The science of oxygen getting to the blood-gas barrier
ANATOMICAL DEAD SPACE - NO GAS EXCHANGE
PHYSIOLOGICAL DEAD SPACE - (ALVEOLAR)
LOWER PORTIONS OF THE LUNGS ARE BETTER VENTILATED
Tidal volume is normal breathing. MAX inspirtion followed by MAX expiration; the exhaled volume is the vital capacity and the gas (of the maximally inspired breath) that remains in the lungs is called the residual volume. The volume of gas remaining in the lung after a normal
expiration is called the functional
residual capacity. FRC and RV
have to be measured by helium dilution
techniques or body plethysmography,
not a spirometer.
Alveolar ventilation is what really matters - is the volume of fresh (non-deadspace) gas that enters the respiratory zone (area of gas exchange) per minute. It can be determined by the CO2 output of the body divided by the fractional concentration of CO2 in the expired gas (alveolar gas equation). The concentration of CO2 (and it's partial pressure) in alveolar gas is inversely related to the alveolar ventilation - this is important because if alveolar ventilation HALVES, then paCO2 doubles. In normal bodies, the pCO2 of arterial blood and in the alveoli are pretty much identical.
This is the volume of the conducting airways. Normal value is around 150ml, but increases with larger breaths because of the traction exerted on the bronchi by the surrounding lung parenchyma. DS also depends on the size and posture of the patient - usually 2mg/kg in upright position. FOWLER'S METHOD estimates dead space volume using a valve box and a continuous nitrogen analyser at the lips. - following a single breath of 100% O2, nitrogen will rises as the dead space gas is increasingly washed out by alveolar gas, finally a uniform conc is seen on the analyser as 'pure' alveolar gas comes out.
DS gives information on how much of total ventilation that reaches both ventilated and perfused alveoli and thus allows gas exchange between alveoli and pulmonary blood.
Realising that CO2 retention can be an effect not only of low total ventilation but also of increased dead space is IMPORTANT.
Moreover, dead space will give insight into the matching of ventilation and perfusion.
Average Tidal Volume = 500ml,
Total vent = 7500ml (RR of 15)
Anatomical dead-space = 150ml
Alveolar gas = ~3000ml,
Alveolar ventilation = 5250ml/min
Pulmonary capilary blood = 70ml
Pulmonary blood flow = 5000ml/min
Va - Ventilation of alveoli
Vco2 = rate of CO2 production
Pco2 = partial pressure of CO2
K = constant of 863 mmHg
West's Physiology + http://www.ncbi.nlm.nih.gov/pubmed/16682925
Professor West has a wonderful repository of online video tutorials on respiratory physiology - http://meded.ucsd.edu/ifp/jwest/resp_phys/
Expired CO2 comes from alveolar gas, not anatomical dead space. Therefore, Bohr's method can calculate physiological dead space. IT IS THE VOLUME OF GAS THAT DOES NOT ELIMINATE CO2. You have to compare arterial and expired CO2 to calculate the mismatch. This is important when considering PE's and many other lung diseases.
- VD = Physiological dead space
- VT = Tidal volume
- PaCO2 = Arterial CO2
- PeCO2 = Expired CO2
The top of the lungs were least well ventilated. This is all due to gravity when standing up.
When supine - apex and base of lung are the same, but the POSTERIOR lobes are better ventilated.
When lateral - the dependent area is better ventilated.
Intrapleural pressure is higher at the bases of the lungs (less negative)
- and expands better on inspiration, therefore ventilation is better.... (large oversimplification!)
This concept is extremely important in sick lungs - and particularly in ICU - which is why some practitioners elect to PRONE people to change the dependent area of the lung, improve ventilation in the healthier portions of lung and improve oxygenation.
Dr. D. Lyness
This was identified when they calculated where radiation of inhaled xenon was detected by counters lined up against the posterior chest wall. Lower portions of the lungs had more xenon detected in them.