Ventilation-Perfusion Flashcards
Effects of 100% O2 correcting V/Q mismatch
- perfusion needed to be okay and ventilation needs to at least be reachign alveoli
- in true cases of shunt, hypoxemia can’t be corrected by giving 100% oxygen (can’t reach if perfusion is bad)
normal ventilation values
ventilation = tidal vol. x resp. rate
total normal: 7500 cc
alveolar ventilation: 5250 cc
Diffusion Basics
- Diffusion = transfer of gas across the blood-gas barrier
- Proportional to the tissue area, the difference in gas partial pressure between two sides, and diffusion constant (property of tissue and the specific gas)
- Inversely proportional to tissue thickness
- Diffusion constant – proportional to solubility of gas and inversely proportional to molecular weight
- CO2 MORE soluble but same size – diffuses 20 x faster than O2
V/Q Mismatch: Respiratory Diseases with High V/Q ratio
- Severe COPD – maladaptive ventilatory overwork of the undamaged lung parenchyma, damaged alveoli decreases surface area, increasing ventilation.
- Pulmonary Embolism
- Compression of pulmonary capillaries (high alveolar pressures)
- Shock (Pulmonary vascular hypotension)
Diffusion vs. Perfusion limitations
DIFFUSION limited
- CO2
- If gas very soluble in blood moves from alveoli into RBC with almost no increase in partial pressure- gas continues to move rapidly across alveolar wall limited only by diffusion properties of blood-gas barrier
PERFUSION limited
- Nitrous Oxide
- If gas not soluble, as moves into RBC partial pressure quickly equals alveolar and no additional movement
- blood flow dependent
BOTH
- Oxygen
- soluble but not fully so still a rise in partial pressure in RBC. Resting conditions- perfusion dependent, abnormal conditions, diffusion dependent.
Zones of Perfusion in Lungs
Zone 1 –
- (top) If pulmonary artery pressure falls below Alveolar pressure then capillaries squashed flat/ No blood flow
- PA > Pa > Pv
- Does NOT occur under normal circumstance
- Causes ALVEOLAR DEAD SPACE
- Ventilated but under perfused
- Example: Hemorrhage (arterial pressure markedly reduced) or PPV (alveolar pressure raised)
Zone 2
- (middle) Pulmonary Artery pressure now higher than Alveolar Pressure (hydrostatic effect) but Alveolar pressure still greater than Venous pressure
- Pa > PA > Pv
- Yes blood flows in capillaries
- Flow determined by difference between Artery and Alveolar (NOT Venous) pressures
- At point where Alveolar pressure is greater than Arterial pressures there is capillary collapse similar to Zone 1 physiology.
Zone 3
- Pulmonary Venous pressure now exceeds Alveolar pressure
- Pa > Pv > PA
- Only Zone where Pulmonary Venous pressure is not the lowest of the three pressures.
- Capillary flow determined by “usual” difference pressures between Artery and Venous pressures
- Capillary flow is greatest in this zone as capillaries relatively “maintained open” or not collapsed by Alveolar pressures as seen in Zone 1
Normal V/Q ratio
Normal V/Q ratio = 1
causes of uneven ventilation
- change in elasticty
- obstruction of alveoli
- regional check valves
- regional disturbances in expansion (from abnormality in alveolar membrane)
Extremes of V/Q ratios
“Dead Space” – area ventilated but not perfused
“Shunt” – area perfused but not ventilated
V/Q Mismatch: COPD
- Increased V/Q mismatch
- Increased V/Q ratio
- Increased areas with “wasted” ventilation
- Increased physiologic dead space
- High frequency, lower tidal volume pattern of ventilation increases dead space even further
Perfusion Discrepancies Causes of Uneven Perfusion
- Embolization
- Occlusion
- Compression
- Fibrosis
- Loss of Capillary Surface
why High V/Q units cannot compensate for Low V/Q units
- Even though the unit on the right has the highest content leaving, the reduced flow means that the relative contribution to O2 Content is proportionately reduced
regional differences of lung in perfusion and ventilation
lower lung perfuses and ventilates much better; upper lung lung has higher V/Q ratio
V/Q Mismatch: Respiratory Diseases with low V/Q ratio
Lower V/Q ratio
- Asthma
- Chronic Bronchitis – bronchospasm, mucus plugs, inflammation, airway obstruction
- Acute Pulmonary Edema
- Airway Obstruction (foreign body aspiration)
- Cystic Fibrosis
size of anatomic dead space
approximately 150 cc/min
