Pneumothorax

Pneumothorax occurs when air enters the pleural space and Ppl falls to zero (^ p. 108), which can lead to collapse of the affected lung due to elastic recoil and respiratory failure (^ B). The contralateral lung is also impaired because a portion of the inspired air travels back and forth between the healthy and collapsed lung and is not available for gas exchange. Closed pneumothorax, i.e., the leakage of air from the alveolar space into the pleural space, can occur spontaneously (e.g., lung rupture due to bullous emphysema) or due to lung injury (e.g., during mechanical ventilation = barotrauma;^ p. 134). Open pneumothorax (^ B2) can be caused by an open chest wound or blunt chest trauma (e.g., penetration of the pleura by a broken rib). Valvular pneumothorax (^ B3) is a life-threatening form of pneumothorax that occurs when air enters the pleural space with every breath and can no longer be expelled. A flap of acts like a valve. Positive pressure develops in the pleural space on the affected side, as well as in the rest of the thoracic cavity. Since the tidal volume increases due to hypoxia, high pressure levels (4 kPa = 30 mmHg) quickly develop. This leads to increasing impairment of cardiac filling and compression of the healthy contralateral lung. Treatment of valvular pneumothorax consists of slow drainage of excess pressure and measures to prevent further valvular action.

|— A. Artificial respiration

Pneumothorax Pressure

O2 if needed Expiration

1 Positive-pressure respiration

O2 if needed Expiration

1 Positive-pressure respiration

Pneumothorax Pressure

2 Negative-pressure respiration

B Mouth-to-mouth resuscitation

B Mouth-to-mouth resuscitation

2 Negative-pressure respiration

Pressure Inspiration Expiration

Gas flow Inspiration Expiration

B. Pneumothorax

Normal Lung Volumes Measurments

1 Normal

Open Pneumothorax

2 Open pneumothorax

1 Normal

2 Open pneumothorax

Perforated tissue acts as valve

Life-threatening

B Valvular pneumothorax complication

Life-threatening

B Valvular pneumothorax complication

Lung Volumes and their Measurement

At the end of normal quiet expiration, the lung-chest system returns to its intrinsic resting position. About 0.5 L of air is taken in with each breath during normal quiet respiration; this is called the resting tidal volume (Vt). Inspiration can be increased by another 3 L or so on forced (maximum) inspiration; this is called the inspiratory reserve volume (IRV). Likewise, expiration can be increased by about 1.7 L more on forced (maximum) expiration. This is called the expiratory reserve volume (ERV). These reserve volumes are used during strenuous physical exercise (^ p. 74) and in other situations where normal tidal volumes are insufficient. Even after forced expiration, § about 1.3 L of air remains in the lungs; this is '■E called the residual volume (RV). Lung capacities are sums of the individual lung volumes. £ The vital capacity (VC) is the maximum ^ volume of air that can be moved in and out in a 10 single breath. Therefore, VC = Vt + IRV + ERV. The average 20-year-old male with a height of 1.80 m has a VC of about 5.3 L. Vital capacity decreases and residual volume increases with age (1.5 ! 3 L). The total lung capacity is the sum of VC and RV—normally 6 to 7 L. The functional residual capacity is the sum of ERV and RV (^ A and p. 114). The inspiratory capacity is the sum of Vt and IRV. All numerical values of these volumes apply under body temperature-pressure saturation (BTPS) conditions (see below).

Spirometry. These lung volumes and capacities (except FRC, RV) can be measured by routine spirometry. The spirometer (^ A) consists usually of a water-filled tank with a bell-shaped floating device. A tube connects the air space within the spirometer (^ A) with the airways of the test subject. A counterweight is placed on the bell. The position of the bell indicates how much air is in the spirometer and is calibrated in volume units (Latps; see below). The bell on the spirometer rises when the test subject blows into the device (expiration), and falls during inspiration (^ A).

If the spirometer is equipped with a recording device (spirograph), it can be also used for graphic measurem.ent of the total ventila-

compliance (^ p. 116), O2 consumption (Vo2), and in dynamic lung function tests (^ p. 118).

Range of normal variation. Lung volumes and capacities vary greatly according to age, height, physical constitution, sex, and degree of physical fitness. The range of normal variation of VC, for example, is 2.5 to 7 L. Empirical formulas were therefore developed to create normative values for better interpretation of lung function tests. For instance, the following formulas are used to calculate the range of normal values for VC in Caucasians: Men: VC = 5.2 h-0.022a-3.6 (± 0.58) Women: VC = 5.2 h-0.018a-4.36 (± 0.42), where h = height (in meters) and a = age (in years); the standard deviation is given in parentheses. Because of the broad range of normal variation, patients with mild pulmonary disease may go undetected. Patients with lung disease should ideally be monitored by recording baseline values and observing changes over the course of time.

Conversion of respiratory volumes. The volume, V, of a gas (in L or m3; 1 m3 = 1000 L) can be obtained from the amount, M, of the gas (in mol), absolute temperature, T (in K), and total pressure, P (in Pa), using the ideal gas equation:

where P is barometric pressure (Pb) minus water partial pressure (PH20; ^ p. 106) and R is the universal gas constant = 8.31 J ■ K-1 ■ mol-1.

Volume conditions

STPD: Standard temperature pressure dry (273 K, 101 kPa, PH2o = 0)

ATPS: Ambient temperature pressure H2O-saturated (Tamb, Pb, Ph20 at TAmb)

BTPS: Body temperature pressure-saturated (310 K, Pb, Ph20 = 6.25 kPa)

It follows that:

Vstpd = M ■ R ■ 273/101 000 [m3] Vatps = M ■ R ■ TAmb/(PB-PH2o) [m3] Vbtps = M ■ R ■ 310/(Pb- 6250) [m3]. Conversion factors are derived from the respective quotients (M ■ R is a reducing factor). Example: VBTPS/ VSTPD = 1.17. If VATPS is measured by spirometry at room temperature (TAmb = 20 °C; PH2osat = 2.3 kPa) and Pb = 101 kPa, Vbtps ~ 1.1 Vatps and Vstpd ~ 0.9 Vatps.

I— A. Lung volumes and their measurement

I— A. Lung volumes and their measurement

Heated Pneumotachygraph

Dead Space, Residual Volume, Airway Resistance

The exchange of gases in the respiratory tract occurs in the alveoli. Only a portion of the tidal volume (Vt) reaches the alveoli; this is known as the alveolar part (Va). The rest goes to dead space (not involved in gas exchange) and is therefore called dead space volume (VD). The oral, nasal, and pharyngeal cavities plus the trachea and bronchi are jointly known as physiological dead space or conducting zone of the airways. The physiological dead space (ca. 0.15 L) is approximately equal to the functional dead space, which becomes larger than physiological dead space when the exchange of gases fails to take place in a portion of the alve-§ oli (^ p. 120). The functions of dead space are to conduct incoming air to the alveoli and to purify (^ p. 110), humidify, and warm inspired £ ambient air. Dead space is also an element of ^ the vocal organ (^ p. 370).

The Bohr equation (^ A) can be used to estimate the dead space.

Derivation: The expired tidal volume Vt is equal to the sum of its alveolar part Va plus dead space Vd (^ A, top). Each of these three variables has a characteristic CO2 fraction (^ p. 376): FeCO2 in Vt, FaCO2 in Va, and FiCO2 in Vd. FiCO2 is extremely small and therefore negligible. The product of each of the three volumes and its corresponding CO2 fraction gives the volume of CO2 for each. The CO2 volume in the expired air (Vt ■ FeCO2) equals the sum of the CO2 volumes in its two components, i.e. in Va and Vd (^ A).

Thus, three values must be known to determine the dead space: Vt, FeCO2 and FaCO2. Vt can be measured using a spirometer, and FeCO2 and FaCO2 can be measured using a Bunte glass burette or an infrared absorption spectrometer. FaCO2 is present in the last expired portion of VT—i.e., in alveolar gas. This value can be measured using a Rahn valve or similar device.

The functional residual capacity (FRC) is the amount of air remaining in the lungs at the end of normal quiet expiration, and the residual volume (RV) is the amount present after forced maximum expiration (^ p. 112). About 0.35 L of air (Va) reaches the alveolar space with each breath during normal quiet respiration. Therefore, only about 12% of the 3 L total FRC is renewed at rest. The composition of gases in the alveolar space therefore remains relatively constant.

Measurement of FRC and RV cannot be per-

indirect techniques such as helium dilution (^ B). Helium (He) is a poorly soluble inert gas. The test subject is instructed to repeatedly inhale and exhale a known volume (VSp) of a helium-containing gas mixture (e.g., FHe0 = 0.1) out of and into a spirometer. The helium distributes evenly in the lungs (Vl) and spirometer (^ B) and is thereby diluted (FHex < FHe0). Since the total helium volume does not change, the known initial helium volume (VSp ■ FHeO) is equal to the final helium volume (Vsp + Vl) ■ FHex. Vl can be determined once FHex in the spirometer has been measured at the end of the test (^ B). Vl will be equivalent to RV ifthe testwas started after a forced expiration, and will be equivalent to FRC if the test was started after normal expiration, i.e. from the resting position of lung and chest. The helium dilution method measures gases in ventilated airways only.

Body plethysmography can also detect gases in encapsulated spaces (e.g., cysts) in the lung. The test subject is placed in an airtight chamber and instructed to breathe through a pneumotachygraph (instrument for recording the flow rate of respired air). At the same time, respiration-dependent changes in air pressure in the subject's mouth and in the chamber are continuously recorded. FRC and RV can be derived from these measurements.

Such measurements can also be used to determine airway resistance, Rl, which is defined as the driving pressure gradient between the alveoli and the atmosphere divided by the air flow per unit time. Airway resistance is very low under normal conditions, especially during inspiration when (a) the lungs become more expanded (lateral traction of the airways), and (b) the transpulmonary pressure (PA-Ppi) rises (^ p. 108). Pa-Pp1 represents the transmural pressure of the airways and widens them more and more as it increases. Airway resistance may become too high when the airway is narrowed by mucus—e.g., in chronic obstructive pulmonary disease, or when its smooth muscle contracts, e.g. in asthma (^ p.118). The residual volume (RV) fraction of the total lung capacity (TLC) is clinically significant (^ p. 112). This fraction normally is no more than 0.25 in healthy subjects and somewhat higher in old age. It can rise to 0.55 and higher when pathological enlargement of the alveoli has occurred due, for example, to emphysema. The RV/TLC fraction is therefore a rough measure of the severity of such diseases. 2003 Thieme and conditions of license.

I— A. Measurement of dead space

I— A. Measurement of dead space

Alveolar CO2

volume

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Responses

  • alicja
    Why is blood pressure high in pneumothorax?
    7 years ago
  • leila mazzanti
    How is residual volume related to pneumothorax?
    7 years ago
  • ileana
    Does frc increase with a pneumothorax?
    7 years ago
  • anssi
    Is residual volume increased with pneumorthorax?
    6 years ago

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