The ABC's of ABG's™

Robert F. Moran, Timothy N. Liesching

• Publisher: Momentum Press
• Year: 2018
• ISBN:
  9781947083493
• Topic: Tecnologia e ingegneria
• Subtopic: Scienza biomedica

When a patient is brought into a trauma center in extremis, or becomes that way during a hospital stay, the intervention mantra is airway, breathing and circulation—the ABC's. We provide the nonspecialist physician, and those in the participating and supporting professions, a quick overview of the meaning of the testing terms in this environment. Many tests, added singly or in small groups over time as clinical and measurement technology evolved, but whose rationale may have become less clear, are defined and explained. Our format is simple—a brief overview of the physiologic environment and then the encyclopedic dictionary of the terminology. We include common terms as well as some of the confusing uses of terms besides lesser known facts influencing their clinical use on the basis of the analytical, medical, and managerial experience of the authors. Keep it in your lab coat pocket—you'll find it useful in understanding and teaching!

CHAPTER 1

In the Beginning . . .

When a patient is brought into a trauma center in extremis or becomes critical for whatever reason, the intervention mantra is Airway, Breathing, and Circulation—the motivation for the title of this book. The limited overview that follows should serve to focus the reader on the main content of this book—the cyclopedic dictionary of the related terminology.

Nearly simultaneously with the initial diagnosis and treatment is the need to assess patient status or the results of treatment with measurements—blood gases and ventilatory status, renal function, acid–base assessment, and other directly related tests such as electrolytes and glucose.

We begin our overview with a brief discussion of the systems and measurements involved, including a brief general description of lung disease, followed by pulmonary function testing (PFT). While not directly a part of “critical” assessment by measurement, the results of PFT are often an important part of the whole patient picture.

Airway: The assessment of the airway is primarily clinical—nothing but patient deterioration can occur if the airway remains blocked! So, we’ll leave that to the clinical texts.

Breathing: Is a process we are all familiar with but generally ignore until it’s significantly impaired, since it occurs automatically and is unconsciously carried out in response to biochemical and neurologic homeostatic systems. Breathing or “external respiration” has as its primary function the introduction of air (oxygen-O₂) and removal of carbon dioxide (CO₂).¹ In subjects with normally functioning lungs and other airways, the variables include environment, activity levels, and a complex interplay of nerves, muscles, and physiology.

The effectiveness of breathing in getting oxygen to the blood and carbon dioxide out of the blood can be determined by the physiologic functioning of the pulmonary tree (nose/mouth, trachea, bronchi, and ultimately to the alveoli of the lungs).

Generally, lung dysfunction may be categorized as airway-based, tissue-based, or blood-circulation-based disease.

Airway-based diseases affect the tubes (airways) that carry oxygen and other gases into and out of the lungs causing narrowing or obstruction of the air passages. Both asthma and chronic obstructive pulmonary disease (COPD) are in this category.

Tissue-based diseases may affect the structure of the lung tissue or surrounding tissues. Scarring and/or inflammation of the tissue reduces the ability of the lungs to fully expand (restrictive lung disease). Pulmonary fibrosis would be an example of a tissue-based restrictive disease. Neuromuscular disease affecting the diaphragm or thoracic cage can also be the cause of restrictive disease, exemplified by myasthenia gravis.

Blood-circulation-based diseases affect the blood vessels in the lungs and could be caused by inflammation and scarring of the pulmonary blood vessels where gas exchange (oxygen and carbon dioxide) occurs. The consequences are many, including impairment of gas exchange, fluid retention, and certain cardiovascular effects. Pulmonary hypertension is an example of this sort of disorder. Specific pulmonary diseases can have a mixture of characteristics.

Disease entities. Most common among lung diseases are asthma, collapse of all or part of the lung (e.g., pneumothorax or atelectasis), bronchitis (swelling and inflammation of airways), COPD, lung cancer, pneumonia, and pulmonary edema.

So, as clinical evaluation warrants, various tests and measurements as well as specific PFTs can be performed, frequently in conjunction with certain cardiovascular tests, to confirm or quantify clinical impressions.

Pulmonary Function Testing

With respect to the “breathing” part of the ABCs, first and foremost are tests to measure volumes and flow of the air in different parts of the airways/lungs as well as the ability of the lungs to allow passage of gases between the airways and the circulating blood. Many results can be obtained by a few simple patient maneuvers in a controlled environment—some even in the physician’s office (albeit a bit less precisely than in a fully equipped PFT laboratory).

Spirometry testing is the most basic of the tests—it simply measures the volume of gas (air) as a function of time, as the gas moves in and out of the lungs in a specified maneuver, a combination of normal (tidal) breathing at rest, followed by a forced, complete inhalation and exhalation. From this one can obtain the volumes referred to as forced vital capacity (FVC), forced vital capacity at one second (FVC1), as well as other quantities.

A more specialized device, but based on a similar patient maneuver, results in the automatic calculation of air flow as well as volume, simultaneously, and presents a graphical representation of the results (as well as a table of values) and a comparison of predicted values (from large population studies). Since the major differentiation of pulmonary function is obstructive vs. restrictive disease or a combination, the flow volume display effectively displays a characteristic pattern compared to the “Normal.” (The two loops shown are a part of the process, to ensure that patient results are reproducible.) The dots show predicted values.

The restrictive pattern shows a smaller size of the normal and the obstructive shows a pronounced “coving” effect on the downslope of the exhalation, with the mixed defect (not shown) having a combination pattern. More detailed evaluation of this pattern is beyond the scope of this work, but greater differentiation is achieved by measurement of flow and volume in different segments of the loop (e.g., FEV₁ or FEV_25–75). Various physical and therapeutic–diagnostic maneuvers coupled with clinical judgments and with cardiovascular evaluations and other technologies may be required for the full picture.

One of the diagnostic tools available in many pulmonary function laboratories is the “body box” (plethysmograph), for whole-body plethysmography. The processes available with this device enable more diagnostic detail of the flow–volume loop (as referred to earlier), as well as being able to measure the residual volume of the lungs.

The measurement of the gas diffusion across the parenchyma of the lung into the circulating blood is probably the best measure of the extent of a fibrotic disease. It may be done as part of the use of the plethysmograph or as a separate test procedure. The diffusion test (D_LCO) mimics gas diffusion through the lung tissue into the blood by using small amounts of carbon monoxide (CO). Carbon monoxide is chosen since it has a similar molecular size in comparison to both oxygen and carbon dioxide and is not typically found in the blood, so interference in the test is near zero.

Once gas exchange occurs at the alveolar–circulatory interface, the biochemistry really begins.

Circulation: Within the organism, several processes are linked to gas transport into and out of the lungs. Oxygen from the external environment ultimately powers the energy production–storage–utilization process and carbon dioxide is the immediate waste product controlled by the lung’s function.

Internally, the acquisition of oxygen to facilitate energy production and the maintenance of a balance between acids, bases, and electrolytes in the human organism has evolved over millions of years. Our premammalian ancestors were living in an oxygen-poor, carbon dioxide-containing ocean. High bicarbonate levels in rainwater came about as that water passed through the limestone deposits and the bicarbonate/carbon dioxide pairing was captured inside the cell when the development of cell membranes occurred.

The primitive base to acid ratio has not only survived inside many species, but has become critical for the proper functioning of the whole organism. This is especially reflected in the key role that hydrogen ion concentration plays in many metabolic systems including the process of extraction of energy from food, the maintenance of cell and tissue integrity, and oxygen transport and delivery.

All these phenomena are reflective of the triad of integrally related homeostatic systems that are critical for both the near and long-term maintenance of life, acid–base balance, gas transport/exchange, and electrolyte/osmotic pressure. While in various texts each system is typically discussed on an individual basis for the sake of conceptual understanding, they are all fundamentally related as shown in the figure.

Since the primary objective of this work is to enable the professionals involved in the diagnosis and treatment of the critically ill to recognize the relationship among the terminology of testing, patient diagnosis, expected results, and obtained results, we won’t go into the intricacies of the interrelationships of acid–base electrolytes and oxygen transport and delivery. However, it is important historically as well as biochemically to understand that these interrelationships exist and to a certain extent the context. For a comprehensive but brief overview, the authors would suggest the monograph by John Toffaletti, Blood Gases and Electrolytes.²

Acid–Base: Hydrogen Ion (pH), Bicarbonate (Hydrogen Carbonate), and Carbonic Acid

The human body continually produces acid as it produces energy. Much of this acid is in the form of carbon dioxide, the end-product of aerobic metabolism which is akin to combustion—the burning metabolically of carbohydrates and related compounds resulting in CO₂ and water. The carbon dioxide is eliminated by the lungs at an approximate rate of almost 300 liters per day.³

In addition to carbon dioxide, a typical “Western” mixed diet produces metabolic acids such as phosphoric and sulfuric acids. These acids are referred to as “fixed” acids as they cannot be converted to CO₂ and “blown off” in the lungs. While the bases found in the diet and produced metabolically neutralize some of these acids, the remainder of the acids must be neutralized by systems in the body so that acid–base homeostasis is maintained.

Acids produced as a part of aerobic metabolism are, in general, either carbonic acid (resu...

Table of contents

• Cover
• Half Title Page
• Title Page
• Copyright
• Dedication
• Contents
• FOREWORD
• ACKNOWLEDGMENTS
• CHAPTER 1 IN THE BEGINNING . . .
• CYCLOPEDIC DICTIONARY OF ACID–BASE HOMEOSTASIS AND OXYGENATION
• BIBLIOGRAPHY
• ABOUT THE AUTHOR
• INDEX