The measurement of respiratory rate is a vital sign. Nurses need to understand the anatomy and physiology of normal breathing in order to measure respiratory rate and interpret the results.
Introduction
To understand the respiratory process, it is important to be familiar with the anatomy of the thoracic cavity and the physiology of the respiratory system. Respiration has two basic components:
Ventilation: the process of physically moving air in and out of the lungs;
Gas exchange: the process of getting oxygen (O2) into the body and getting rid of carbon dioxide (CO2).
Anatomy and physiology
The lungs are located within the thoracic cavity surrounded by two layers of pleura. The diaphragm is located at the base of the thoracic cavity and separates it from the abdominal cavity. It is the main muscle for inspiration and is innervated by the phrenic nerve.
The lungs consist of large and small airways - the trachea is the largest and the first of 23 levels of airways, each arising from an irregular dichotomous system from the previous airway (Davies and Moore, 2010). The smaller airways (respiratory fine bronchi) contain alveoli in their walls. The alveoli are the site of gas exchange and their presence increases as the airway becomes smaller. This allows the total surface area of the lungs to increase exponentially, thus permitting the greatest opportunity for gas exchange.
Central and peripheral chemoreceptors that are sensitive to hypoxia (low O2 levels) and hypercapnia (increased CO2) control the drive to breathe (Davies and Moore, 2010).
Ventilation
Air moves naturally from areas of high pressure to areas of low pressure. During normal respiration, inspiration is generated by contraction and flattening of the diaphragm and contraction of the external intercostal muscles, which causes a rising and outward movement of the thoracic cavity. This increases the size of the thoracic cavity. These changes cause the pleural layer of the lung wall to move along with the thoracic cavity and diaphragm, thus creating negative pressure. The dirty pleural layer attached to the surface of the lung follows, and the lung expands, drawing in air.
Exhalation at rest is primarily a passive process; the inspiratory muscles stretch and the lungs elastically retract, causing a state of pressure equilibrium before the cycle begins again (Bourke and Burns, 2015). This movement of the chest wall is observed when respiratory rate (RR) is measured.Changes in RR occur during exercise, mood, and sleep; RR changes associated with exercise and anxiety may be greater than 25 beats per minute, but usually return to normal in a rested, calm state.
Gas exchange
The ventilation process transports air to the alveoli where gas exchange occurs through a simple diffusion process. Gases will move from areas of high concentration to areas of low concentration. The partial pressure of O2 in the atmosphere is higher than the partial pressure in the body, and the partial pressure of CO2 in the blood is higher than the atmosphere. For efficient gas exchange to occur, air breathed into the lungs must flow to the alveolar membrane, where the capillary walls are thin and the overall surface area is large.
What is baseline RR?
When ventilation and gas exchange occur, the normal range for oxygen saturation (SpO2) is 94-98% (O'Driscoll et al., 2017) and can be maintained at a RR of 12-20 breaths per minute at rest.
Figure 2 shows the oxyhemoglobin dissociation curve. This illustrates how physiological factors can lead to changes in RR due to changes in SpO2. For example, if available atmospheric O2 (PO2) decreases at altitude, SpO2 will decrease, thus triggering an increase in RR. In diseases where changes in temperature or blood pH shift the oxyhemoglobin dissociation curve to the right or left, the RR will be affected as the body attempts to restore homeostasis in the body.
Effect of poor health on baseline RR
It is important to question whether RR as part of the National Early Warning System (NEWS) (Royal College of Physicians, 2017) is more useful for patients without known respiratory disease, where a score of 0 (12-20 breaths/min) is the true baseline.
In lung disease with impaired gas exchange and/or ventilation at rest, hypoxia and hypercapnia drive increased RR to maintain SpO2. poor gas exchange, such as pulmonary fibrosis or emphysema (caused by alveolar wall thickening and lung tissue destruction, respectively), results in higher resting RR. therefore, it is important to consider the patient's "normal " baseline.
Common obstructive lung disease, such as COPD or asthma, is characterized by increased airflow resistance as the small airways narrow, thereby reducing oxygen delivery to the alveoli. During acute exacerbations, this resistance increases, resulting in an elevated RR. The use of bronchodilators relaxes the smooth muscle in the airway wall, thereby reducing resistance and restoring the RR to normal levels.
Neuromuscular disease affecting the lungs usually leads to hypoventilation because the mechanisms required for normal ventilation are not working properly. In such cases, a low RR (respiratory delay) can lead to respiratory failure.
RR must be closely monitored during surgery and postoperative recovery because anesthetics, which usually contain opioids, depress breathing and reduce RR (Koo and Eikermann, 2011). They act on the central chemoreceptors that inhibit respiratory drive.
It is important to remember that pulse oximetry measures oxygen saturation, while RR measures ventilation. In the early stages of deterioration, a patient's SpO2 may appear to be in the normal range, but the RR will increase due to inadequate gas exchange. changes in RR are usually the first sign of deterioration.
Future
RR is an early sign of patient deterioration and early detection of changes ensures that patients receive meaningful clinical intervention. For the RR to serve as an early warning sign in a patient with known respiratory disease, we need to know what is normal for that patient.
As will be discussed later in this series, there are techniques to objectively measure a patient's resting RR, and we need to consider whether these techniques should be used routinely in practice in the same way as measuring SpO2 or blood pressure.
