemia
• Will partially correct with supplemental oxygen

Question 10

Treatment of Acute Mountain Sickness

Answer 10

Supplemental oxygen, Acetazolamide (diuretic that induces metabolic acidosis which increases ventilation), Descent to lower elevation

Question 11

What to ask if we see that an A-a gradient is affected

Answer 11

• Is it only present with activity?
• Does PaO₂ improve with 100% O2?

Question 12

Confounding Factors in Pulse Oximetry

Answer 12

• Anemia, Vasoconstriction, low BP
• Increased venous pulsation
• External light sources
• Dyes and pigments – methylene blue, nail polish
• Dyshemoglobinemias: CarboxyHb, MetHb
• Methemoglobin skews the reading to appear lower than it is
• Carboxyhemoglobin skews the reading to appear higher than it is

Question 13

Measurements of Oxygen

Answer 13

Oxyhemoglobin: Hb-O2

• Measured through co-oximetry and reported as a % Saturation; % of hemoglobin that is bound to oxygen

Dissolved Oxygen: PaO2

• Measured through blood gas; blood gathered an evaluated in lab

SaO2: Arterial O2 Saturation

PaO2: partial pressure O2 in blood

SpO2: Peripheral O2 Saturation – most common measurement

Question 14

Pulse Oximetry

Answer 14

• Can determine the proportion of hemoglobin that is saturated with oxygen using spectrophotometry
• Takes into account the Beer-Lambert Law
• This works because deoxyhemoglobin and oxyhemoglobin absorb light at two different ends of the spectrum
• Deoxyhemoglobin: Absorbs Red (600-750nm)
• Oxyhemoglobin: Absorbs Infrared (8500-1000nm)
• Emitters on probe emit light at 660nm and 940nm

Question 15

Periodic Breathing of Altitude

Answer 15

Cheyne Stokes breathing, or PB) is common at high altitude and becomes more frequent with increasing altitude. Periodic breathing involves alternating periods of deepbreathing and shallow breathing.

Question 16

Alveolar-arterial gradient (A-a Gradient)

Answer 16

• a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. It is used in diagnosing the source of hypoxemia. It helps to assess the integrity of alveolar capillary unit
• Normal range = ( Age in years ÷ 4 ) + 4
• Anything beyond this is considered to be a widened A-a gradient
• At this level or less means that A-a gradient is not affected

Question 17

Acute Mountain Sickness (AMS)

Answer 17

• Headache, fatigue, lightheadedness, anorexia, nausea/vomiting
• Proposed etiology: vasogenic brain edema due to disruption to the blood-brain barrier induced by hypoxemia at high elevation
• Onset 6-12hrs after ascension
• Typically Above 2000m (6500ft)
• Denver 5280ft (1609m), Lake Tahoe 5817ft (1773m), Santa Fe 7198ft (2194m), Machu Picchu 7972ft (2430m), Mt Rainier 14,409ft (4392m)
• Dependent on individual, elevation, and rate of ascent • Not protected by youth or physical fitness but obesity and heavy exertion increase risk

Question 18

Oxygen Content (CaO₂)

Answer 18

Oxygen Content (CaO₂) = (1.36 x Hgb x SaO₂/100) + 0.003 X PaO₂

• Constant of 1.36 is amount of O2 (ml at 1atm) bound per gram of Hgb
• Constant of 0.003 is amount of O2 dissolved in plasma

Question 19

Oxygen Delivery (DO₂)

Answer 19

Oxygen Delivery (DO₂) = Cardiac Output x CaO₂

• Volume of oxygen delivered to systemic vascular bed per minute
• Normal CO is 5-6 L/min and O2 extraction is 50ml per liter
• Normal DO2 is 1000mL/min or 500mL/min/m2

Question 20

Curve of Hemoglobin Saturation vs. Partial Pressure of Oxygen

Answer 20

Steep decline once PaO2 reaches 60 and lower

Very little change in hemoglobin saturation at partial pressure over 60 mmHG

Question 21

High Altitude Pulmonary Edema

Answer 21

• Occurs 2-4days after ascent above 2500m (8000ft)
• Most common cause of death at high altitude
• High recurrence risk if return to that elevation
• Treatment: supplemental oxygen and descent to lower elevation