Cardiovascular pharamacology

• β-blockers


  • Used to slow the heart rate to below 65 beats per minute (bpm) for CT coronary angiography
  • Reduces coronary artery movement artefacts by increasing the length of the diastolic phase
  • Effectively increases the time that the coronary arteries are motion-free
• Higher heart rates, the diastolic phase shortens, and the temporal resolution of most current scanners is insufficient to capture the coronary arteries without blurring artefact
• With newer CT systems, such as dual-source scanners, the temporal resolution is as low as 83 ms, and motion-free images may be obtained at higher heart rates
• Metoprolol


  • The most commonly used b-blocker in cardiac imaging
  • It is a cardioselective b1 antagonist
  • Has a rapid onset, and can be given either orally or intravenously
  • Has the shortest half-life (T1/2)
• Oral β-blockers


  1. 50-100 mg to be taken the evening before and the morning of the scan

  2. Patient attends the radiology department an hour before their scan and takes 50-100 mg metoprolol orally

  
• IV β-blockers


  1. Commonly 2.5 or 5 mg aliquots of metoprolol followed by a saline flush given to the patient while they are on the scanner

  2. Further 2.5-5mg doses are given if no response has occurred after 5 min up to a total administered dose of 15 mg

  3. Blood pressure and continuous ECG monitoring should always be used when giving IV metoprolol
• Esmolol


  • Ultra-short acting b1-selective β-blocker
  • Quick acting with an extremely short half-life of 9 min
  • Expensive compared with metoprolol and it is usually given as an infusion
• Verapamil and diltiazem


  • Calcium channel blockers that are available both in oral and IV preparations
  • Can be used particularly in patients in whom b-blockers are contraindicated
  • Contraindicated where there is a history of heart failure or significantly impaired LV function because they can reduce myocardial contractility
• Glyceryl trinitrate (GTN)


  • Causes smooth muscle relaxation of vessel walls resulting in vasodilatation
  • In CT coronary angiography it is used to improve image quality by increasing the luminal diameter of the coronary arteries
  • Particularly improves visualisation of the right coronary artery and septal branches
  • Fast-acting nitrate and is quickly metabolised by the body
  • Typically given as two sprays of 0.4 mg sublingual GTN just before the scan is performed, ideally with the patient on the scanner table
• Patient may experience a headache but this usually resolves quickly. Uncommonly, GTN may precipitate vasovagal symptoms that usually require simple supportive measures, such as elevating the patient's legs
• Adenosine


  • Used as a stress agent in cardiac MRI and nuclear cardiology studies
  • At high doses, adenosine causes a transient blockade of the atrioventricular (AV) node
  • At the lower doses used in diagnostic imaging, adenosine causes smooth muscle relaxation and significant vasodilatation
• Contraindication: Severe asthma, sick sinus syndrome, heart block, myocardial infarction within previous 3 days
• Patient preparation: Stop nitrates, caffeine, and dipyridamole for 24 hours. Theophylline drugs should be stopped for 48 hours

  
• Side effects: Flushing sensation, shortness of breath, chest pain, headache. Rarely bronchospasm, complete heart block
• Dobutamine


  • Synthetic catecholamine that increases myocardial contractility and heart rate
  • This results in increased myocardial oxygen demand and a rise in blood pressure
  • Mainly works as a β1 agonist but it also has some β2 agonist activity, which causes coronary artery vasodilatation
  • Dose: lower doses 10 mg/kg/min increasing by 10 mg/kg/min to a maximum of 40 mg/kg/min
• Contraindications: Severe hypertension (>220/120 mmHg), moderate to severe aortic stenosis, severe hypertrophic cardiomyopathy, recent myocardial infarction, complex dysrhythmia, unstable angina, active endo/myo/pericarditis
• Patient preparation: Stop b-blockers for 48 h
• Side effects: Anxiety, chest pain, nausea, ventricular ectopics, atrial fibrillation. Myocardial infarction and ventricular dysrhythmias are rare
• Dipyridamole


  • Commonly used in patients with ischaemic heart disease who are unable to take aspirin
  • It is also used with aspirin for secondary prevention in stroke patients
  • Administered intravenously for stress imaging
  • It causes vasodilatation by inhibiting of cellular uptake of adenosine and inhibition of adenosine breakdown
  • Adenosine is now the more widely used agent in stress MR because there tend to be more significant adverse effects with dipyridamole
  • It has a longer half-life than adenosine (10 h)
• Contraindication: patients with myasthenia gravis and when the systolic blood pressure is less than 90 mmHg
• Atropine


  • It blocks the activity of the vagus nerve causing a rise in heart rate due to increased conduction through the sinoatrial (SA) node
  • It is used routinely in dobutamine stress tests if the target heart rate has not been achieved at the peak dose of dobutamine
  • It may also be required in cardiac CT if β-blocker administration results in excessive bradycardia
• Contraindication: glaucoma, prostatic hypertrophy, myasthenia gravis
• COPD stands for


  Chronic pulmonary disease
• Caution/remark to be taken for B-blocker

  • Pregnant
  • Breast feeding
  • 1st degree heart block
  • COPD
  • max total dose 15mg metoprolol IV and titrate slowly