1 What is the functional residual capacity? What factors affect it?

The functional residual capacity (FRC) is the volume in the lungs at the end of passive expiration. It is determined by opposing forces of the expanding chest wall and the elastic recoil of the lung. A normal FRC = 1.7 to 3.5 L. FRC is increased by:

Body size (FRC increases with height)

Age (FRC increases slightly with age)

Certain lung diseases, including asthma and chronic obstructive pulmonary disease (COPD).

FRC is decreased by:

Sex (woman have a 10% decrease in FRC when compared to men)

Diaphragmatic muscle tone (individuals with paralyzed diaphragms have less FRC when compared to normal individuals)

Posture (FRC greatest standing > sitting > prone > lateral > supine)

Certain lung diseases in which elastic recoil is diminished (e.g., interstitial lung disease, thoracic burns, and kyphoscoliosis)

Increased abdominal pressure (e.g., obesity, ascites)

2 What is closing capacity? What factors affect the closing capacity? What is the relationship between closing capacity and functional residual capacity?

Closing capacity is the point during expiration when small airways begin to close. In young individuals with average body mass index, closing capacity is approximately half the FRC when upright and approximately two thirds of the FRC when supine.

Closing capacity increases with age and is equal to FRC in the supine individual at approximately 44 years and in the upright individual at approximately 66 years. The FRC depends on position; the closing capacity is independent of position. Closing capacity increases with increasing intraabdominal pressure, age, decreased pulmonary blood flow, and pulmonary parenchymal disease, which decreases compliance.

3 What muscles are responsible for inspiration and expiration?

The respiratory muscles include the diaphragm, internal and external intercostals, abdominal musculature, cervical strap muscles, sternocleidomastoid muscle, and large back and intervertebral muscles of the shoulder girdle. During normal breathing inspiration requires work, whereas expiration is passive. The diaphragm, scalene muscles, and external intercostal muscles provide most of the work during normal breathing. However, as the work of breathing increases, abdominal musculature and internal intercostal muscles become active during expiration, and the scalene and sternocleidomastoid muscles become increasingly important for inspiration.

4 What is the physiologic work of breathing?

The physiologic work of breathing involves the work of overcoming the elastic recoil of the lung (compliance and tissue resistance work) and the resistance to gas flow. The elastic recoil is altered in certain pathologic states, including pulmonary edema, pulmonary fibrosis, thoracic burns, and COPD. The resistance to gas flow is increased dramatically during labored breathing. In addition to the physiologic work of breathing, a patient on a ventilator must also overcome the resistance of the endotracheal tube.

5 Discuss the factors that affect the resistance to gas flow. What is laminar and turbulent gas flow?

The resistance to flow can be separated into the properties of the tube and the properties of the gas. At low flow, or laminar flow (nonobstructed breathing), the viscosity is the major property of the gas that affects flow. Clearly the major determining factor is the radius of the tube. This can be shown by the Hagen-Poiseuille relationship:

where R is resistance, L is the length of the tube, μ is the viscosity, and r is the radius of the tube. At higher flow rate (in obstructed airways and heavy breathing), the flow is turbulent. At these flows the major determinants of resistance to flow are the density of the gas (ρ) and the radius of the tube, r.

6 Suppose a patient has an indwelling 7-mm endotracheal tube and cannot be weaned because of the increased work of breathing. What would be of greater benefit, cutting off 4 cm of endotracheal tube or replacing the tube with one of greater internal diameter?

According to the Hagen-Poiseuille relationship discussed previously, if the radius is halved, the resistance within the tube increases to sixteenfold; but if the length of the tube is doubled, the resistance is only doubled. Cutting the length of the tube minimally affects resistance, but increasing the tube diameter dramatically decreases resistance. Therefore, to reduce the work of breathing the endotracheal tube should be changed to a larger size.

7 Why might helium be of benefit to a stridorous patient?

When flow is turbulent, as is the case in a stridorous patient, driving pressure is mostly related to gas density. Use of low-density gas mixtures containing helium and oxygen lowers the driving pressure needed to move gas in and out of the area, decreasing the work of breathing.

8 Discuss dynamic and static compliance

Compliance describes the elastic properties of the lung. It is a measure of the change in volume of the lung when pressure is applied. The lung is an elastic body that exhibits elastic hysteresis. When the lung is rapidly inflated and held at a given volume, the pressure peaks and then exponentially falls to a plateau pressure. The volume change of the lung per the initial peak pressure change is the dynamic compliance. The volume change per the plateau pressure represents the static compliance of the lung.

9 How does surface tension affect the forces in the small airways and alveoli?

Laplace’s law describes the relationship between pressure (P), tension (T), and the radius (R) of a bubble and can be applied to the alveoli.

As the radius decreases, the pressure increases. In a lung without surfactant present, as the alveoli decrease in size, the pressure is higher in small alveoli, causing gas to move from the small to larger airways, collapsing in the process. Surfactant, a phospholipid substance produced in the lung by type II alveolar epithelium, reduces the surface tension of the small airways, thus decreasing the pressure as the airways decrease in size. This important substance helps keep the small airways open during expiration.

10 Review the different zones (of West) in the lung with regard to perfusion and ventilation

West described three areas of perfusion in an upright lung, and a fourth was later added. Beginning at the apices, they are:

Zone 1: Alveolar pressure (P_Alv) exceeds pulmonary artery pressure (P_pa) and pulmonary venous pressure (P_pv), leading to ventilation without perfusion (alveolar dead space) (P_Alv > P_pa > P_pv).

Zone 2: Pulmonary arterial pressure exceeds alveolar pressure, but alveolar pressure still exceeds venous pressure (P_pa > P_Alv > P_pv). Blood flow in zone 2 is determined by arterial-alveolar pressure difference.

Zone 3: Pulmonary venous pressure exceeds alveolar pressure, and flow is determined by the arterial-venous pressure difference (P_pa > P_pv >P_Alv).