What is the amount of air that can be forcefully inhaled after a normal inhalation called?

Lung Volumes

The volume of air in the lung at any given time can be partitioned (Fig. 79-2). The air that remains in the lung after a maximal expiratory effort is the residual volume. The amount of air in the lungs at the relaxation point, when muscle effort is minimized and the inward recoil of the lung is balanced by the outward recoil of the chest wall, is the functional residual capacity (FRC). The difference between FRC and residual volume is the expiratory reserve volume. The volume exhaled in a normal breath is the tidal volume. The volume that can be inhaled above tidal volume is the inspiratory reserve volume.

A series ofcapacities consist of the sum of two or more different volumes. FRC is the sum of expiratory reserve volume plus residual volume. Inspiratory capacity is the sum of tidal volume plus inspiratory reserve volume. Vital capacity is the sum of tidal volume plus inspiratory reserve volume plus expiratory reserve volume. Total lung capacity is the sum of residual volume plus expiratory reserve volume plus tidal volume plus inspiratory reserve volume.

Three of the volumes (tidal volume, inspiratory reserve volume, expiratory reserve volume) can be measured with a spirometer. Measurement of residual volume or any of the capacities that include it, so-called absolute lung volumes, requires more sophisticated methods, such as body plethysmography, the inert gas dilution technique, or the nitrogen washout technique.

Abstract

Spirometers can measure three of four lung volumes, inspiratory reserve volume, tidal volume, expiratory reserve volume, but cannot measure residual volume. Four lung capacities are also defined: inspiratory capacity, vital capacity, functional residual capacity, and the total lung capacity. Pulmonary ventilation is the product of tidal volume and respiratory frequency. The maximum voluntary ventilation is the maximum air that can be moved per minute. Spirometry also provides a measure of airway resistance by use of the forced expiratory volume test. The clinical spirogram presents the forced vital capacity differently. In laminar flow, pressure necessary to drive flow increases linearly with the flow. In turbulent flow, pressure increases with the square of the flow. The Reynolds number is used to estimate whether flow is laminar or turbulent. Airway resistance also increases inversely with lung volume because stretch of the lungs opens airways. Dynamic compression limits flow at high expiratory effort.

Usage.

Diagnosis and monitor the progress of pulmonary dysfunction (asthma, bronchitis, bronchiolitis obliterans, emphysema, and myasthenia gravis); quantify the severity of known lung disease; evaluate the effectiveness of medications (bronchodilators); determination of whether a functional abnormality is obstructive or restrictive; identification of clients at high risk for postoperative pulmonary complications; evaluation of the risk of pulmonary resection; used in conjunction with a cardiopulmonary exercise stress test for evaluation of functional ability; serial measurements used to evaluate response to treatment in cardiopulmonary vascular disease.

VENTILATION

Air movement during both inspiration and expiration requires the creation of a pressure gradient. The initial event in inspiration is contraction of the diaphragm, which causes an increase in the volume of the thoracic space and a decrease in the interpleural pressure (B1 to B2 in Fig. 10-3). The expansion of the lungs causes alveolar pressure to drop below atmospheric pressure (A2), creating a pressure gradient that is diminished (A3) as air flows into the alveoli (C1 to C2). Inspiration (air flow) ends when intra-alveolar pressure equals atmospheric pressure. By the end of inspiration, interpleural pressure is at its most negative, but alveolar pressure has returned to atmospheric pressure because of the increase in lung volume.

The sequence is reversed during expiration as air moves from the alveoli to the atmosphere. Relaxation of the diaphragm causes a decrease in the volume of the thoracic cage, and interpleural pressure becomes less negative. Compression of the lungs causes alveolar pressure to become positive (1 cm H2O) relative to the atmosphere. Again, air moves down the pressure gradient, now exiting the lungs. Expiration ends when intra-alveolar pressure equals atmospheric pressure.

Lung Volumes and Capacities

Lung volumes can be measured using body plethysmography or by inert gas techniques with slightly differing results.74 By term, total lung capacity is slightly reduced,75 whereas tidal volume increases by 45%, with approximately half the change occurring during the first trimester (Table 2.3 andFig. 2.6). The early change in tidal volume is associated with a transient reduction in inspiratory reserve volume. Residual volume tends to decrease slightly, a change that maintains vital capacity. Inspiratory capacity increases by 15% during the third trimester because of increases in tidal volume and inspiratory reserve volume.76,77 There is a corresponding decrease in expiratory reserve volume.76,77 The functional residual capacity (FRC) begins to decrease by the fifth month of pregnancy with uterine enlargement and diaphragm elevation, and is decreased by 400 to 700 mL to 80% of the prepregnancy value at term.76,77 The overall reduction is caused by a 25% reduction in expiratory reserve volume (200 to 300 mL) and a 15% reduction in residual volume (200 to 400 mL). Assumption of the supine position causes the FRC to decrease further to 70% of the prepregnancy value. The supine FRC can be increased by 10% (approximately 188 mL) by placing the patient in a 30-degree head-up position.78

RESPIRATORY ANATOMY AND PHYSIOLOGY

The function of the respiratory system is to supply O2 and remove CO2 from blood in order to maintain a state of homeostasis. The respiratory system consists of a network of many airway branches or generations. The trachea is the first-generation and largest airway opening. The trachea divides into two main branches, the right and left bronchi (second-generation passages), which further subdivide into bronchioles that branch approximately 23 times before terminating in the smallest passageway, the alveoli. Alveoli are minute sacs that make up the lungs and provide the site for gas exchange.

Respiratory airways can be classified as part of the conducting zone or the respiratory zone. The conducting zone is the part of the respiratory system that purifies, humidifies, and transports air to the lower respiratory system. No gas exchange occurs in these regions. The conducting zone originates at the nasal passages, travels through the pharynx and trachea (first-generation passageway), and terminates at the terminal bronchioles (generation 16). The respiratory zone is the zone of gas exchange. Generation 17, or the first generation of the respiratory zone, is known as the respiratory bronchioles. The respiratory zone terminates at the alveoli.

In the alveoli the movement of O2 and CO2 occurs by the process of simple diffusion. O2and CO2 move along pressure gradients, from areas of high pressure to areas of low pressure between the alveoli and capillaries. At the site of gas exchange, O2 is taken up by the capillaries and CO2 is removed from the blood to be excreted during exhalation. The gas exchange process is known as respiration. As O2 is used to create energy, CO2 is given off as a by-product (as demonstrated in the following equation).

As CO2 is taken up by the blood to be excreted by the body, blood pH rises, making the blood more acidic (as demonstrated in the following equation).

Ventilation is a dynamic, time-dependent process involving the mechanical movement of air based on the passive elastic properties of the lungs and the function of accessory muscles of inspiration and exhalation. The diaphragm is the primary muscle of respiration, separating the thoracic and abdominal cavities. In its resting position the diaphragm is dome shaped. Contraction of the diaphragm within the chest cavity during inspiration creates a negative pressure, causing the thorax and lungs to expand and air to flow into the lungs. During exhalation the diaphragm relaxes and air is expelled by the elastic recoil of the lungs, chest wall, and abdomen. During exercise and heavy breathing, forces of elastic recoil are not sufficient to inhale the necessary amount of air. Accessory muscles must be recruited to assist in the processes of inhalation and exhalation to enhance O2delivery and CO2 removal. The muscles of inspiration, external intercostals, sternocleidomastoid, serratus anterior, and scalenes assist in lung expansion by contracting and raising the rib cage. The muscles of expiration—rectus abdominis, internal obliques, external obliques, transverse abdominis, and internal intercostals—depress the rib cage and assist with exhalation.

Lower brain centers, specifically the medulla oblongata and the pons, assist in breath initiation and regulate the volume of each breath. Therefore the nervous system is responsible for controlling the rate and depth of ventilation to meet the demand of the body maintaining relatively constant concentrations of O2 and CO2. If the respiratory rate is too slow, O2 delivery is inadequate to meet the metabolic requirements of the body. This breathing state, referred to as hypoventilation, is characterized by slow, shallow breathing leading to increased levels of CO2 in the blood. Conversely, increased depth and rate of breathing is referred to as hyperventilation. Hyperventilation results in abnormally low levels of CO2in the blood, disrupting blood homeostasis. As a result, blood pressure (BP) significantly drops and individuals may experience symptoms of dizziness, tingling, and possible fainting spells.

Lungs differ in both size and capacity, significantly contributing to the overall functional capacity of the respiratory system. Normative values of static, anatomical measurements of the respiratory system have been recorded in healthy adults (see the following box). Functional measurements have also been determined for dynamic components of respiration. These values are important determinants of aerobic capacity determining the efficiency of the cardiorespiratory system. Pulmonary minute volume (VE) is the amount of air moved in 1 minute. Minute alveolar ventilation (VA) is the amount of air capable of participating in gas exchange or the volume of air breathed each minute. During exercise, VAincreases with increases in metabolic rate and CO2 production. VE increases with the onset of exercise to meet the demands of VA to remove excess CO2. When exercise intensity reaches a particular level, blood flow to the exercising muscles becomes inadequate to provide the necessary O2.This is termed the anaerobic threshold and is the point at which anaerobic pathways become the primary source of energy production.

Evidence-Based Clinical Application: Lung Volumes

Lung Volumes: Anatomic Determinants

The nomenclature of the lung volumes was originally based on four independent volumes: residual volume (RV), expiratory reserves volume (ERV), tidal volume (VT), and inspiratory reserve volume (IRV). A fifth, overlapping lung volume closing volume (CV) has been added. Two or more volumes may be added to obtain a capacity.

Functional residual capacity (FRC) = RV + ERV

Inspiratory capacity (IC) = VT + IRV

Vital capacity (VC) = ERV + VT + IRV

Total lung capacity (TLC) = RV + ERV + VT + IRV

Closing capacity (CC) = CV + RV

When the lungs and viscera are removed from the body and all muscles are relaxed or paralyzed, the volume of the thoracic cage is several hundred milliliters larger than the FRC. Similarly, when the lungs are removed from the thoracic cavity and opened to the atmosphere, they decrease their volume by up to several hundred milliliters.

The resting position, the FRC, is set by equating the forces of the parenchyma to further collapse, with the force of the thorax tending to reexpand. At this point, the expiratory muscles are stretched slightly beyond their rest length and can contract to decrease the gas volume in the chest from the FRC to the RV, but the limit of this contraction differs somewhat in children and youths from older adults (see later) (Fig. 2.4).

The VC is determined by the maximal excursion of the thoracic girdle, rib cage, spine, and diaphragm. From the FRC, the IC is limited by muscle shortening and rib excursion, not by lung compliance. The ERV and the RV are limited differently at different ages, however. In adults, the RV represents the volume of gas in the lung when all small airways have closed because of loss of tethering effect (see later). In children, the RV of the excised lung is somewhat smaller than the pleural cavity volume during maximum expiratory effort. Consequently, the pleural pressure is always negative. With increasing age, the increase of the closing capacity of lung tissue makes it higher than the minimal volume of the bony thorax at maximum expiration. Now expiratory effort produces a positive pleural pressure.5,6

Publisher Summary

This chapter describes different aspects of lung volumes and airway resistance. Spirometry can be used to measure three lung volumes and two capacities, which include the tidal volume (TV), inspiratory reserve volume, and expiratory reserve volume. The residual volume cannot be measured by spirometry. Combinations of these four volumes define the lung capacities. Pulmonary ventilation is the product of the TV and respiratory rate. The maximum voluntary ventilation can also be measured using a spirometer. It usually exceeds the maximum ventilation during exercise. The presence of turbulence in the airways depends on the velocity of airflow, the diameter of the airways, and the density and viscosity of the air. The velocity of the air and diameter of the airways vary considerably, whereas density and viscosity of the air are nearly constant. The diameter of the airways decreases nearly exponentially with generation number, whereas the total cross-sectional area of the airways increases because the number of airways increases with generation number. It is found that airway resistance is also modified by smooth muscle contraction of the muscles surrounding the bronchioles.

Total Lung Capacity

The total lung capacity (TLC) is the maximal volume of gas in the lungs after a maximal inhalation; thus it is the sum of the RV, ERV, VT, and IRV. TLC is approximately 6 L for a healthy 70-kg adult. The vital capacity (VC) is the maximal volume of gas exhaled during a forced exhalation after a forced inhalation. Thus VC is the sum of the VT, IRV, and ERV. The VC is approximately 4.5 L in a healthy 70-kg adult.5

Fig. 5.3 shows a summary of how the standard lung volumes and capacities relate, along with average values for a 70-kg adult.

Pulmonary Function

Pulmonary function tests give clinicians information about the mechanical function of the lungs. Lung volumes that can be tested and analyzed include total lung capacity (TLC), VC, residual volume, inspiratory capacity, functional residual capacity, inspiratory reserve volume, and expiratory reserve volume (Figure 4-12). These volumes estimate unassisted inspiratory and expiratory muscle function. TLC is the volume of air in the lungs at the end of maximal inspiration (Box 4-1). VC represents the patient's maximum breathing ability and is commonly monitored, especially for patients with high cervical injuries to help determine their potential to be weaned from ventilatory support.

Maximal insufflation capacity (MIC) is another parameter used by clinicians working with patients with SCI. MIC is the maximum volume of air that a patient can hold with a closed glottis, and the difference between the MIC and VC strongly correlates with glottic function. This will be important to glossopharyngeal breathing (i.e., air stacking).

Normal values for pulmonary function including values for lung volumes, ventilation, mechanical breathing, gas exchange, alveolar gas, and arterial blood are listed in Table 4-2. Patients with SCI may have lower values depending on the level of injury. Pulmonary functions do not have a single normal value because these are based on an interaction between body surface area, age, height, weight, sex, and race. The normal values listed in Table 4-2 provide a frame of reference based on a young male with a body surface area of 1.7m2. There are no universally accepted criteria for determining abnormalities.9

Although a subset of standard pulmonary function tests are typically performed on patients with SCI, it should be remembered that the lungs of these patients are normal unless an overriding disease process such as chronic obstructive pulmonary disease (COPD) or tumor exist. Therefore, the primary limitation to lung function in SCI is chest wall muscle paralysis.