• it causes there to be a greater distending pressure (transpulmonary pressure) for alveoli at the top of the lung, and is a larger size than bottom
• pleural pressure more negative at top of thoracic cavity than bottom

• fill at different rates operate at different points on the pressure volume loop
• at the top, the alveoli are very distended, so it takes a lot of pressure to cause movement of air into those alveoli

Hydrostatic pressure in different parts of the lungs

Hydrostatic pressure gets higher as we get down the lung, and this affects blood flow

alveolar P greater than arterial P which is greater than venous pressure

arterial pressure is greater than alveolar pressure which is greater than venous pressure

arterial pressure greater than venous pressure which is greater than alveolar pressure

DOES NOT EXIST IN NORMAL LUNG

• positive pressure breathing (ex: ventilator)
• hemorrhage

• blood flow driven by pulmonary arterial and venous pressure differences

• apex has high ventilation:perfusion ratio
• base has low ventilation: perfusion ratio (even though it techincally has a higher ventilation and perfusion than apex)

Describe effects of V/Q on alveolar gas tensions

• if there is no ventilation, but continued perfusion, the alveolar pressure of oxygen and carbon dioxide will be equal to that of mixed venous blood
  • less fresh air, less oxygen, more carbon dioxide (change in pressures)
• if no perfusion with ventilation, alveolar oxygen pressure much greater and alveolar CO2 pressure much lower

Relation btw physiologic dead space and vent/perfusion ratio

Types of V/Q ratio seen in high CO2 partial pressure areas in alveoli

Caues of hypoxemia

• decrease alveolar oxygen tension
  • decrease PO2 in inspired gas
  • hypoventilation
• V/Q mismatch (low V/Q)
• shunts
• impaired diffusion

• hypoxia- low oxygen delivery to tissue
• hypoxemia- low blood oxygen

• P(A-a)DO2
• physiologic shunt
• physiologic dead space

• difference btw calculate alveolar oxygen pressure and measured arterial oxygen pressure from ABG analysis
• estimated 1/4 patients age (mine would be 95 while Dr. Delany's would be 85)

What separates shunt from cardiac output

this is the part of the cardiac output not fully oxygenated

right to left

left to right

• bronchial blood flow (about 2%)
• thebesian veins of coronary circulation

Contributes to normal A-a gradient

What can be said about the level of oxygenation of alveolar units with low V/Q

• part of their blood flow not being oxygenated enough
• includes fraction of normal LV output desaturated (bronchial venous return, thebesian venous return)

Clinical conclusion of high V/A ratios

• part of ventilation is not effective for exchange
• include anatomic dead space

oxygenation does not improve

• high right sided pressures
• hypoxemia always occurs
• cannot be corrected by breathing oxygen
• does not lead to big changes in arterial CO2 pressure (increased ventilation rate)
• increased A-a gradient

• no hypoxemia
• ex: patent ductus arteriosus

Vd/Vt = alveolar CO2 pressure - (PeCO2/alveolar CO2 pressure)

measure volume of lungs that doesn't eliminate CO2 (normal: 0.2-0.35)

diffusion defect (ex: fibrosis)

V/Q defect

Right to left shunt