Back To The Basics: Ventilation V. Respiration

You were taught from day one of taking vital signs to count respirations.  However, I am here to say that your stellar EMT instructor let you down because she/he was wrong.  The same incorrect terminology is also written in numerous textbooks, on run reports, and spoken everyday between medical professionals.  So now you’re asking: Ok, smartypants, then what is the proper term? The appropriate term, my fellow professionals, is, you count – ventilations.

Aren’t these terms essentially the same?  The easy answer is no.  The harder question is: Well, why not?  This installment of Back to the Basics discusses these physiological processes and how they differ.  So please, read on.

Ventilation

Simply put, ventilation is breathing – the physical movement of air between the outside environment and the lungs.  Air travels through the mouth and nasal passages, then down the pharynx.  Upon reaching the vocal cords, air flows into the trachea, transitioning from the upper airway into the lower airway.  Here, it continues distally to the carina, then through the primary bronchi, various branches of bronchioles, and eventually arriving in the alveoli.  This is inhalation.  Air movement in a reverse pathway from alveoli to mouth and nose, is exhalation.  Inhalation, followed by exhalation, equals one ventilation.  This is what you observe (chest rise and fall) when determining the breathing rate.

A ventilation can only take place if the brainstem, cranial, and associated peripheral nerves, the diaphragm, intercostal musculature, and lungs are all functional.  Combining the function of all these structures, the pulmonary ventilation mechanism establishes two gas pressure gradients.  One, in which the pressure within the alveoli is lower than atmospheric pressure – this produces inhalation.  The other, in which the pressure in the alveoli is higher than atmospheric pressure – this produces exhalation.  These necessary changes in intrapulmonary pressure occur because of changes in lung volume.

So, how does the lung volume change?  Quite simply, it is a combination of muscle contractions stimulated by the central nervous system, and the movement of a serous membrane within the thorax called the pleura.  The pleura is made of two layers:  a parietal layer that lines the inside of the thorax and a visceral layer that covers the lungs and adjoining structures (blood vessels, bronchi, and nerves). Between the visceral and parietal layers is a small, fluid-filled space, called the pleural cavity.

The initiation of ventilation begins with the brainstem, where impulses (action potentials) generate within the medulla oblongata, then travel distally within the spinal cord.  The impulse traverses individually through cervical nerves three, four, and five until just above the clavicle.  Here, the three cervical nerves merge into one large nerve called the phrenic nerve, which attaches distally to the diaphragm.  Imagine these two nerves resembling a pair of suspenders on the anterior chest.  The delivered impulse from the phrenic nerve initiates diaphragm contraction.

The intercostal muscles are a group of intrinsic chest wall muscles occupying the intercostal spaces. They are arranged separately in three distinct layers (external intercostal muscles, internal intercostal muscles, and innermost intercostal muscles).  The intercostal nerves that stimulate these muscles originate from the spinal cord thoracic nerves 1-11.

Inhalation is initiated as the dome-shaped diaphragm is stimulated. As it contracts and flattens, the thorax expands inferiorly.  The internal and innermost intercostal muscles relax, while the external intercostal muscles contract from stimulus by the thoracic nerves. This produces an upward and outward movement of the ribs (similar to the movement of a bucket handle), and the sternum (similar to when pulling upward on a handle of a water pump).  The fluid in the pleural cavity acts like glue, adhering the thorax to the lungs.  Hence, as the thorax expands vertically and laterally, the parietal layer drags the visceral layer along with it, causing the lungs to expand. Adequate expansion of the lungs results in a decreased pressure within the alveoli.  Therefore, when the alveolar pressure drops below atmospheric pressure, air rushes into the lungs.

Remember, inhalation requires a stimulus initiated from the central nervous system.  Think of it like turning on a light.  The light stays unlit until you flip a switch (CNS), releasing electricity and stimulating the components of the light bulb.  As long as the switch is on and there is an impulse, the light stays lit.  However, if you turn off the switch, the stimulus ceases, and the light shuts down.  Exhalation is akin to turning off the switch, so to speak.

Thoracic stretch receptors constantly monitor chest expansion.  Once an acceptable limit of expansion develops, they send a message to the central nervous system to “turn off the switch.”  All the nerves stimulating diaphragmatic and external intercostal muscle contraction temporarily stop conducting.  Consequently, the diaphragm and the external intercostal muscles relax, decreasing the thoracic volume – like letting air out of a balloon.  Assisting with this passive process, the internal and innermost intercostal muscles are stimulated.  Their contraction pulls the ribcage and attached pleura further downward and inward, compressing the lungs and increasing the air pressure within the alveoli.  Once the alveolar pressure exceeds the atmospheric pressure, air moves out of the lungs.

That is all there is to it – simple, right?  Adults normally ventilate between 12 to 20 times per minute, thanks to the autonomic nervous system.  We do not even have to think about it!  Nonetheless, what becomes a problem (and why EMS gets a call) is when the nervous system, the thoracic musculature, or the lungs become diseased or disabled.  Here is a partial list of pathologies that impair ventilation:

• Nervous system:  brain stem injury/traumatic brain injury, cervical spinal injury, Myasthenia Gravis, ALS (Lou Gehrig’s disease), Guillain-Barré syndrome
• Thorax:  blunt chest trauma, rib fracture/flail chest, diaphragmatic tear/hernia, penetrating chest trauma/pneumothorax, hemothorax, pleural effusion, compression of the chest
• Lungs: emphysema, chronic bronchitis, asthma, foreign body airway obstruction, cystic fibrosis, lung cancer/tumor