The Physiology of Respiratory Sensations

Learning Objectives

To describe the multiple physiologic sources of respiratory sensations.
To illustrate links between receptors that are important in ventilatory control and respiratory sensation.
To describe the sources of sensory information arising from the lung that are important to the perception of breathing.
To describe the importance of the relationship between the neural output of the controller and the mechanical response of the ventilatory pump in modulating the intensity of respiratory discomfort.
To demonstrate that dyspnea, or respiratory discomfort, is composed of multiple, qualitatively distinct sensations.
Using asthma as a model, to demonstrate that within a patient suffering from a single pathologic problem, there may be several physiological sources of breathing discomfort.

Breathe in. Breathe out. We do it every day, all day long. For the most part, we are unaware of our breathing, and if we asked you to describe what it feels like to breathe, you would have to think about the question for a few moments before answering. Certainly, when you exercise hard, you notice your breathing and, if you push yourself to your physiological limits (more on this in Chapter 9), you will likely experience breathing discomfort, or dyspnea. If you are among the 5% to 10% of the population with asthma, you undoubtedly have noted abnormal breathing sensations at some time. Are these sensations the same as when you exercise? If not, what are the differing physiological mechanisms responsible for the disparate sensations? The answer to the first question is “no” for the majority of individuals with asthma who describe a sensation of “chest tightness” with their asthma flare-ups. This sensation contrasts with the sensation of “huffing and puffing” or “heavy breathing” most commonly seen with mild to moderate exercise in the absence of respiratory system disease. Studies of respiratory sensations, which are described in this chapter, provide data on the quality of dyspnea in a variety of disease states and physiological conditions and suggest hypotheses regarding the physiological origins of the sensations.

Before you started your study of respiratory physiology, you might have responded to the question, “Why do we get short of breath?” by answering, “Because oxygen levels decrease or carbon dioxide levels increase.” You learned in Chapter 6, however, that the control of ventilation is more complex than a simple response to hypoxemia or hypercapnia. Similarly, respiratory sensations and dyspnea cannot be explained fully by changes in P_aO₂ or P_aCO₂, either. Rather, these sensations and dyspnea reflect the processing of neurological information arising from receptors throughout the respiratory system, along with an apparent comparison of the outgoing motor commands from the controller with the mechanical response of the ventilatory pump.

Acknowledging that we are walking a fine line between physiology and pathophysiology in the discussion of respiratory sensations and dyspnea, we believe that this is an appropriate topic for you to consider at this time because it reinforces many of the concepts you have been learning about the controller and the ventilatory pump. In addition, the topic is rarely addressed in pathophysiology texts, and many clinicians do not have a firm understanding of the physiological mechanisms that underlie dyspnea. Finally, an appreciation of the links between physiology and respiratory sensations will make you a better diagnostician when confronted with patients who complain of shortness of breath.

We will approach the physiology of respiratory sensations by using the following framework. First, we will examine the relationships between ventilatory control and respiratory sensations. Second, we will explore the role of the lungs as sensory organs. Finally, we will address the issue of dissociation between outgoing motor command from the controller and the subsequent response of the ventilatory pump. Much of what we have learned about respiratory sensations in recent years comes from studies of the language of dyspnea. These studies have developed and used dyspnea questionnaires to systematically elicit the words and phrases that patients use to describe their breathing discomfort¹^,². In contrast to pain, which includes sensations that we all experience from time to time as part of our lives, even if we are essentially healthy, most healthy people only experience dyspnea with exercise. Thus, we do not grow up with a readily available vocabulary to describe our breathing discomfort when we develop a cardiopulmonary disease that interferes with normal physiology.

Relative to the other elements of respiratory physiology described in this book, the study of the physiology of breathing sensations is fairly new, the concepts we present here are less well substantiated by experimental data and are, therefore, more controversial, and the physicians with whom you work in the future may be unaware of the principles we outline. For these reasons, we will provide several key references in case you wish to explore this topic in more detail in the future or have the opportunity to share your knowledge in a discussion with your colleagues.