Chapter 5
Ventilation and Ventilatory Control Tests
1. Describe the measurement of tidal volume and minute ventilation.
2. Identify at least two causes of decreased minute ventilation.
3. Calculate the VD/VT ratio, using the modified Bohr equation.
1. Compare the calculation of VD/VT, using Paco2 and Petco2.
2. List at least two causes for an increased VD/VT ratio.
3. Explain the function of a variable CO2 scrubber in a circuit for measuring ventilatory response to hypoxia.
Tidal volume, rate, and minute ventilation
Description
Tidal volume (VT) is the volume of gas inspired or expired during each respiratory cycle (see Figure 2-1). It is usually measured in liters or milliliters and corrected to BTPS. Conventionally, the volume expired is expressed as VT. The respiratory rate is the number of breaths per minute (sometimes called breathing frequency or respiratory frequency or fb). The total volume of gas expired per minute is E, or minute ventilation.
E includes alveolar and dead space ventilation and is recorded in liters per minute, BTPS.
Technique
VT can be measured directly by simple spirometry (see Figure 2-1). The patient breathes into a volume-displacement or flow-sensing spirometer (see Chapter 11). Volume change may be measured directly from the excursions of a volume spirometer. VT may also be measured from an integrated flow signal (see Chapter 11). A graphic representation of tidal breathing can be displayed on a computer screen. Because no two breaths are the same, inhaled or exhaled tidal breaths should be measured for at least 1 minute and then divided by the rate to determine an average volume:
![image](/wp-content/uploads/2015/06/B9780323085052000058_si1.gif)
= volume expired or inspired per minute (usually the
E )
fb = number of breaths for the same interval (i.e., the respiratory rate)
Respiratory frequency (fb) may be determined by counting chest movements, noting the excursions of a volume displacement spirometer (Criteria for Acceptability 5-1), or most commonly by measuring flow changes while the subject breathes through a flow-sensing spirometer. Counting the rate for several minutes and taking an average produces a more accurate value than shorter measurements. Prolonged measurement of VT and rate with a volume-displacement spirometer requires a means of removing CO2. This is called a rebreathing system and uses a chemical CO2 absorber (see Figure 4-1, B). Sodium hydroxide crystals (Sodasorb) or barium hydroxide crystals (Baralyme) are commonly used to scrub CO2 from rebreathing systems. Flow-sensing spirometers usually do not require a chemical absorber.
Significance and Pathophysiology
See Interpretive Strategies 5-1. Average VT for healthy adults at rest ranges between 400 and 700 mL, but there is considerable variation. Decreased VT occurs in many types of pulmonary disorders, particularly those that cause severe restrictive patterns. Pulmonary fibrosis and neuromuscular diseases (e.g., myasthenia gravis) often cause reduced VT. Decreased tidal breathing usually accompanies changes in the mechanical properties of the lungs or chest wall (i.e., compliance and resistance). These changes usually produce an increased respiratory rate (fb) required to maintain an adequate A. Decreases in both VT and respiratory rate are often associated with respiratory center depression because of drugs or pathologic conditions affecting the brain stem. Low VT and rate usually result in alveolar hypoventilation.
Decreased ventilation may result from hypocapnia, metabolic alkalosis, respiratory center depression, or neuromuscular disorders that involve the ventilatory muscles. Hypoventilation is defined as inadequate ventilation to maintain a normal arterial Pco2, with respiratory acidosis as the result. The diagnosis of either hyperventilation or hypoventilation requires blood gas analysis (see Chapter 6).
Respiratory dead space and alveolar ventilation
Description
A is the volume of gas that participates in gas exchange in the lungs. It can be expressed as:
![image](/wp-content/uploads/2015/06/B9780323085052000058_si2.gif)
D= dead space ventilation per minute
Technique
Dead Space
![image](/wp-content/uploads/2015/06/B9780323085052000058_si3.gif)
FAco2 = fraction of CO2 in alveolar gas
Fco2 = fraction of CO2 in mixed expired gas
![image](/wp-content/uploads/2015/06/B9780323085052000058_si5.gif)
Pco2 = Pco2 of mixed expired gas sample
The VD/VT ratio can be calculated if arterial and mixed-expired Pco2 values are known. It can also be estimated noninvasively. End-tidal Pco2 (see the section on capnography in Chapter 6) can be used to estimate Paco2. The main advantage of this method is that it is not necessary to obtain an arterial blood sample. This technique is often used in systems that monitor expired CO2 continuously and in breath-by-breath metabolic measurement devices. VD/VT may be calculated as follows:
![image](/wp-content/uploads/2015/06/B9780323085052000058_si7.gif)
Pco2= Pco2 of mixed-expired gas sample
Alveolar Ventilation
A can be calculated in two ways:
![image](/wp-content/uploads/2015/06/B9780323085052000058_si9.gif)
Because atmospheric gas contains almost no CO2, A can be calculated on the basis of CO2 elimination from the lungs. A volume of expired gas may be collected in a bag, balloon, or spirometer and analyzed to determine the volume of CO2 (see Chapter 7). The following equation can then be used:
![image](/wp-content/uploads/2015/06/B9780323085052000058_si10.gif)
co2= volume of CO2 produced in liters per minute (STPD)
FAco2= fractional concentration of CO2 in alveola
![image](/wp-content/uploads/2015/06/B9780323085052000058_si11.gif)
End-tidal CO2 may not equal alveolar CO2 in patients with grossly abnormal patterns of ventilation-perfusion (see Chapter 6).
![image](/wp-content/uploads/2015/06/B9780323085052000058_si12.gif)
co2= CO2 production in mL/min (STPD)
Paco2= partial pressure of arterial CO2
0.863= conversion factor (concentration to partial pressure, correcting co2 to BTPS)
Significance and Pathophysiology
See Interpretive Strategies 5-2. Measurement of VD yields important information regarding the ventilation-perfusion characteristics of the lungs. Anatomic dead space is larger in men than in women because of differences in body size. It increases along with the VT