Brady is pleased to share excerpts from Transition Series: Topics for the EMT by Joseph Mistovich and Daniel Limmer, which provide both an overview of new information contained within the Education Standards at the EMT level and a source of continuing education for practicing EMTs. Intended for a new generation of EMTs, the text integrates new “topics” that were not contained in the U.S. DOT 1994 EMT-Basic National Standard Curriculum and existing “topics” at a much greater depth and breadth than what was contained in the typical EMT-Basic education program. Visit www.bradybooks.com for more information.
The pathophysiology included in this topic is among the most formative and challenging changes to the new education standards. Previously, EMTs had to know only about the air moving in and out of the body—and, quite frankly, in very simple terms and concepts. It was the belief, in creating the standards that include this information, that EMTs would understand the internal processes of perfusion and gas transport that would make assessment and care performed more intuitive and effective.
Regulation of Ventilation
Although breathing can be altered voluntarily, it is primarily controlled involuntarily by the autonomic nervous system. A large part of the regulation is related to maintaining normal gas exchange and normal blood gas levels. Receptors within the body constantly measure the amount of oxygen (O2), carbon dioxide (CO2), and hydrogen ions (pH) and signal the brain to adjust the rate and depth of respiration (Figure 1). Centers responsible for ventilatory control are the chemoreceptors, lung receptors, and specialized centers in the brainstem.
Chemoreceptors are specialized receptors that monitor the number of hydrogen ions (pH) and the carbon dioxide and oxygen levels in the arterial blood. There are two different types of chemoreceptors: central and peripheral.
The central chemoreceptors are located near the respiratory center in the medulla. These receptors are most sensitive to changes in the amount of carbon dioxide in arterial blood and the pH of cerebrospinal fluid (CSF). The pH of CSF is directly related to the amount of carbon dioxide in the arterial blood. Carbon dioxide readily crosses the blood–brain barrier and moves into the CSF. In the CSF, the CO2 combines with water (H2O) to form carbonic acid (H2CO3). Thus there is an association between CO2 and the level of acid in the body as follows:
- An increase in the amount of CO2 in the blood will increase the amount of acid in the blood.
- A decrease in the amount of CO2 in the blood will decrease the amount of acid in the blood.
The central chemoreceptors are highly sensitive to the amount of hydrogen in the CSF. After the CO2 and H2O molecules combine to form H2CO3, the hydrogen ions (H+) disassociate from the H2CO3, enter the CSF, and stimulate the central chemoreceptors. Small changes in the H+ level in the CSF will stimulate a change in respirations. Because CO2 is needed to produce H2CO3, the changes in the breathing rate and depth are geared toward increasing or decreasing the CO2 level in the arterial blood. The response of ventilation can be summarized as follows:
- An increase in arterial CO2 will increase the number of hydrogen ions in the CSF, stimulating an increase in the rate and depth of respiration to blow off more CO2.
- A decrease in arterial CO2 will decrease the number of hydrogen ions in the CSF, causing a decrease in the rate and depth of respiration to blow off less CO2.
The peripheral chemoreceptors are located in the aortic arch and carotid artery bodies in the neck. These chemoreceptors are also sensitive to CO2 and pH; however, the arterial oxygen level is the strongest stimulus. Thus, a change in the arterial oxygen level is what stimulates the brain to increase or decrease ventilation. It takes a significant decrease in the arterial oxygen content to trigger the peripheral chemoreceptors to stimulate the respiratory center to increase rate and depth of respiration. The activity of the peripheral chemoreceptors can be summarized as follows: