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Capnography: Clinical Aspects.

Roy, Tuhin K. M.D., Ph.D.
Section Editor(s): Warner, David O. M.D., Editor

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Capnography: Clinical Aspects. Edited by J. S. Gravenstein, M.D., Dr. med. h.c., Michael B. Jaffe, Ph.D., and David A. Paulus, M.D. Cambridge, United Kingdom, Cambridge University Press, 2004. Pages: 441. Price: $120.00.
Practicing anesthesiologists and intensivists have come to take capnography for granted in the monitoring of surgical and critically ill patients. Although many standard anesthesiology texts contain a chapter about this important and useful technique, a comprehensive up-to-date treatment of the subject is not easy to find. Capnography: Clinical Aspects fills this void.
The book is a multiauthored effort edited by two academicians and an engineer working in industry. The editors acknowledge significant overlap between chapters and characterize the book as more of a “symposium” than a textbook. There is adequate continuity of style between chapters, but as with any book written in this format, some chapters are more interesting to read than others.
The book is organized into four parts. The first part is meant to be clinical and describes the interaction of respiratory, cardiovascular, and metabolic systems in determining the amount of exhaled carbon dioxide as measured by capnography. This is followed by parts on basic carbon dioxide physiology, the history of capnography, and the technology of capnography.
The clinical part is divided into four sections: Ventilation, Circulation, Metabolism, and Organ Effects. The ventilation section is further divided into subsections on breathing assessment, airway management, monitoring of ventilation, weaning, and special situations. The first chapter (written by two of the editors) is a well-written introduction to time-based capnogram interpretation, the most commonly used form of capnography in the operating room setting. Of particular value is the introduction to the volume-based capnogram, a topic not commonly detailed in anesthesia texts. Subsequent chapters discuss capnography outside the operating room and in the prehospital setting for airway management, in particular to confirm tracheal intubation. The chapter on airway management in the intensive care unit includes a section on using capnography to confirm proper orogastric and nasogastric tube placement. The chapter on airway management in the operating room includes sections on confirming tracheal intubation and recognizing endobronchial tube placement.
The chapter describing the use of capnography to monitor ventilation during anesthesia includes interesting comments on the Food and Drug Administration checkout relevant to capnography. This chapter also includes sections on equipment troubleshooting and how capnograms can be affected by positioning, pulmonary pathology, and several particular situations such as one-lung ventilation, laparoscopy, neurosurgery, cardiac surgery, tourniquet release, and high-frequency jet ventilation. Other chapters in this section focus on the use of capnography during transport and how it can be used in the field as a way to avoid deleterious effects of unintentional hyperventilation after intubation.
A particularly comprehensive chapter describes the unique physiology and technological limitations of capnography in neonates and infants. Other chapters describe capnography in the sleep laboratory, capnography as a feedback tool for behavioral therapy in various disorders, and how the capnogram is affected by alterations in physiologic and technical limitations in high- and low-pressure environments.
Chapters are also included on sedation and noninvasive ventilation. These chapters are valuable for their descriptions of how end-tidal carbon dioxide can be sampled during spontaneous ventilation in nonintubated patients and the clinical utility and limitations of end-tidal carbon dioxide as a method of estimating arterial carbon dioxide tension (PCO2) in noninvasive ventilation.
Chapters relevant to critical care describe the use capnography to optimize tidal volume, alveolar minute ventilation, and positive end-expiratory pressure to wean patients from mechanical ventilation. These chapters also describe the use of volumetric capnography to assess carbon dioxide production and how the capnogram is affected by positive end-expiratory pressure, unilateral lung injury, tracheal gas insufflation, and various high-frequency ventilation modes.
The circulation subsection includes chapters on how end-tidal carbon dioxide monitoring can be used to assess circulatory status during cardiopulmonary resuscitation and for prognostication during cardiac arrest in medical patients as well as the use of end-tidal and tissue carbon dioxide monitoring techniques to assess oxygen delivery in shock states. This section includes an elegant physiologic description of changes in alveolar dead space with pulmonary embolism and the use of capnography in diagnosis and treatment of pulmonary emboli and gas embolization in addition to a chapter on the utility of volumetric capnography for estimating arterial PCO2 in patients with acute respiratory distress syndrome.
The chapter on noninvasive pulmonary blood flow measurement describes complete and partial carbon dioxide rebreathing techniques as alternatives to invasive cardiac output monitoring. A variety of clinical scenarios illustrating the use of these techniques sets this chapter apart from other descriptions of this topic.
The metabolism subsection includes a single chapter describing alterations in normal physiology induced by surgery and anesthesia that affect carbon dioxide elimination. The chapter discusses alterations in ventilation, circulation, and carbon dioxide metabolism that are influenced by temperature alterations, various anesthetic techniques, and pharmacologic agents as well as particular intraoperative situations such as laparoscopy, tourniquet release, vascular cross clamping, and cardiopulmonary bypass.
The final chapter of the ventilation section describes the effects of hypercapnia and hypocapnia on tissue oxygenation and perfusion, focusing on the central nervous system, respiratory system, and cardiovascular system. This is an excellent introduction to the effect of carbon dioxide at the organ, tissue, and cellular/molecular level and could have been included in the section on physiology.
The physiology section includes a chapter on carbon dioxide pathophysiology, which describes inherited and acquired mitochondrial and enzyme disorders as well as pharmacologic agents that alter carbon dioxide production. The chapter also discusses carbon dioxide embolism and the increase in PCO2 during apnea testing for brain death. There is a complete if somewhat standard chapter on acid–base physiology, followed by an excellent description of how capnography can provide information on ventilation/perfusion mismatch from a physiologic standpoint, including examples of various disease states. Subsequent chapters describe clinical correlates of alterations in normal time and volume capnographic tracings and how capnograms can provide clues to the underlying pathophysiology.
A particularly interesting chapter in this section summarizes a biomedical engineering approach to illustrate the underlying anatomical and physiologic processes that result in a normal volumetric capnogram. A mathematical model that accounts for bronchial airway structure, gas convection and diffusion, and the carbon dioxide release from alveolar capillary blood is shown to generate a computed washout curve that shows remarkable agreement with an experimentally measured capnogram from a healthy human subject. This illustrates the utility of physiologic modeling as a useful tool for investigating potentially complex pathophysiologies without placing patients at risk.
A unique historical section describes the evolution of time and volumetric capnography with many interesting anecdotes, as well as a first-person account by Smalhout, an early proponent of capnography. A selection of capnographic tracings corresponding to clinical events that he made over a 20-yr period is one of the highlights of this book. Without reading this section of the book, few people would realize that the impetus for carbon dioxide analyzer development was to investigate the cause of death in patients who turned out to be rebreathing due to a channeling issue through carbon dioxide absorption devices, or that carbon dioxide analyzers enabled a reduction in mortality for polio patients by allowing clinicians to titrate ventilation to expired carbon dioxide instead of adjusting ventilation based on their weight.
The technological section fulfills the editors’ wishes for providing clinicians with information necessary to appreciate the mechanism, design, and limitations of devices for measuring carbon dioxide. Various chapters address technical specifications and standards (e.g., accuracy, range, drift, response time, interfering gases, alarm systems, calibration) for carbon dioxide analyzers and describe technological limitations for flow measurement, required to estimate carbon dioxide production. Another chapter describes various methods for carbon dioxide detection, including infrared, photoacoustic, colorimetric, and mass spectrometry methods. Unfortunately, Raman spectroscopy is not included simply because it is not currently commercially available. This chapter also includes a discussion of mainstream versus sidestream carbon dioxide analyzers.
The book ends with a mini-atlas of capnographic waveforms typifying various physiologic states, which is useful although not exhaustive.
As the editors acknowledge, there is a fair amount of redundancy; as an example, the fact that highly sensitive colorimetric carbon dioxide indicators can yield false positives with esophageal intubation is mentioned in multiple chapters along with the fact that false negatives in cardiac arrest have led to the removal of correctly placed endotracheal tubes. Other recurring themes include the predictive value of end-tidal carbon dioxide in assessing arterial PCO2 and the utility of volumetric capnography. In general, I found the multiple perspectives to be helpful instead of confusing or irritating. As with any book, the onus is on the reader to formulate his or her judgment with the assistance of the most recent literature.
The overall introduction to the book and the introduction chapters for each section are very short and could have been used to provide the reader with a more substantial description of the basic concepts or objectives of each section. The section and subsection titles are somewhat arbitrary, and some chapters are in fact assigned to their own sections. Although the terminology is relatively consistent, the book could also use a more comprehensive list of abbreviations and acronyms used in various chapters. I found most of the typographical and page-setting errors to be minor (with the exception of a reference to “title” volumes). In spite of these limitations, the book admirably maintains its focus on capnography; readers interested in the latest tissue oxygen tension (PO2) monitoring techniques, for example, will have to look elsewhere.
In summary, Capnography: Clinical Aspects is a very readable introduction to a topic addressed by few textbooks. It is useful as a reference primarily because of its comprehensive index and contains much information useful to the practitioner of critical care as well as anesthesiology. It addresses the physiologic and technological considerations that need to be understood to make capnography a clinically useful tool and should be standard reading for those who depend on it as a basic anesthetic monitor.
Tuhin K. Roy, M.D., Ph.D.,
David O. Warner M.D., Editor
Mayo Clinic, Rochester, Minnesota. roy.tk@mayo.edu

© 2006 American Society of Anesthesiologists, Inc.

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