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1 The Russell H. Morgan Department of Radiology and Radiological Science, Johns
Hopkins Medical Institutions, Baltimore, MD, and Department of Radiology,
Johns Hopkins Outpatient Center, JHOC 3235A, 601 N Caroline St., Baltimore, MD
21287.
2 Department of Pharmacy, Johns Hopkins Medical Institutions, Baltimore,
MD.
Received December 22, 2004;
accepted after revision March 23, 2005.
Address correspondence to H. K. Pannu.
Abstract
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CONCLUSION. Beta-blockers can be safely given, orally or IV, to most patients to lower the heart rate for cardiac CT. A protocol can be implemented and patients can be screened for certain contraindications to allow successful administration of these medications by radiologists.
Keywords: ß-blocker • cardiac imaging • coronary arteries • CT arteriography • MDCT
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For those patients for whom ß-blockers are considered, certain guidelines are suggested to avoid complications [1]. The protocol can include oral, IV, or a combination of oral and IV administration [2]. We present our protocol for administering metoprolol, a ß1-antagonist and cardioselective ß-blocker, for cardiac CT studies (Table 1). The determination to give ß-blockers and the dosage to be used should be individualized for each patient on the basis of the practitioner's assessment of the patient and the patient's history and consultation with a pharmacist as necessary.
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ß-Blocker Administration Protocol
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II: Screening Patients for Contraindications to Giving ß-Blockers
Patients are screened for medical conditions that may preclude them from
receiving ß-blockers. These contraindications are sinus bradycardia,
which is defined as a heart rate of less than 60 bpm; systolic blood pressure
of less than 100 mm Hg; allergy to the medication or its constituents;
decompensated cardiac failure; asthma on ß-agonist inhalers; active
bronchospasm; and second- or third-degree atrioventricular block. We also do
not administer ß-blockers to pregnant patients. Sinus bradycardia and
hypotension are excluded by obtaining the patient's vital signs. The patient
is then asked if he or she has known allergies or a history of asthma or
chronic obstructive pulmonary disease (COPD).
Although metoprolol has relative ß1 selectivity, especially at low doses, according to the Physicians' Desk Reference, "patients with bronchospastic diseases, in general, should not receive beta blockers" [1]. In patients with COPD, the use of ß-blockers has been somewhat controversial. In patients with mild COPD, the use of cardioselective ß-blockers—which include agents such as atenolol, esmolol, bisoprolol, and metoprolol—has been shown to be safe [3–5]. However, the patient with severe COPD who is also dependent on the use of ß2-agonist inhalers, such as albuterol, should not receive ß-blockers. Therefore, patients with asthma or COPD on ß-agonist inhalers are not given ß-blockers. Patients who report a history of asthma, such as childhood asthma, but no current asthma and who do not take asthma medications are given ß-blockers. Lastly, patients are evaluated for possible second- or third-degree atrioventricular block by generating a single-lead ECG strip [6] (Figs. 1A, 1B, and 1C).
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IV: Precautions for Administering IV Metoprolol
IV ß-blockers are given with caution if the patient has mild COPD and
is being treated with oral steroids but is not using ß-agonist inhalers.
Caution is also used for patients who are on other atrioventricular nodal
blocking agents such as calcium channel blockers (e.g., diltiazem, verapamil),
digoxin, and other ß-blockers. However, if the heart rate is not in the
desired range, consideration is given to administering ß-blockers.
V: Administration of IV Metoprolol
The patient is initially given one 2.5-mg dose of metoprolol IV over 1 min.
If the heart rate remains more than 65 bpm after 5 min, a second dose of 2.5
mg of metoprolol is given. If the heart rate continues to remain elevated, up
to two additional doses of 5 mg each of metoprolol can be given IV, each over
1 min, with a 5-min interval between doses. The patient's blood pressure and
heart rate are checked before each dose is administered. The maximum total
dose of metoprolol given is 15 mg IV with the sequence being 2.5, 2.5, 5, and
5 mg at 5-min intervals.
VI: Postprocedure Care
After the CT examination, all patients who are given IV or two oral doses
of metoprolol are observed for 30 min. If the patient has bronchospasm, two
puffs of an albuterol inhaler are given from a 17-g albuterol inhaler
canister. If the patient's heart rate drops to less than 45 bpm, consideration
is given to administering atropine. If the patient is atropine-resistant and
has a low heart rate, resuscitative measures and administration of IV
ß1-agonists such as dopamine or epinephrine may become
necessary.
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An essential part of performing a successful CT coronary angiography examination is to limit the effect of cardiac motion on the coronary arteries. Normally, single-segment reconstruction is done using scan data from a single cardiac cycle to create the image, but this method is optimal only for patients with a low heart rate [7]. For those with a high heart rate, data from more than one cardiac cycle can be used to reconstruct the image; this method is called "multisegment reconstruction" [7, 8]. Typically, the data from two cardiac cycles are used, which improves the temporal resolution to gantry rotation time divided by 4. However, a drawback of using multiple cardiac cycles to reconstruct images is that the spatial resolution in the z-axis can decrease if the pitch is too high for the patient's heart rate because there are gaps in the acquired data [7]. Volume coverage or longitudinal resolution is compromised with multisegment reconstruction and the quality of the images may not be better [9, 10]. There-fore, lowering the heart rate with ß-blockers is suggested as the preferred approach over using multisegment reconstruction [9, 11].
When comparing low heart rates and single-segment reconstructions with higher heart rates (> 65 bpm) and multisegment reconstructions, investigators found that vessel visibility was highest when the heart rate was below 65 bpm and single-segment reconstruction was used [12]. The quality of the CT angiogram, especially for visualization of the right coronary artery, has been shown to improve with the administration of ß-blockers [13]. Detection of vessel stenoses has also been shown to be higher in patients with lower heart rates [14]. The proportion of the cardiac cycle spent in diastole increases as the heart rate decreases; therefore, medications such as ß-blockers are given to increase diastole [15].
In general, ß-blockers are helpful in patients with irregular heart rates, as seen with premature atrial or ventricular contractions; supraventricular tachycardias; and arrhythmias, such as atrial fibrillation. There is an alternating sinus bradycardia and atrial tachycardia due to sinus node dysfunction in the tachycardia-bradycardia syndrome form of sinus node dysfunction. With atrial fibrillation, the negative chronotropic and dromotropic effects of the ß-blocker lengthen the diastolic portion of the cardiac cycle. Beta-blockers can be given to patients with pacemakers if the heart rate is higher than the paced rhythm and no pacer spikes are seen in the ECG tracing. Once the pacer spikes are identified, the heart rate cannot be lowered any further.
Beta-blockers can be given for short-term use, such as for a CT study, in patients with diabetes, psoriasis, controlled congestive heart failure, and ablated Wolff-Parkinson-White syndrome. Beta-blockers can also be given for a CT study to patients who are on medications such as reserpine, monoamine oxidase (MAO) inhibitors, clonidine, quinidine, fluoxetine, paroxetine, and propafenone. For patients who are on long-term treatment with ß-blockers, reserpine and MAO inhibitors can have an additive effect, and the serum concentration can be increased if quinidine, fluoxetine, paroxetine, and propafenone are also present. Less than 1 mg of metoprolol is excreted per liter of breast milk and breast feeding may be held for 12 hr after administration [16].
After a single oral dose of metoprolol, plasma levels of metoprolol are detectable at 10 min. The effects of an oral dose are seen within 1 hr after administration. Peak plasma concentrations are seen at 90 min. Plasma concentrations of metoprolol after an IV dose are approximately twice that seen with the oral route of administration. The peak effect of IV-push metoprolol occurs between 5 and 10 min after administration. The plasma half-life for oral and IV metoprolol in healthy volunteers ranges from 3 to 4 hr [16].
Although ß-blockers can help lower the heart rate, they also have a negative inotropic effect and can decrease left ventricular contractility [17, 18]. This may impact assessment of ventricular function; however, currently ventricular contractility is typically evaluated by echocardiography or nuclear medicine studies and the role of CT is primarily to assess the coronary arteries.
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