Question

L.V. is a 68-year-old male admitted to the coronary care unit 24 hours ago with an anteroseptal myocardial infarction (MI). His past medical history includes two other MIs within the last 5 years, obesity, hypertension, hyperlipidemia, and sleep apnea. L.V. had chest pain at home for 12 hours before seeking medical treatment. Lab results note troponin I at 5.2 mcg/L. L.V. is currently pain free with stable VS. The heart monitor shows sinus rhythm with occasional, unifocal premature ventricular contractions and a heart rate (HR) in the 90s. His blood pressure (BP) is 130/70, respiratory rate (RR) is 24 breaths/minute and O2 saturation is 93% on O2 via nasal cannula at 2 L/min. He has a heparin drip infusing at 1200 U/hr and IV nitroglycerin infusing at 20 mcg/min. You are assigned to care for L.V. as part of a two-patient assignment. Question 5: L.V. does respond to the IV bolus of furosemide. As you continue to monitor him, which of the following changes would indicate a therapeutic response specific to the dobutamine infusion?

Increased cardiac output and cardiac index

Increased PAWP

Increased systemic vascular resistance (SVR)

Increased systolic BP

Answer 1

Correct answer: Increase in cardiac output and cardiac index

Explanation:

• Dobutamine is sympathomimetic drug.
• It acts on beta-1 adrenergic receptor on the heart. (weak beta 2 and alpha-1 activity)
• Dobutamine increases the force of contraction of the heart (inotropic effect)
• This leads to increase in the stroke volume. Stroke volume is the amount of blood pump out of the heart with each beat.
• Cardiac output = stroke volume x heart rate. Therefore the cardiac output is directly proportional to the stroke volume. So dobutamine increases the cardiac output (as it increases the stroke volume)
• Cardiac index = cardiac out / body surface area. This helps to assess the patient's cardiac output in relation to the body surface area of the patient. From the formula, it is evident that the cardiac index is directly proportional to the cardiac output. So dobutamine increases the cardiac index.

Incorrect answers:

• Increase in pulmonary capillary wedge pressure
  • This pressure is reflex the left ventricular end diastolic pressure.
  • In other words it represents the preload on the left ventricle.
  • Increase in PCWP indicates severe left ventricular failure or severe mitral stenosis
• Increase systemic vascular resistance
  • Dobutamine has a weak Beta-2 activity. It does reduce the systemic vascular resistance to a lesser extent
• Increase in systolic blood pressure
  • Dobutamine increases the cardiac output.
  • This stimulates the baroreceptors and causes decrease in the systemic vascular resistance.
  • Therefore, there is no effect of dobutamine on blood pressure.