Neural Control

Ventilation is regulated by two separate neural mechanisms: one controls automatic ventilation, and the other controls voluntary ventilation. The medullary respiratory center in the brain stem, which is responsible for the rhythmicity of breathing, controls automatic ventilation. The pneumotaxic center, located in the pons, controls ventilation rate and depth. The cerebral cortex, which sends impulses directly to the motor neurons of ventilatory muscles, mediates voluntary ventilation.³

Chemical Control

Arterial levels of CO₂ (PCO₂), hydrogen ions (H⁺), and O₂ (PO₂) can modify the rate and depth of respiration. To maintain homeostasis in the body, specialized chemoreceptors on the carotid arteries and aortic arch (carotid and aortic bodies, respectively) respond to either a rise in PCO₂ and H⁺ or a fall in PO₂. Stimulation of these chemoreceptors results in transmission of impulses to the respiratory centers to increase or decrease the rate or depth, or both, of respiration. For example, an increase in PCO₂ would increase the ventilation rate to help increase the amount of CO₂ exhaled and ultimately lower the PCO₂ levels in arterial blood. The respiratory center found in the medulla primarily responds to a rise in PCO₂ and H⁺.4,5

Nonchemical Influences

Coughing, bronchoconstriction, and mucus secretion occur in the lungs as protective reflexes to irritants such as smoke or dust. Emotions, stressors, pain, and visceral reflexes from lung tissue and other organ systems also can influence ventilation rate and depth.

Mechanics of Ventilation

Ventilation occurs as a result of changes in the potential space (volume) and subsequent pressures within the thoracic cavity created by the muscles of ventilation. The largest primary muscle of inhalation, the diaphragm, compresses the contents of the abdominal cavity as it contracts and descends, increasing the volume of the thoracic cavity.

The contraction of the intercostal muscles results in two motions simultaneously: bucket and pump handle. The combined motions further increase the volume of the thorax. The overall increase in the volume of the thoracic cavity creates a negative intrathoracic pressure compared with outside the body. As a result, air is pulled into the body and lungs via the pulmonary tree, stretching the lung parenchyma, to equalize the pressures within the thorax with those outside the body.

Accessory muscles of inspiration, noted in Table 4-2, are generally not active during quiet breathing. Although not the primary actions of the individual muscles, their contractions can increase the depth and rate of ventilation during progressive activity by increasing the expansion of the thorax. Increased expansion results in greater negative pressures being generated and subsequent larger volumes of air entering the lungs.

Although inhalation is an active process, exhalation is a generally passive process. The muscles relax, causing a decrease in the thoracic volume while the lungs deflate to their natural resting state. The combined effects of these actions result in an increase of intrathoracic pressure and flow of air out of the lungs. Contraction of the primary and accessory muscles of exhalation, found in Table 4-2, results in an increase in intrathoracic pressure and a faster rate of decrease in thoracic size, which forces air out of the lungs. These motions are outlined schematically in Figure 4-2.^6,7

In persons with primary or secondary chronic pulmonary health conditions, changes in tissue and mechanical properties in the pulmonary system can result in accessory muscle use being observed earlier in activity or may even be present at rest. Determination of the impairment(s) resulting in the observed activity limitation can help a clinician focus a plan of care. In addition, clinicians should consider the reversibility, or the degree to which the impairment can be improved, when determining a patient’s prognosis for improvement with physical therapy. If reversing a patient’s ventilatory impairments is unlikely, facilitation of accessory muscle use can be promoted during functional activities and strengthening of these accessory muscles (e.g., use of a four-wheeled rolling walker with a seat and accompanying arm exercises).

• History of smoking, including packs per day or pack years (packs per day × number of years smoked) and the amount of time that smoking has been discontinued (if applicable)

• Presence, history, and amount of O₂ therapy at rest, with activity and at night

• Exposure to environmental or occupational toxins (e.g., asbestos)

• History of pneumonia, thoracic procedures, or surgery

• History of assisted ventilation or intubation with mechanical ventilation

• History or current reports of dyspnea either at rest or with exertion. Dyspnea is the subjective complaint of difficulty with respiration, also known as shortness of breath. A visual analog scale or ratio scale (Modified Borg scale) can be used to obtain a measurement of dyspnea. The American Thoracic Society Dyspnea Scale can be found in Table 4-3. Note: The abbreviation DOE represents “dyspnea on exertion”

• Level of activity before admittance

• History of baseline sputum production, including color (e.g., yellow, green), consistency (e.g., thick, thin), and amount. Familiar or broad terms can be applied as units of measure for sputum (e.g., quarter-sized, tablespoon, or copious)

• Sleeping position and number of pillows used

• General appearance and level of alertness

• Ease of phonation

• Skin color

• Posture and chest shape

• Ventilatory or breathing pattern

• Presence of digital clubbing

• Presence of supplemental O₂ and other medical equipment (refer to Chapter 18)

• Presence and location of surgical incisions

Observation of Breathing Patterns

Breathing patterns vary among individuals and may be influenced by pain, emotion, body temperature, sleep, body position, activity level, and the presence of pulmonary, cardiac, metabolic, or nervous system disease (Table 4-4). The optimal time, clinically, to examine a patient’s breathing pattern is when he or she is unaware of the inspection because knowledge of the physical examination can influence the patient’s respiratory pattern.

Observation of breathing pattern should include an assessment of rate (12 to 20 breaths per minute is normal), depth, ratio of inspiration to expiration (one to two is normal), sequence of chest wall movement during inspiration and expiration, comfort, presence accessory muscle use, and symmetry.

Auscultation

Auscultation is the process of listening to the sounds of air passing through the tracheobronchial tree and alveolar spaces. The sounds of airflow normally dissipate from proximal to distal airways, making the sounds less audible in the periphery than the central airways. Alterations in airflow and ventilation effort result in distinctive sounds within the thoracic cavity that may indicate pulmonary disease or dysfunction.

Auscultation proceeds in a systematic, side-to-side, and cephalocaudal fashion. Breath sounds on the left and right sides are compared in the anterior, lateral, and posterior segments of the chest wall, as shown in Figure 4-4. The diaphragm (flat side) of the stethoscope should be used for auscultation. The patient should be seated or lying comfortably in a position that allows access to all lung fields. Full inspirations and expirations are performed by the patient through the mouth, as the clinician listens to the entire cycle of respiration before moving the stethoscope to another lung segment.

All of the following ensure accurate auscultation:

Breath sounds may be normal or abnormal (adventitious or added) breath sounds; all breath sounds should be documented according to the location and the phase of respiration (i.e., inspiration, expiration, or both) and in comparison with the opposite lung. Several strategies can be used to reduce the chance of false-positive adventitious breath sound findings, including the following:

Palpation

The third component of the physical examination is palpation of the chest wall, which is performed in a cephalocaudal direction. Figure 4-5 demonstrates hand placement for chest wall palpation of the upper, middle, and lower lung fields. Palpation is performed to examine the following: