|Bhavani Kodali||Hypoventilation does not per se increase dead space but rather decrease alveolar ventilation. Because of cone front effect due to laminar flow of gases (flow is relatively more in the center of the airway tubes than in the periphery. Because of this phenomenon, the alveolar ventilation does occur with tidal volume less than anatomical dead space. Therefore as hypoventilation occurs, end tidal CO2 will increase. But when hypoventilation is significant enough, the end-tidal gases may not represent alveolar gases and end tidal CO2 can be significantly lower than PaCO2. Under these circumstances a deep ventilation given via positive pressure bag will enable true alveolar sampling (squeeze PCO2) and will reflect PETCO2 closer to PaCO2. I hope you understand now. |
During hyperventilation, there is no such issue and therefore end tidal gases represent decreased arterial PCO2 and hence decreased PETCO2.