Gases arteriales

Review Article

Analysis of Arterial Blood Gases — A Comprehensive Approach
MS Barthwal

Abstract
The arterial blood gases is one of the most important investigation for assessment of clinical oxygenation and acid-base status in critically ill patients. It provides us with information about ventilation, oxygenation and acid-base status, the three closely interrelated physiology parameters,which maintain the pH homeostasis. Its correct interpretation and application necessitates the knowledge of basic applied physiology in relation to these parameters. Through this review article an attempt has been made to formulate a comprehensive approach by first describing the basic physiology in relation to these parameters followed by stepwise approach to analyse arterial blood gases. ©INTRODUCTION
he analysis of arterial blood gases (ABG) plays a pivotal role in making correct diagnosis and deciding management strategies in critically ill patients. This role can be well exemplified by considering the concept of respiratory failure. This term remains nonspecific and does not help much in deciding whether the respiratory failure is due to primary oxygenation failure or ventilatoryfailure. ABG is the only investigation, which can document, specify and quantitate the respiratory failure. High PaCO2, moderately low PaO2 and acidic pH indicate ventilatory failure and low PaCO2, low PaO2 and alkaline pH indicate primary oxygenation failure. The management plan will be ventilatory support in the former case and oxygen therapy in the later case. Since ABG provides a rapid andaccurate assessment of oxygenation, ventilatory and acid-base status, the prerequisite for understanding and correctly interpreting ABG provides a rapid and accurate assessment of oxygenation, ventilatory and acid-base status, the pre-requisite for understanding and correctly interpreting ABG is knowledge of basic physiology in relation to ventilation, oxygenation and acid base status. Through thisreview an attempt has been made to formulate a comprehensive approach by first describing the basic physiology in relation to ABG, followed by stepwise approach to analyse ABG.

homeostasis : 1. Alveolar ventilation, 2. Oxygenation, 3. Acid-base balance These three processes are closely interrelated with each other and alteration in one process affects other process. For the sake of simplicityand better understanding the basic physiology in relation to each will be discussed separately avoiding the more complex details. 1. Alveolar ventilation : The maintenance of CO2 level reflected by arterial CO2 tension (PaCO2) at any given moment depends on the quantity of CO2 produced in body and its excertion through alveolar ventilation (VA) and can be expressed by the equation, PaCO2 ~ CO2/VA.The alveolar ventilation is that portion of total ventilation that participates in gas exchange with pulmonary blood. It it is presumed that CO2 production is constant, then CO2 homeostasis can be simplified to 1/VA ~ PaCo2. Thus PaCO2 is the best index for assessment of alveolar ventilation. High PaCO2 (> 45 mmHg) indicates alveolar hypoventilation and low PaCO2 (< 35 mmHg) implies alveolarhyperventilation. 2. Oxygenation : The ultimate aim of oxygenation is to provide adequate delivery of oxygen to tissues. This is a function of cardiopulmonary system and various factors like arterial oxygen tension (PaO2), oxygen concentration of inspired air (FiO2) and hemoglobin content with its affinity and saturation with oxygen contribute towards normal tissue oxygenation. The PaO 2 and SaO 2 isprimarily used for assessment of oxygenation status since PaO2 accurately assesses oxygenation from 30 to 200 mmHg, whereas SaO2 is normally reliable only in the range of PaO2 from 30 to 60 mmHg.1 Oxygen saturation measurement by pulse oximetry (SpO2) or by ABG analysis (SaO2) is better indicator of arterial oxygen content than PaO2, since approximately 98% of oxygen
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BASIC PHYSIOLOGY...