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Exercise
- Example
- Increase ventilation matches Vdot(O2) and Vdot(CO2)
- e.g. Fit young subject:
- Vdot(O2) from 0.25 to 2.5 L/min (X10)
- Vdot(CO2) from 0.2 to 3.0 L/min (X15) (R=1.2)
- Ve from 8 to 120 L/min (X15)
- Arterial PCO2
- Mild/moderate unchanged
- Severe might fall
- Arterial PO2
- Might rise slightly
- Arterial pH
- Mild/moderate unchanged
- Severe might fall
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Stimuli for Increased Ventilation
- LIMB MOVEMENTS-Proprioceptors
- OSCILLATIONS OF PAO2/PACO2 -Increase with increase VT, Although mean level constant
- CENTRAL CHEMORECEPTOR SERVOMECHANISM
- Response to PaCO2, Yet PaCO2 falls @ end extreme exercise
- BODY TEMPERATURE
- CORTICAL IMPULSES, Motor cortex
- Altitude
- Exponential decrease in PB with altitude
- Altitude PB PAO2
- (ft) (mm Hg) (mm Hg)
- 0 760 150
- 10,000 500 (67%) 100 Ski resort
- 18,000 380 (50%) 70
- 29,000 250 (33%) 42 Summit of Everest
- 33,000 Aircraft cruising
- 63,000 47 0
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Primary Increase in Ventilation (Altitude)
- Hypoxia stimulates ventilation
- Lower PACO2 maintains higher PAO2
- (since PACO2+PAO2=PIO2)
- But lower PaCO2 and alkalosis decrease excitation of peripheral chemoreceptors and limit this increase in ventilation
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Secondary Increase in Ventilation (Altitude)
- After 2-3 days
- (A) RENAL EXCRETION OF HCO3-
- Returns blood pH towards normal
- (B) CSF HCO3- REDUCED
- Active transport?
- Passive redistribution?
- (C) CEREBRAL LACTIC ACIDOSIS
- Consumes HCO3-, reduces CSF pH
- (D) ENHANCED CO2 RESPONSIVENESS
- Reduced PaO2 causes left shift of -PaCO2 relationship
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Diminished Hypoxic Responsiveness
- Residents @ high altitude
- More likely if resident since infancy
- (Also seen in chronic hypoxemia of congenital heart disease)
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Polycythemia
- Increased erythropoetin
- Increased 2,3-DPG
- Decreased Hb affinity for oxygen
- (left shift or lower P50)
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Monge’s Disease
- Chronic mountain sickness
- Poor hypoxemic ventilatory response
- Polycythemia
- Pulmonary hypertension
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Sleep
- Ventilation is decreased during sleep.
- This is in response to reduced metabolic rate, diminished cortical and reticular activating system activity and respiratory muscle relaxation during rapid eye movement (REM) sleep.
- There is a slight tendency for PaO2 to fall and PaCO2 to rise.
- Disturbances of ventilatory control during sleep or alternatively obstruction of the hypophangeal airway can cause sleep apnea (defined as cessation of breathing for more than 10 seconds). Some patients have frequent sleep apneas (or hypopneas) leading to sleep disruption, nocturnal hypoxemia, early morning hypercapnia and sleepiness during the day.
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