Ventilation and lung volumes

Question 1

Total lung capacity

Answer 1

• maximum volume of gas that the lungs can contain
• TLC is divided into tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume
• normally 6-7 L

Question 2

Tidal volume

Answer 2

• the volume of gas which flows into and then out of the lung in one breath
• normally 500-600 ml, and increase with exercise
• may be measured with a spirometer

Question 3

Inspiratory reverse volume

Answer 3

• IRV

- the maximum volume of gas that can be inhaled from the end-tidal inspiratory position

Question 4

Expiratory reverse volume

Answer 4

• ERV

- the volume of gas that can be exhaled from the end-tidal expiratory position

Question 5

Residual volume

Answer 5

• is the volume of gas contained in the lungs after a maximal forced expiration
• cannot be exhaled

Question 6

Vital capacity

Answer 6

• is the maximum volume of gas that can be exhaled after a maximal inspiration
• VC= IRV +VT +ERV = TLC - RV

Question 7

Inspiratory reserve capacity

Answer 7

• the maximum volume of gas that can be inhaled from the resting expiratory position
• IC= VT + IRV = TLC - FRC

Question 8

Functional residual capacity

Answer 8

• is the volume of gas in the lungs after a normal expiration when the diaphragm and chest muscles are relaxes, that is, when the lungs and chest wall are at mechanical equilibrium
• cannot be measured with a spirometer since this capacity includes RV

Question 9

Effect of Compliance on FRC

Answer 9

• increased lung compliance increased FRC
• decreased lung compliance decreases FRC
• lung compliance increases during aging, as does FRC
• hyperinflation is characteristic of emphysema

Question 10

Open Circuit nitrogen washout for measuring FRC

Answer 10

• subject breaths air, an alveolar gas sample is taken an the initial N2 fraction is measured
• then at the end of eupneic expiration, with the lung at FRC, the subject breaths 100% oxygen for at least 7 min to wash out all of the N2 from the lung
• the expired gas is collected in spirometer
• the volume expired and N2 fraction in the collected gas are measured
• a conservation of mass equation may be used to estimate FRC

Question 11

Body Plethysmograph FRC

Answer 11

• subject sits in a gas tight chamber, similar in size to a telephone booth, and breathes through a tube leading to the outside
• the tube is shut off, closed by solenoid when the lung is at FRC
• the subject then makes an expiratory effort against a pressure transducer which records the pressure within the lung
• the expiratory effort compresses the volume of the lung and raises the pressure from its initial level of P1 by an amount of change in P
• a second transducer B records decrease in pressure of the box
• FRC may be calculated from the compressibility of the gas

Question 12

Total ventilation

Answer 12

• conducting zone: anatomic dead space
• respiratory zone: where gas exchange occurs
• Vt= VD + Va
• alveolar ventilation is the difference between total ventilation and dead space ventilation

Question 13

Alveolar Ventilation

Answer 13

• hypoventilation results in alveolar hypercapnea (increased PaCO2) and hypoxia (decreased PaO2)
• hyperventilation results in alveolar hypocapnea (decreased PaCO2) and hyperoxia (increased PaO2)

Question 14

Alveolar Gas Equation for CO2

Answer 14

• increasing alveolar ventilation decreases steady state alveolar PaCO2
• increasing carbon dioxide production increases steady state PaCO2
• PACO2 = VCO2 x PT/VA
• PVCO2= 46
• PaCO2= 40
• PACO2= 40

Question 15

Alveolar Gas Equation for O2

Answer 15

• increasing alveolar ventilation increases alveolar oxygen
• increasing oxygen consumption decreases alveolar oxygen
• increasing the partial pressure of oxygen in the inspired gas increases alveolar oxygen
• PAO2 = PIO2- VO2 x PT/VA = PIO2- PACO2/R

Question 16

Alveolar O2 and CO2 during a single breath

Answer 16

• VT is only 10% of TLC, the volume of air going into and out of the lung with each breath is a small fraction of the volume of air present in the lung
• alveolar gas composition does not change much with each breath
• the oscillations of PO2 and PCO2 in the alveoli are of the order of 1-2 mm Hg during eupnea
• the large amount of air that is not exchanged acts as a buffer to minimize these oscillations
• theres of hitch in PO2 because of the last breaths conducting zone has to go first

Question 17

Single breath analysis of anatomic dead space

Answer 17

• inspires air containing negligible CO2 and then exhales into a spirometer while the fraction of CO2 in the expired gas is being measured
• plots of volume expired and of FECO2 allow measurement of Vd
• VA= VE- VD = (VE- VD) x f
• expired CO2 comes exclusively from the alveoli and not from the anatomic dead space
• as subject exhales, the FECO2 rises from a negligibly low level to plateau near 0.05
• the first gas to be expired has very low CO2 because the gas comes from anatomic dead space
• due to mixing FECO2 risesi n a sigmoid manner with time
• the time of the midpoint of the rise of FECO2 defines the time at which all of the dead space air would be expired if there were a sharp boundary between dead space gas and alveolar gas