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The Chemoreflex Control of Breathing and Its Measurement

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933 Continuing Medical Education The chemoreflex control of breathing James Duffin PhD and measurement The chemoreflex control of breathing is described in terms of a Le contrrle chimiorEflexe de la respiration est dEcrit comme un graphical model. The central chemoreflex, the ventilatory modEle graphique. Le centre chimiordflexe, #1 rEponse respira- response to carbon dioxide mediated by the central chemorecep- toire au C02 mEdiEe par les chEmorEcepteurs centrmLr sont tors, is modelled as a straight-line relation between the dEcrits comme une relation directe entre la rEponse respira- ventilatory response and the arterial level of carbon dioxide. toire et les niveaux de C02 artdriels. Le chimior~flexe ptariph~- The peripheral chemoreflex, the ventilatory response to carbon rique, la r~ponse ventilatoire au C02 et I'hypoxie mddi~s par les dioxide and hypoxia mediated by the peripheral chemorecep- chEmorEcepteurs p~riph~riques sont subdivis~es en deux rela- tors, is broken into two relations. First, a straight.line relation tions. PremiErement, une relation directe entre la rdponse between the ventilatory response and the arterial level of carbon ventilatoire et le niveau artEriel de COz dont la pente (sensiti- dioxide whose slope (sensitivity) increases as the oxygen level vitE) augmente a vec les variations du niveau d' oxygEne d' hyper- varies from hyperoxic to hypoxic. Second, a rectangular oxique c} hypoxique. Deuxidmement, une relation hyperbolique hyperbolic relation between the ventilatory response and the rectangulaire entre la rdponse respiratoire et le niveau artEriel arterial level of oxygen with ventilation increasing with increas- d' oxygOne avecla ventilation augmentant avec I' augmentation ing hypoxia. The three ventilatory response relations (one de l'hypoxie. Les trois relations de la rdponse respiratoire central and two peripheral) add to produce the total chemoreflex (une centrale et deux pdriphEriques) s' ajoutent pour produire la ventilatory response which forms the feedback part of the rEponse respiratoire chEmordflexe totale qui forme la portion de respiratory regulator. The forward part consists of the relation ~,feedback ,, du r~gulateur respiratoire. L'autre partie consiste between the arterial level of carbon dioxide and ventilation dune relation entre le niveau artdriel de C02 et la ventilation when ventilation is controlled (the metabolic hyperbola). The lorsque cette dernidre est contrrlEe (l'hyperbole mdtabolique ). forward and feedback parts of the respiratory regulator can be Ces deux parties du rEgulateur respiratoire peuvent Etre combined so as to predict resting ventilation and carbon diaxide combinEes afin de pr~dire la ventilation au repos et le niveau de levels under a number of circumstances. Methods of measure. C02 dans d'autres circonstances. Des mEthodes de mesure et ment of these chemoreflex ventilatory responses are also des rdponses de ces chdmor~cepteurs respiratoires sont aussi described so as to illustrate the physiological principles in- ddcrites afin d'illustrer les principes physiologiques impliquEs volved in the model. dans ce moddle. This article presents a view of the control of breathing by the central and peripheral chemoreflexes as a graphical Key words model of this feedback regulator, and also presents REFLEXES: chemoreceptor; methods by which the regulator performance may be VENTILATION: carbon dioxide response, control, hypoxic assessed. It is not a review of the literature relating to response. either the underlying physiology or models of it, but is intended as a practical approach to understanding. For the Departments of Anaesthesia and Physiology, University of background research involved the reader is referred to the Toronto. review by Cunningham et al. Address correspondence to: Dr. J. Duffin,Department of In the awake patient, breathing is subject to conflicting Physiology, University of Toronto, Toronto, Ontario, Canada, demands and conditions. These may include diverse M5S I A8. influences such as anxiety hyperventilation, posturally CAN J ANAESTH 1990 ! 37: S / pp933-42 934 CANADIAN JOURNAL OF ANAESTHESIA FORWARD LOOP 24 Pco2 Pulmonary and [H +1 Cardiovascular [HCO3"] Systems where: is the Hydrogen ion concentration [H +] in nanomoles per litre Peripheral 1~ and Central Pco 2 is the partial pressure of Carbon Chemorcnexes Dioxide In mm Hg FEEDBACK LOOP is the Bicarbonate ion eoHeentration [HCO3-] in millimoles per litre FIGURE I A block diagram of the control of breathing by the chemoreflexes. FIGURE 2 The linear form of Ihe Henderson-Hasselbalch equation. Normal resting values are: hydrogen ion concentration = 40 nmol. L -j, partial pressure of carbon dioxide = 40 mmHg, and induced changes in pulmonary mechanics, and metabolic bicarbonate ion concentration = 24 mmol. L -j. requirements. While the control of breathing in the anaesthetized patient is somewhat simplified by the elimination or suppression of many of these infuences, arterial blood, but the partial pressure of carbon dioxide the physiology of breathing will be further simplified in does. In addition, the bicarbonate ion concentration this article by assuming that breathing is under the sole within the tissues of the central chemoreceptors may alter control of the respiration chemoreflexes. over time and differ from that of arterial blood; usually in The term chemoreflexes refers to the effects of the such a way as to restore central nervous system hydrogen central and peripheral chemoreceptors on pulmonary ion concentration to normal values. 4 For these reasons, ventilation. The chemoreflexes form the feedback portion the hydrogen ion concentration sensed by the central of a control loop. The forward portion, which completes chemoreceptors is likely to be more closely related to the the control loop, refers to the effects that changing arterial partial pressure of carbon dioxide than to the pulmonary ventilation has upon the stimuli sensed by the arterial hydrogen ion concentration in the short term chemoreceptors and is usually termed the metabolic (minutes), and furthermore that relationship may be hyperbola because of its shape. The complete control loop altered by changes in central nervous system bicarbonate is the classical, negative feedback regulator which main- ion concentration (hours). tains respiratory homeostasis, pictured in Figure I. The second consequences of the location of the central chemoreceptors concerns the time course for changes in The central chemoreflex the partial pressure of carbon dioxide at the central The central chemoreceptors respond only to the hydrogen chemoreceptors, relative to such changes in arterial ion concentration of their environment, and therefore this blood. chemoreflex might be expected to be relatively simple to Because the central chemoreceptors are located in brain understand. However, the complexities associaled with tissue which is supplied with blood at a flow of about 0.01 the central chemoreflex are due to the location of the ml. sec- ~for every ml of tissue, the changes in the partial central chemoreceptors, somewhere within the brain pressure of carbon dioxide at the central chemoreceptors tissue in the medulla. 2 This location has three conse- lag behind those in arterial blood. In experiments where quences for the operation of the central chemoreflex. the arterial level of the partial pressure of carbon dioxide The first consequence concerns the relation between is abruptly increased (a step change), the partial pressure hydrogen ion concentration and the partial pressure of of carbon dioxide at the central chemoreceptors gradually carbon dioxide. This relation can be expressed as the increases to a new level following the time course of a linear form of the Henderson-Hasselbalch equation wash-in exponential function. 5 The time constant of this shown in Figure 2 (but also see an alternative approach to exponential function can be estimated as the reciprocal of acid-base chemistry using fundamental physical and the blood perfusion per volume of chemoreceptor tissue, chemical principles). 3 Because of the blood-brain barrier i.e., 100 sec. Since a wash-in exponential process is for polar solutes, hydrogen ion concentration does not almost complete in about three time constants, it therefore easily equilibrate between the central chemoreceptors and takes five minutes for the partial pressure of carbon Duffin: CHEMOREFLEX CONTROL 935 rVENTILATION litres/minute Vcos 50 Paco~ - PIco 2 VA 4O where: 30 is the arterial partial pressure of Paco= Carbon Dioxide in turn Hg 20 is the inspired partial pressure of P[co~ Carbon Dioxide in mm Hg 10 V is the rate of Carbon Dioxide 0 CO s output in litres per minute 30 40 610 is the alveolar ventilation in Pco~ mm Hg VA litres per minute FIGURE 4 The metabolic hyperbola, the relation between ventilation FIGURE 3 The alveolar ventilation response to arterial carbon and carbon dioxide when ventilation is controlled. dioxide mediated by the central chemoreceptors. dioxide at the central chemoreceptors to reach its new The relationship between the partial pressure of carbon level. dioxide in arterial blood and alveolar ventilation is that It should be understood that the partial pressure of shown in Figure 3, when only the central chemoreflex is carbon dioxide at the central chemoreceptors is not the operating. This straight-line relationship with a threshold same as that of arterial blood, even when equilibration is of approximately 5.3 kPa (40 mmHg) partial pressure of complete, but is close to
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