Page 398 - Cardiac Nursing

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374 P A R T III / Assessment of Heart Disease
(Haldol), especially when it is given IV or in doses higher than and the United States. Brugada syndrome is an autosomal domi-
recommended. Hypokalemia and hypomagnesemia are the elec- nant genetically transmitted abnormality in a gene that is respon-
trolyte abnormalities most often associated with QT prolonga- sible for proper operation of the sodium channel in the cardiac cell
tion. The risk of developing TdP from drugs increases in the pres- membrane. 8,77,90,91
ence of hypokalemia, hypomagnesemia, or when taking other The typical ECG characteristic of Brugada syndrome is a
drugs that also prolong the QT interval or slow drug metabolism. coved ST segment elevation 2 mm with T-wave inversion in the
Grapefruit juice can slow metabolism of many drugs and can also right precordial leads (V 1 –V 3 ), presenting as “pseudo RBBB” (see
increase the QT interval directly. Other factors that can lead to Fig. 15-48 in Chapter 15 for an example of Brugada syndrome).
delayed repolarization include cerebral events such as cerebral vas- This typical pattern can occur transiently and is sometimes only
cular accidents and subarachnoid hemorrhage and liquid protein manifest in the presence of sodium channel blocking drugs (e.g.,
weight loss diets or starvation. 22,83,84,86 procainamide, flecainide) or with fever, although many other
The congenital type of LQTS is an inherited condition associ- drugs and electrolyte imbalances that are known to precipitate
ated with TdP and SCD. Congenital LQTS is due to mutations LQTS can also unmask Brugada syndrome. 90,91 Two other ECG
of at least five genes identified so far, all of which modulate the patterns involving a “saddle back” type of ST elevation in right
function of sodium or potassium ion channels in the cardiac cell precordial leads have been identified but are not considered diag-
membrane. 9,77,87,88 These derangements in ion flow across the nostic of Brugada syndrome unless they convert to the typical pat-
cardiac cell membrane lead to prolongation of action potential tern with sodium channel blocker administration. If a person has
duration which precipitates the development of EADs and trig- the typical ECG pattern but no clinical criteria they have “Bru-
gered activity. It is also possible that derangements in sympathetic gada pattern.” If they have associated clinical criteria they have the
innervation of the heart may contribute to development of TdP in syndrome. The diagnostic criteria for Brugada syndrome include
85
congenital LQTS. (See Chapter 4 for more detailed information the typical ECG pattern of coved ST elevation in more than one
on genetics related to cardiac diseases.) right precordial lead (V 1 –V 3 ) in the presence or absence of a
Characteristic ECG findings of TdP include (1) markedly pro- sodium channel blocking agent in conjunction with one of the
longed QT intervals with wide TU waves; (2) initiation of the ar- following: documented VF or PVT, a family history of SCD at an
rhythmia by an R-on-T PVC with a long coupling interval; and (3) age 45 years, similar ECG patterns in family members, syncope,
wide, bizarre, multiform QRS complexes that change direction fre- nocturnal agonal respiration, or inducibility of VT with pro-
quently, appearing to twist around the isoelectric line (see Figs. grammed stimulation during electrophysiology study. 90
16-18 and 16-19). The acquired type of TdP is usually associated The clinical significance of Brugada syndrome is its association
with bradycardia and is “pause dependent,” meaning that it tends with lethal ventricular arrhythmias and SCD. Often cardiac arrest
to occur after pauses produced by a PVC or sudden slowing of the is the initial presentation, although patients may present with un-
heart rate. TdP is often initiated by a “long–short” cycle sequence explained syncope that is most likely due to self-terminating
in which episodes begin on the T wave of a beat that terminates a episodes of PVT. Arrhythmias are more likely to occur at night
long cycle. The congenital type of TdP frequently occurs with a and during sleep. The only effective therapy for preventing SCD
sudden surge in sympathetic tone, such as with loud noises, emo- in patients with Brugada syndrome is ICD implantation. Most
tional stress, or physical activity. In this case TdP occurs without a antiarrhythmic drugs are contraindicated with the possible excep-
change in preceding cycle length and is not pause dependent or tion of quinidine, which has been effective in some paients. 90,91
bradycardia dependent. Ventricular rate during TdP is commonly
200 to 250 beats per minute. TdP is usually self-terminating and Differential Diagnosis of Wide QRS
occurs in repeated episodes, but it can deteriorate into VF. Beats and Tachycardias
The differentiation of TdP from PVT and VF is extremely im-
portant because TdP does not respond to conventional antiar- One of the most frequently encountered problems in working
rhythmic therapy and is usually made worse by the drugs used to with cardiac patients is differentiating VT from aberrantly con-
treat ordinary VT. Treatment of TdP is aimed at shortening the re- ducted supraventricular rhythms, both of which can cause a wide
fractory period and unifying repolarization by increasing the heart QRS complex. Establishing the correct diagnosis is important in
rate and correcting any contributing causes, such as electrolyte choosing the correct therapy for the acute event as well as deter-
imbalances, or discontinuing causative drugs. Cardiac pacing at mining long-term therapy for the arrhythmia. Because aberrantly
rates of 100 to 110 beats per minute can be instituted until the conducted supraventricular beats and tachycardias can look al-
underlying cause is corrected. Magnesium can suppress the ar- most identical to ventricular ectopic beats or VT, it is sometimes
rhythmia in both the acquired and congenital forms by reducing impossible to tell them apart.
the amplitude of afterdepolarizations thought to cause TdP. Drugs There are three major causes of wide QRS beats or tachycar-
such as quinidine, procainamide, disopyramide, sotalol, and dias: (1) ventricular origin of the beat or rhythm, (2) aberrant
amiodarone are contraindicated because they prolong the refrac- conduction of a supraventricular beat or tachycardia through the
tory period and contribute to the abnormal repolarization that bundle-branch system (temporary or permanent bundle-branch
causes TdP. block), (3) preexcitation of the ventricle through an accessory
pathway. VT is the most common cause of a wide complex tachy-
Brugada Syndrome. Brugada syndrome was first described 89 cardia, accounting for approximately 80% of cases; aberrant con-
in 1992 and has since been recognized as a common cause of SCD, duction of an SVT occurs in 15% to 30% of cases of wide com-
accounting for up to 20% of SCD in people with structurally nor- plex tachycardia; and accessory pathway conduction accounts for
90
mal hearts. It was first discovered in young men in Southeast Asia 1% to 5% of cases. 92 Other conditions that can also cause the
and is most commonly found there, although now that it is a clini- QRS to widen include antiarrhythmic drugs; electrolyte abnor-
cally defined syndrome it is seen with increasing frequency in Europe malities, especially hyperkalemia; and ventricular paced rhythms.