A 66-year-old man with poorly controlled type 2 diabetes is admitted to the hospital due to a 2-day history of fever and confusion. Blood pressure is 110/50 mm Hg. Pulse oximetry shows an oxygen saturation of 97% on room air. Examination reveals warm extremities with full peripheral pulses, an infected neuropathic foot ulcer with surrounding cellulitis, and normal lung sounds. A central venous catheter terminating in the superior vena cava is placed. Blood aspirated from the catheter appears red. Blood gas analysis of this venous sample reveals an oxygen saturation of 87% (normal: 65%-70%), and lactate is moderately elevated. Which of the following is the most likely cause of this patient's abnormal blood gas findings?

Question

A 66-year-old man with poorly controlled type 2 diabetes is admitted to the hospital due to a 2-day history of fever and confusion.  Blood pressure is 110/50 mm Hg.  Pulse oximetry shows an oxygen saturation of 97% on room air.  Examination reveals warm extremities with full peripheral pulses, an infected neuropathic foot ulcer with surrounding cellulitis, and normal lung sounds.  A central venous catheter terminating in the superior vena cava is placed.  Blood aspirated from the catheter appears red.  Blood gas analysis of this venous sample reveals an oxygen saturation of 87% (normal: 65%-70%), and lactate is moderately elevated.  Which of the following is the most likely cause of this patient's abnormal blood gas findings?

Quizplus · Accepted Answer

11ec160b_3b4f_a76f_9249_27a5d16ba910_MD0001_00 This patient has sepsis due to an infected diabetic foot ulcer with surrounding cellulitis. Sepsis is a florid host inflammatory response to infection that can lead to multiple organ system dysfunction. Septic organ dysfunction is driven mainly by poor tissue oxygen use. This cellular dysoxia is caused by 3 major mechanisms: Because oxygen use decreases, the transorgan arteriovenous oxygen gradient is small; therefore, oxygen saturation of central venous blood (ScvO₂) (superior vena cava) - returning from the periphery - increases. Central venous catheters are often placed therapeutically to deliver medications; they can be used diagnostically to differentiate septic (↑ ScvO₂) from cardiogenic (↓ ScvO₂) shock. (Choice A) Cardiac output (oxygen delivery) is typically increased in sepsis due to systemic vasodilation (warm extremities, wide pulse pressure). Decreased cardiac output is the hallmark of severe heart failure (cold extremities, narrow pulse pressure), and ScvO₂ decreases with heart failure because peripheral oxygen consumption outstrips its delivery. (Choice C) Sepsis is associated with microvascular (rather than macrovascular) thrombosis due to endothelial injury and focal or disseminated intravascular coagulation, further impairing oxygenation. Widespread large-vessel thrombosis and infarction are seen in conditions such as catastrophic antiphospholipid-antibody syndrome. (Choice D) Sepsis can induce capillary permeability in the lung, leading to pulmonary edema with resultant arterial hypoxemia (acute respiratory distress syndrome). However, this patient's pulmonary function and oxygenation are intact, as evidenced by normal lung examination (no crackles) and arterial oxygen saturation. (Choice E) Hemoglobin oxygen unloading to peripheral tissues is facilitated by sepsis-induced (lactic) acidosis and fever, which lead to rightward shifting of the oxyhemoglobin dissociation curve (ie, weaker oxygen-hemoglobin binding). Educational objective: Sepsis is a host inflammatory response to infection that can lead to multiple organ dysfunction due to defective mitochondrial oxidative respiration, resulting in a widespread dissociation between oxygen delivery and extraction. This manifests as elevated central venous oxygen saturation. __________ References: Oxidative stress and mitochondrial dysfunction in sepsis. (http://www.ncbi.nlm.nih.gov/pubmed/21596843)

A 66-Year-Old Man with Poorly Controlled Type 2 Diabetes Is