Why do we breathe? Take in O 2 (which we need to make ATP) Get rid of CO 2 (which is a waste product of ATP synthesis)

Thought questions What happens to our cells and our bodies if we don’t get enough O 2, or if CO 2 is made but never removed from the body? Is the amount we breathe always the same? What kinds of things change it?

Partial pressure of a gas (P) P = F x P atm F = fraction of the gas in the atmosphere that is that gas P atm : atmospheric pressure

Partial pressure of a gas (P) P = F x P atm F = fraction of the gas in the atmosphere that is that gas P atm : atmospheric pressure Example: partial pressure of O 2 in this room 21% of the atmosphere is O 2 Atmospheric pressure in Boston (see level) is 760 mmHg PO 2 = 0.21 x 760 = 160 mmHg

Hypercarbia: PaCO 2 greater than set point of 40 mmHg What determines the PaCO 2 ?

Answer: The P A CO 2 What determines the P A CO 2 ?

What determines the PaCO 2 ? Answer: The P A CO 2 What determines the P A CO 2 ? Answer: Alveolar ventilation

Time (sec) Minute Ventilation = tidal volume (V T ) x frequency Breathing frequency: 1 breath/ 4 sec = 15 breaths/min V T = 0.6 liters Change in volume (liters) VTVT

Is all the air that we take in useful?

A useful model of the lung – balloon on a tube Tube: airways Balloon: alveoli

V D = volume of cylinder End Expiration Begin InspirationMid Inspiration

End Inspiration V T = total volume of white area Mid ExpirationEnd Expiration

PANTING: high frequency but low volume breathing (assume a dead space of 0.1 L) Breathing at rest: V T = 0.5 L f = 10 breaths/min V E =5 L/min V A = 4 L/min Panting: V T = 0.14 L f = 100 breaths/min V E = 14 L/min V A = 4 L/min

Examples of things that can cause hypoventilation drugs (i.e. morphine) that suppress respiratory drive diseases of the muscles (like myasthenia gravis) diseases of the nerves diseases that affect central respiratory drive: - CCHS (Ondine’s curse).

CCHS

What causes changes in dead space? 1) Breathing through a piece of equipment Gas mask Snorkel Ventilator circuit 2) Loss of blood flow to a ventilated region of lung pulmonary embolism Certain lung diseases (emphysema)

What causes changes in dead space? 1) Breathing through a piece of equipment Gas mask Snorkel Ventilator circuit 2) Loss of blood flow to a ventilated region of lung pulmonary embolism Certain lung diseases (emphysema)

Jeffrey et al, Am. J. Respir. Crit. Care Med : 28S-38S EMPHYSEMA

Hypoxemia: PaO 2 less than the set point of about 80 mmHg

CAUSES OF HYPOXEMIA Hypoventilation Diffusion Impairment Shunt V/Q abnormalities

What determines how fast O 2 (or CO 2 ) diffuses across the alveolar wall? Diffusion Impairment

. VO 2 = K x A x (P A O 2 – P a O 2 )/T. VO 2 = flux of O 2 across the lung K = a constant A = surface area of the lung T = thickness of the lung (distance between air and red blood cell) P A O 2 and P a O 2 = partial pressure for O 2 in alveolus and in capillary blood

Jeffrey et al, Am. J. Respir. Crit. Care Med : 28S-38S EMPHYSEMA

Shunt: blood passes from the right heart to the left heart without becoming oxygenated 1. Blood goes from the right to the left heart without going through the lungs Bronchial circulation Foramen ovale, ductus arteriosus 2. Blood goes through the lungs but never comes into close contact with alveolar gas Foreign object lodged in an airway Pneumonia (accumulation of fluid and pus in the alveoli)

How much does a shunt affect PaO 2 ? Pus and fluid Air “Good” lung “Bad” lung 60% of C.O. 40% of C.O.

How much does a shunt affect PaO 2 ? Pus and fluid Air “Good” lung “Bad” lung 60% of C.O. 40% of C.O. PO 2 = 40 mmHg PO 2 = 100 mmHg

How much does a shunt affect PaO 2 ? Pus and fluid Air “Good” lung “Bad” lung 60% of C.O. 40% of C.O. PO 2 = 40 mmHg PO 2 = 100 mmHg PO 2 In Out 40 mmHg In Out 40 mmHg 100 mmHg

O 2 carrying capacity of hemoglobin 1 g Hemoglobin (Hg) can carry 1.34 ml O 2 (when fully saturated) Normal: 15 g Hg/ 100 ml blood x 1.34 = 20 ml O 2 /100 ml blood = 200 ml O 2 /L blood

100% x 200 ml O 2 /L blood x 3L “Good” lung “Bad” lung 75% x 200 ml O 2 /L blood x 2L Assume the cardiac output is 5 L/min: 60% to good lung and 40% to bad lung 3 L/min 2 L/min 600 ml O ml O 2 Blood mixes in left atrium 900 ml O 2 in 5L blood 180 ml O 2 /L (Fully saturated 200 mlO 2 /L) 90% saturatedPaO 2 of 60 mmHg

Pressure (cm H 2 O) Volume (L)