A 28-year-old man is the unrestrained driver in a motor vehicle collision. He is conscious immediately after the collision and refuses transfer to a hospital for detailed evaluation. Two hours later, he is brought to the emergency department due to progressively worsening mental status. Systolic blood pressure is 60 mm Hg, pulse is 130/min, and respirations are 30/min. On examination, the patient is stuporous and responsive only to noxious stimuli. There are bruises over the extremities and upper abdomen. The trachea is midline, neck veins are flat, and abdomen is distended but nontender. Large-bore peripheral intravenous lines are secured to begin fluid resuscitation while the trachea is intubated. Immediately after the patient is placed on mechanical ventilation, the cardiac monitor shows asystole and no pulse is palpable. Which of the following is the most likely underlying mechanism leading to cardiac arrest in this patient?

Question

A 28-year-old man is the unrestrained driver in a motor vehicle collision.  He is conscious immediately after the collision and refuses transfer to a hospital for detailed evaluation.  Two hours later, he is brought to the emergency department due to progressively worsening mental status.  Systolic blood pressure is 60 mm Hg, pulse is 130/min, and respirations are 30/min.  On examination, the patient is stuporous and responsive only to noxious stimuli.  There are bruises over the extremities and upper abdomen.  The trachea is midline, neck veins are flat, and abdomen is distended but nontender.  Large-bore peripheral intravenous lines are secured to begin fluid resuscitation while the trachea is intubated.  Immediately after the patient is placed on mechanical ventilation, the cardiac monitor shows asystole and no pulse is palpable.  Which of the following is the most likely underlying mechanism leading to cardiac arrest in this patient?

Quizplus · Accepted Answer

This patient with evidence of hypovolemia (eg, flat neck veins), abdominal trauma (eg, bruising, distension), and poor organ perfusion (eg, altered mental status) is likely in hypovolemic shock due to internal hemorrhage. Severe hypovolemia lowers central venous pressure, leading to decreased venous return to the right atrium and therefore decreased cardiac output (CO). Positive pressure mechanical ventilation causes an acute increase in intrathoracic pressure, which, in a severely hypovolemic patient with low central venous pressure, can collapse venous capacitance vessels (eg, inferior vena cava) and cut off venous return. This sudden loss of right ventricular preload can cause acute circulatory failure and cardiac arrest. In addition, sedatives used prior to intubation cause relaxation of venous capacitance vessels and can also contribute to decreased venous return. (Choice B) Decreased systemic vascular resistance occurs in distributive shock (eg, sepsis, anaphylaxis, central nervous system injury) due to peripheral vasodilation. Recent trauma with evidence of abdominal injury and absence of neurologic signs (eg, focal weakness) make hypovolemic shock much more likely in this patient. In hypovolemic shock, decreased CO leads to peripheral vasoconstriction and increased systemic vascular resistance. (Choice C) Vasovagal syncope results from a sudden increase in parasympathetic activity, leading to bradycardia, peripheral vasodilation, and a temporary decrease in CO. Cardiac arrest does not occur. This patient in hypovolemic shock has a marked increase in sympathetic activity, leading to tachycardia and peripheral vasoconstriction. (Choice D) Acute pulmonary embolism can cause cardiac arrest due to a rapid increase in pulmonary vascular resistance, leading to acute right-sided heart failure and loss of left ventricular preload. Jugular venous distension would be expected. In addition, it would be unusual for an acute pulmonary embolism to occur at exactly the time of mechanical ventilation. (Choice E) Reduced myocardial contraction occurs in myocardial infarction and can lead to cardiogenic shock. When severe, loss of CO due to cardiogenic shock can cause cardiac arrest. However, jugular venous distension would be expected. Educational objective: Positive pressure mechanical ventilation causes an acute increase in intrathoracic pressure. In the setting of decreased central venous pressure (eg, hypovolemic shock), initiation of mechanical ventilation can cause acute loss of right ventricular preload, loss of cardiac output, and cardiac arrest. References: Cardiovascular issues in respiratory care. (http://www.ncbi.nlm.nih.gov/pubmed/16306058)

A 28-Year-Old Man Is the Unrestrained Driver in a Motor