A 34-year-old woman with chronic pelvic pain and dyspareunia due to endometriosis undergoes elective laparoscopic endometriosis resection. The patient has no other medical conditions, and preoperative medical evaluation shows no cardiac, pulmonary, or renal abnormalities. Surgery is performed under general anesthesia and is prolonged due to excessive pelvic adhesions. During surgery, the patient has a moderate amount of blood loss and receives a large-volume infusion of isotonic saline. After completion of the surgery and reversal of anesthesia, laboratory results are as follows: Which of the following best explains this patient's postoperative acid-base imbalance?

Question

A 34-year-old woman with chronic pelvic pain and dyspareunia due to endometriosis undergoes elective laparoscopic endometriosis resection.  The patient has no other medical conditions, and preoperative medical evaluation shows no cardiac, pulmonary, or renal abnormalities.  Surgery is performed under general anesthesia and is prolonged due to excessive pelvic adhesions.  During surgery, the patient has a moderate amount of blood loss and receives a large-volume infusion of isotonic saline.  After completion of the surgery and reversal of anesthesia, laboratory results are as follows:  Which of the following best explains this patient's postoperative acid-base imbalance?

Quizplus · Accepted Answer

11ec25c8_d959_0760_bc81_4bfb4a52da6f_MD0003_00 This patient's postoperative laboratory results show the development of metabolic acidosis (pH <7.35, serum bicarbonate <24 mEq/L) with respiratory compensation (PaCO₂ <40 mm Hg) and a normal anion gap (10-14 mEq/L). This acid-base disturbance is most likely due to the infusion of excess normal saline, a common cause of nonanion gap metabolic acidosis (NAGMA). metabolic acidosis Because chloride (Cl⁻) and bicarbonate (HCO₃⁻) are the predominant anions in the body, they are readily exchanged between the extracellular and intracellular spaces to maintain electronegative balance. Infusion of excess normal saline (NaCl) markedly increases intravascular Cl⁻, which drives intracellular shifting of HCO₃⁻. The loss of HCO₃⁻ from the bloodstream decreases blood pH. ⁻ ⁻ ⁻ ⁻ ⁻ Other causes of NAGMA typically involve the loss of HCO₃⁻ from the kidneys (eg, renal tubular acidosis) or gastrointestinal tract (eg, severe diarrhea). With these etiologies, Cl⁻ is shifted extracellularly to compensate for the loss of HCO₃⁻. Because of the inverse relationship between Cl⁻ and HCO₃⁻, NAGMA of any etiology is also referred to as hyperchloremic acidosis. ⁻ ⁻ ⁻ ⁻ ⁻ (Choice A) To facilitate visibility and maneuverability, CO₂ insufflation into the abdominal cavity is sometimes performed during laparoscopic surgery. Absorption of this CO₂ into the bloodstream can lead to hypercapnia (PaCO₂ >40), similar to that occurring with hypoventilation. (Choices C and E) Anion gap metabolic acidosis (AGMA) results from the addition of unmeasured acidic compounds into the bloodstream. The unmeasured compounds donate H⁺ to bind HCO₃⁻ and cause acidemia, and the remaining anionic component increases the anion gap. In renal failure, AGMA results from reduced excretion of acidic compounds, including hydrogen phosphate, hydrogen sulfate, and uric acid. Prolonged systemic hypoperfusion causes organ and tissue ischemia, leading to AGMA due to increased production of lactic acid. AGMA ⁻ (Choice D) Inadequate mechanical ventilation (hypoventilation) leads to CO₂ retention and respiratory acidosis. Educational objective: Infusion of excess normal saline is a common cause of nonanion gap metabolic acidosis. The increase in intravascular chloride (Cl⁻) drives intracellular shifting of bicarbonate (HCO₃⁻) to reduce serum HCO₃⁻ and decrease blood pH. ⁻ ⁻ ⁻ References: Hyperchloremic acidosis. (http://www.ncbi.nlm.nih.gov/pubmed/29493965)

A 34-Year-Old Woman with Chronic Pelvic Pain and Dyspareunia Due