A 38-year-old man is evaluated for tachypnea. The patient, a construction worker, fell off a roof while working yesterday. He sustained an open femur fracture, which required urgent operative repair. Medical history includes chronic low back pain for which he has been taking opioids. Temperature is 36.7 C (98.1 F), blood pressure is 130/90 mm Hg, pulse is 110/min, and respirations are 30/min. Pulse oximetry is 99%. On physical examination, the lungs are clear to auscultation. Heart sounds are regular. The abdomen is soft and nontender to palpation. Arterial blood gas while the patient is breathing room air shows the following: Which of the following best explains the laboratory findings in this patient?

Question

A 38-year-old man is evaluated for tachypnea.  The patient, a construction worker, fell off a roof while working yesterday.  He sustained an open femur fracture, which required urgent operative repair.  Medical history includes chronic low back pain for which he has been taking opioids.  Temperature is 36.7 C (98.1 F), blood pressure is 130/90 mm Hg, pulse is 110/min, and respirations are 30/min.  Pulse oximetry is 99%.  On physical examination, the lungs are clear to auscultation.  Heart sounds are regular.  The abdomen is soft and nontender to palpation.  Arterial blood gas while the patient is breathing room air shows the following:  Which of the following best explains the laboratory findings in this patient?

Quizplus · Accepted Answer

11ec25c8_d95d_4d29_bc81_f91b0aba07d7_MD0003_00 This patient has a high pH of 7.53 with a low PaCO₂, which reflects respiratory alkalosis. His serum bicarbonate of 20 mEq/L indicates that there has been partial compensation. Renal compensation for primary respiratory alkalosis begins after several hours via increased bicarbonate excretion and requires approximately 72 hours to achieve full compensation. compensation In this patient, who also has a high respiratory rate and a normal oxygen saturation and has recently undergone surgery, the most likely reason for respiratory alkalosis is inadequate pain control. Patients who use opioids chronically develop tolerance and therefore require higher doses of opioids or multimodal strategies (eg, regional anesthesia, nonsteroidal anti-inflammatory drugs) to appropriately control acute pain. (Choice A) Excessive infusion of normal saline (NaCl) causes hyperchloremic metabolic acidosis (formerly called dilutional acidosis). Because chloride and bicarbonate are the most abundant anions in the blood, an increase in chloride ions drives bicarbonate intracellularly to maintain appropriate electronegativity, which decreases buffering capacity and causes non-anion gap metabolic acidosis (low pH and low bicarbonate). (Choice B) Patients with acute opioid intoxication develop respiratory acidosis due to hypoventilation from a decreased central respiratory drive. Laboratory results would show a low pH and high PaCO₂. (Choice D) Postoperative patients are at increased risk of a pulmonary embolism, which can result in acute ventilation/perfusion mismatch. Patients become hypoxic, which triggers hyperventilation and can lead to respiratory alkalosis. However, patients with pulmonary embolism have persistent hypoxemia, and this patient's oxygen saturation and PaO₂ are normal. (Choice E) Sepsis commonly causes primary anion gap metabolic acidosis due to lactic acidosis. Although respiratory compensation causes an increased respiratory rate and a decreased PaCO₂, the laboratory values would be consistent with the primary disorder (metabolic acidosis) and show a low pH and low bicarbonate. Educational objective: Patients can develop respiratory alkalosis with a high respiratory rate and normal oxygen saturation due to inadequate pain control. Patients who take opioids chronically often develop tolerance and require increased doses of opioids or multimodal strategies to appropriately control acute pain. References: Intravenous sodium and chloride: not too much, not too quick, and only to healthy kidneys! (http://www.ncbi.nlm.nih.gov/pubmed/31245078)

A 38-Year-Old Man Is Evaluated for Tachypnea