Anatomy Relevant to Acute Respiratory Distress Syndrome

by Adam Lawson BA, MSc and Terra Doucette Hiller BA, BSN, RN

Nervous System Control of Breathing

Ventilation control is not completely understood because of its complexity. The brain, muscles, chemoreceptors, mechanoreceptors, and nerve fibers all play a role in ventilation regulation.

Brain Regulation
- Cerebral cortex - allows voluntary ventilation
  - It can temporarily override automatic controls of the medulla and pons during intense emotional stress, chemical stress, or changes in lung inflation.
  - It controls the muscles of ventilation in a coordinated fashion.
- Brainstem - regulates automatic ventilation. Ventilation depends on the rhythmic function signaled by the brainstem and its intact nervous pathways to the respiratory muscles.
  - Medulla oblongata - sends signals through the phrenic nerves which initiate the diaphragm contraction.
    - Dorsal respiratory group - controls the basic rhythm of ventilation; the cells will automatically fire and trigger inhalation.
    - Ventral respiratory group - controls voluntary or increased rates inspiration and expiration.
  - Pons - modulates ventilatory patterns and coordinates transition from inhalation to exhalation.
    - Pneumotaxic center - controls rate and pattern of respiration by limiting inhalation and triggering exhalation.
    - Apneustic center - controls depth of inspiration by working antagonistically with limiting signals of the pneumotaxic center.
Chemoreceptors - regulate ventilation by detecting chemical changes in blood and other fluid and sending signals to the cerebral cortex. There are two types of chemoreceptors:
- Central chemoreceptors - located on the ventrolateral surface of the medulla.
  - Indirectly responsive to changes in PaCO₂.
    
    Central chemoreceptors are directly responsive to increases in hydrogen ion concentrations. Increases in PaCO₂ result in the movement of CO₂ across the blood-brain barrier into the CSF; a higher concentration of CO₂ in the CSF decreases the concentration of hydrogen ions and results in CSF acidity.
    
    Acidic CSF stimulates the respiratory center to increase ventilation and blow off the excess CO₂ in the blood. Alkaline CSF will inhibit the respiratory center.
  Signals from the central chemoreceptors are capable of overriding voluntary control of respiration (e.g., holding breath, slowing respiration) before the CSF becomes too acidic.
- Peripheral chemoreceptors - send signals to the respiratory control centers to immediately regulate breathing and blood pressure.
  - Carotid body - located at the bifurcation of the common carotid artery. They are responsive to changes in oxygen levels, CO₂ levels, pH, and temperature; sends signals via the glossopharyngeal nerve.
  - Aortic body - located in the aortic arch between the brachiocephalic trunk and the left common carotid artery. They are responsive to changes in oxygen and CO₂ levels; sends signals via the vagus nerve.
    
    Rotate the man incrementally to a lateral view and examine the branching of the brachiocephalic trunk and the left common carotid.
  Peripheral chemoreceptors increase ventilation in response to arterial oxygen levels. Their effect on ventilation rates is usually markedly less than the effect of central chemoreceptors.
  
  Patients with COPD often have chronically elevated CO₂ levels. Under such conditions, central chemoreceptors become de-sensitized to PaCO₂ levels, and a patient's ventilation rate becomes primarily controlled by peripheral chemoreceptors.
Spinal cord - processes information and conducts signals from the brain and peripheral receptors to the muscles of ventilation.
- The phrenic nerve, which innervates the diaphragm, is formed from fibers of the C3, C4, and C5 spinal nerves.
  
  A spinal cord injury at level C3 to C5 or above often leads to death or severe difficulty in breathing because it damages or severs the efferent pathway running from the medullary respiratory centers to the diaphragm.
- Intercostal nerves originate between the T1 and T12 vertebrae and innervate the intercostal muscles.
Miscellaneous Lung Receptors
- Irritant receptors - lie in the epithelial cells of the bronchioles. They respond to irritants by inducing bronchoconstriction and rapid, shallow breathing (i.e., tachypnea).
- Stretch receptors - located along the entire airway. Changes in lung volume cause them to inhibit inhalation and prevent lung over inflation by altering respiratory rate and tidal volume.
- J receptors - lie in the alveolar walls close to the capillaries. They respond to engorgement of the pulmonary capillaries and an increase interstitial fluid volume by inducing rapid, shallow breathing.

Lymphatic Circulation

Lymphatic tissue within and around the lungs protects ventilatory pathways to the alveoli by facilitating cell- and antibody- mediated immune response and by clearing the bronchi of excess fluid. The lungs, when compared with other organs, are associated with the greatest quantity of lymphatic tissue.

Afferent lymphatic vessels are first found around and above the level of the terminal and respiratory bronchioles.
Vessels from the bronchioles converge at the bronchopulmonary nodes surrounding the bronchi of the lungs.
Lymph travels upward from the bronchopulmonary nodes to the tracheobronchial nodes or the paratracheal nodes.

Swollen paratracheal lymph nodes immediately above the clavicle are often a good indicator of lung cancer or other lung disease.
Lymph eventually drains into the left thoracic duct and flows back into blood circulation when the duct drains into the left subclavian vein.

Rotate the man incrementally to 180° to examine the thoracic duct as it drains into the subclavian vein. The drainage can also be seen around cross-sections 1305.

Anatomy involved during Intubation

Trachea vs. Esophagus

Several methods can be used to help the airway function of a compromised patient:

Bag Valve Mask
(Click to view animation)

Bag valve mask (a.k.a., BVM, Ambu bag) - a mixture of air and oxygen is forced from a bag into the patient's lungs through a valved mask covering the nose and mouth.

Oropharyngeal Airway
(Click to view animation)

Oropharyngeal airway (a.k.a. oral airway) - helps to prevent the tongue from covering the epiglottis and airway.
- An oral airway should only be used with unconscious patients.
- Typically, the length of the tube will run from the corner of the mouth to the tip of the ear lobe.
- To place the tube, insert it with the J pointing upward toward the rood of the mouth. When the tube is halfway in, twist the tube 180° so the J points down toward the trachea and finish insertion.

Endotracheal Tube
(Click to view animation)

Endotracheal tube (a.k.a., ET tube, ETT) - inserted through the mouth, larynx, and vocal cords into the trachea of an unconscious patient.
- Typically performed by a physician.
- The ET tube may also be placed down a patients nose; this is called nasoendotracheal intubation.
A physician performing ET intubation will often look for the pearly white color of vocal cords to confirm correct placement. Look at the cords in the cross-section below.

Complications can include intubation of the esophagus or the mainstem right bronchus; a tube which runs into the right bronchus will only inflate the right lung. It is important to auscultate for lung sounds in both lungs and to get a chest x-ray post-placement to ensure that the ET tube in the correct location.

Tracheostomy - insertion of a specially-designed tube directly into the trachea through an tracheotomy incision. The tube is typically inserted through:
- A midline incision on the neck between the infrahyoid muscles.
- An incision made on a region of the trachea in between the second and third tracheal rings known as the Björk flap.
Patients who will require ventilation for longer than 7 days, according to the Council on Critical care of the American College of Chest Physicians, will require a tracheostomy. It may also be indicated for patients who are difficult to wean from ventilation due to: severe COPD, acute respiratory distress syndrome (ARDS), severe brain injury, or multiple organ dysfunction syndrome (MODS).

Rotate the man to 0° to see the infrahyoid muscles bilaterally.

Count the tracheal rings to locate the Björk flap. Note this location and increase the skin opacity to see how it relates to the skin surface.
Complications of endotracheal insertion include:
- Hemorrhage - Some individuals have an additional artery travelling midline down the trachea which, if cut, can lead to severe bleeding into the airway.
- Laryngeal nerve injury - accidental severing of the laryngeal nerve during the tracheotomy or tube insertion can result in vocal cord paralysis; this paralysis can cause hoarseness, difficulty speaking, or difficulty swallowing.

References