Abnormal Ventilation, Abnormal Gas Exchange Robert C. Basner, MD Associate Professor of Clinical Medicine Director, Adult Pulmonary Diagnostic Unit Di...
Abnormal Ventilation, Abnormal Gas Exchange Robert C. Basner, MD Associate Professor of Clinical Medicine Director, Adult Pulmonary Diagnostic Unit Director, Cardiopulmonary Sleep and Ventilatory Disorders Center Columbia University College of Physicians and Surgeons
Ventilation and Gas Exchange Ventilation and Gas Exchange • Objective: to achieve adequate tissue oxygenation j q yg and remove metabolically produced CO2. • Ventilation: concerned with delivery of fresh volume of air to gas exchanging units, and the removal of a f h d h l f sufficient volume of mixed gas out • Gas Exchange: the ability to move gas across the Gas Exchange: the ability to move gas across the alveolar‐capillary membrane
Ventilation and Gas Exchange Ventilation and Gas Exchange • The failure of either or both results in impaired p arterial blood gases and ultimately respiratory failure. • Ventilatory failure: l f l Hypercapnic respiratory failure f l • Gas exchange failure: Hypoxemic respiratory failure • Hypoxemia is the inevitable result of both H i i th i it bl lt f b th
Ventilation
Ventilation = Breathing Ventilation Breathing • Ventilation Ventilation is the process of moving gases is the process of moving gases between the atmosphere and the alveoli
Normal breathing Normal breathing • Respiratory rate = the number of breaths per minute – About 12 to 15 per minute – Abbreviated RR
• Tidal volume = volume of gas inspired in a single breath – About 0.5 liters – Abbreviated VT
• Minute ventilation = volume of gas inspired per minute = RR x VT – About 6 liters per minute – Abbreviate VE
Only Some of the Tidal Volume Reaches Alveoli h l l
“A t i Dead “Anatomic D d Space” S ”
“Alveolar Space”
Dead Space Dead Space • Anatomic Dead Space Anatomic Dead Space – Normal – About 1ml per lb. body weight ( About 1ml per lb body weight (~150 150 ml) ml)
• Physiologic Dead Space Physiologic Dead Space – Abnormal – Areas not participating in gas exchange (more later) A t ti i ti i h ( l t )
Dead Space Dead Space
VT = VD + VA 500 ml = 150 ml + 350 ml
Alveolar and Dead Space Ventilation Alveolar and Dead Space Ventilation
VT = VD + VA
VT × RR = (VD × RR ) + (VA × RR )
& & & VE = VD + VA
Volumes and flows Volumes and flows
Total and Alveolar Ventilation Total and Alveolar Ventilation
& & & VCO2 = VA × FACO2 = VA × V&A =
V&CO2 PACO2
×K ≈
V&CO2 PaCO2
PACO
2
K ×K
Total and Alveolar Ventilation Total and Alveolar Ventilation
V&CO2
P CO A 2 & & = VA × FACO2 = VA × K
V&A =
V&CO2 PACO2
×K ≈
V&CO2 PaCO2
×K
Total and Alveolar Ventilation Total and Alveolar Ventilation
V&CO2
P CO A 2 & & = VA × FACO2 = VA × K
V&A =
V&CO2 PACO2
×K ≈
V&CO2 PaCO2
×K
Total and Alveolar Ventilation Total and Alveolar Ventilation
V&CO2
P CO A 2 & & = VA × FACO2 = VA × K
V&A =
V&CO2 PACO2
×K ≈
V&CO2 PaCO2
×K
Application of the Alveolar Ventilation Equation l l l
& VCO2 PaCO2 ∝ V& A
What happens if… 1. Dead space increases (minute ventilation held constant) 2. Minute ventilation increases (VD is constant) 3. CO2 production increases
PaCO2 is used to determine alveolar ventilation l • Normal PaCO2 = 37 to 42 mm Hg Normal PaCO2 = 37 to 42 mm Hg • PaCO2 > 42 mm Hg = alveolar hypoventilation • PaCO2