Advanced EMT (AEMT)

The upper airway runs from the nose and mouth to the vocal cords. Its jobs are to warm, filter and humidify inhaled air and to protect the lower airway from food, fluid and foreign material. Air enters through the nasal cavity, where turbinates and mucous membranes trap particles, or through the mouth when the nose is blocked or demand is high.

The pharynx (throat) has three parts: the nasopharynx behind the nasal cavity, the oropharynx behind the mouth, and the laryngopharynx (hypopharynx) just above the larynx and esophagus. In an unresponsive supine patient the tongue relaxes and falls against the back of the pharynx; it is the most common cause of airway obstruction in an unresponsive patient, which is why manual maneuvers and airway adjuncts work.

Below the vocal cords the trachea carries air to the carina, where it divides into the right and left mainstem bronchi. The right mainstem bronchus is shorter, wider and more vertical, so aspirated material and deeply placed tubes tend to enter the right lung. Bronchi divide into smaller bronchioles, which have smooth muscle in their walls; bronchoconstriction in asthma and anaphylaxis happens here.

The bronchioles end in clusters of alveoli, the tiny air sacs where gas exchange takes place. Alveoli are lined with surfactant, which lowers surface tension and keeps them from collapsing at the end of exhalation. Each lung is wrapped in pleura: the visceral pleura covers the lung, the parietal pleura lines the chest wall, and a thin film of fluid between them lets the lungs glide. Air or blood entering this potential space causes a pneumothorax or hemothorax.

Ventilation is the movement of air into and out of the lungs. Inhalation is active: the diaphragm contracts and moves down while the external intercostal muscles lift the ribs, enlarging the chest. Pressure inside the chest falls below atmospheric pressure and air flows in. Exhalation is normally passive: the muscles relax and the elastic lungs recoil, pushing air out.

In respiratory distress the patient recruits accessory muscles (sternocleidomastoid and scalene muscles in the neck, abdominal muscles to force exhalation). Visible accessory muscle use, retractions and the tripod position tell you the work of breathing is high and the patient may tire. Positive-pressure ventilation with a BVM reverses normal mechanics: air is pushed in, which raises pressure inside the chest instead of lowering it.

Respiration is the exchange of gases. External respiration happens in the lungs, where oxygen diffuses from the alveoli into the pulmonary capillaries and carbon dioxide diffuses out of the blood into the alveoli. Internal respiration happens at the tissues, where oxygen leaves the blood and carbon dioxide enters it. Cellular respiration is the use of oxygen by the cells to make energy; without oxygen, cells switch to anaerobic metabolism and produce lactic acid.

Gas exchange needs three things: air reaching the alveoli (ventilation), blood reaching the alveoli (perfusion), and a thin membrane between them. A mismatch causes hypoxia: in pneumonia or pulmonary edema the alveoli are filled with fluid (ventilation problem); in pulmonary embolism the alveoli fill with air but the blood flow is blocked (perfusion problem). Most oxygen is carried bound to hemoglobin, which is why pulse oximetry measures hemoglobin saturation.

Breathing is controlled by respiratory centers in the brainstem (medulla and pons). The main stimulus to breathe is the level of carbon dioxide, sensed as a change in pH by chemoreceptors in the brainstem: when CO2 rises, rate and depth increase. Chemoreceptors in the aortic arch and carotid bodies also respond to low oxygen, which serves as a backup stimulus called the hypoxic drive.

Children are not small adults. Their airways are smaller and softer, they use more oxygen per kilogram, and they have less oxygen reserve, so they desaturate quickly and can move from respiratory distress to respiratory failure and arrest fast. Most pediatric cardiac arrests begin as respiratory problems, which is why airway and ventilation come first in a sick child.