Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 4: Ventilation

4-1	Water vapor pressure (PH2O) accounts for more of the total pressure in tracheal gas than it does in atmospheric gas; this leaves less total pressure available for the rest of the atmospheric gases, and their pressures (including the PO2) must therefore decrease; that is, in the trachea PO2 = FIO2 (PB  PH2O) rather than PO2 = FIO2 (PB).

4-2	Dead-space volume (VD) in a single breath is fixed, determined by the dimensions of the conducting airways. Therefore increasing the tidal volume increases the single-breath alveolar volume (VA) but not the single-breath VD.

4-3	 =    thus an increase in  with a constant  causes  to decrease. Because only  affects gas exchange, its decrease causes alveolar PCO2 to rise and alveolar PO2 to decrease.

4-4	Use the following equation:
				                        
After calculating , subtract it from . Remember,  equals  minus . Calculate  as follows:
	 = 200 mL/min  0.863  40 mm Hg = 4.315 L/min, or 4315 mL/min
	 = f  VT
	 = 10/min  600 mL = 6000 mL/min
	 = 6000 mL/min  4315 mL/min = 1685 mL/min

4-5	The only component of minute ventilation that directly affects the PaCO2 is alveolar ventilation, because only  is in contact with blood. Because the PaCO2 is normal, alveolar ventilation must be appropriate. However, this is not consistent with the greatly elevated  of 18 L/min. This high minute ventilation coupled with a normal PaCO2 means that a large proportion of the minute ventilation must be ventilating alveoli that have lost their blood flow. This constitutes alveolar dead space, as might occur in pulmonary embolus or a severe drop in blood pressure (shock). Increased dead space explains the reason why  must increase so much to maintain a normal PaCO2. High  with a normal PaCO2 is the hallmark of increased dead space.