Respiratory abnormalities are prevalent in critically ill patients and are often unveiled through arterial blood gas (ABG) analysis. Accurate interpretation of ABGs is essential for diagnosing and managing a variety of respiratory conditions. This chapter provides a comprehensive approach to assessing respiratory acidosis and alkalosis, emphasizing the alveolar-arterial (A–a) gradient to differentiate underlying causes and guide appropriate management. Understanding the physiological mechanisms, compensation processes, and clinical implications enhances the clinician’s ability to make informed decisions [1, 2]. [Ref: Algorithm 17.1].