Page 268 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

172 PART 2: General Management of the Patient

The exact mechanism of defibrillation remains uncertain. Whether THE THREE-PHASE TIME-SENSITIVE MODEL
a critical number of myocardial cells require membrane depolariza- OF CARDIAC ARREST THERAPY
tion to overcome ventricular fibrillation or whether certain regions of
the heart must achieve a critical current density remains a subject of There is hope that in the coming years resuscitation science will offer
active study. Several mechanistic aspects are clear, however. The energy substantially improved survival for victims of cardiac arrest. With
discharged (measured in joules, or watt-seconds) appears to have both the success of early defibrillation programs in airports, casinos, and
dose-response and therapeutic window characteristics. That is, the other public places, survival rates in these special locales have soared
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chance of successful defibrillation rises with increasing energies deli- to greater than 50%. However, a new paradigm has emerged in our
vered; however, as energy is increased further, functional myocardial understanding of sudden death. The three-phase time-sensitive model
injury predominates over useful resuscitative properties. With standard of cardiac arrest, based on data from the past several years, offers
biphasic defibrillators, 150 to 200 J is generally the recommended energy the hope of better survival with therapy tailored to the time after
for all shocks, though it is suggested to follow manufacture guidelines; initial arrest. 33
with monophasic devices, 200 J is recommended energy for the initial This model proposes that time in cardiac arrest can be divided into
shock, 300 and 360 J are accepted levels for subsequent attempts to defi- different phases: the electrical phase (the first 4 minutes after arrest),
brillate. For children, the recommended initial dose is 2 to 4 J/kg, with the circulatory phase (minutes 4 through 10), and the metabolic phase
additional shocks at 4 J/kg but not to exceed10 J/kg. Most defibrillators (after 10 minutes), with each requiring different therapeutic approaches
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in use today generate biphasic waveform shocks. These defibrillators (see Fig. 25-4). The electrical phase calls for defibrillation as the first
have been shown to be equally effective as monophasic devices at lower therapy for VF/VT and is currently our standard of care regardless of
energies, which may optimize the benefits of the shock while minimiz- time spent in cardiac arrest. This fits well with national efforts to get
ing myocardial injury. 4 more rapid defibrillation with AEDs—because the evidence suggests
Technique of defibrillation is also important. Firm pressure must be that defibrillation within the first few minutes is associated with a
applied with defibrillation paddles to ensure proper delivery of energy better than 50% chance of initial survival. However, a challenge during
without electrical arc or skin burn. Similarly, defibrillation pads must be this electrical phase is the need to get defibrillators rapidly to victims
well applied to the chest. Positioning of paddles or pads must ensure that at home, where over 70% of cardiac arrests take place. The circulatory
the imaginary line connecting the two electrodes runs through the heart. phase appears to be best treated initially with chest compressions and
That is, in one standard approach, an electrode should be placed at the ventilations and then followed by defibrillation after several minutes of
right upper sternal border and the other at the left midaxillary line near CPR. Using this “CPR first” algorithm, paramedic services in Norway
the apex of the heart. have improved survival rates from 4% to 20% over standard advanced
Perhaps the most important observation regarding electrical defibril- cardiac life support during this circulatory phase. However, another
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lation is that the longer the delay before a shock is delivered, the less challenge becomes apparent during this phase: our current quality of
chance there is for a successful resuscitation. Ventricular fibrillation or CPR remains unmeasured and poorly controlled. Recent studies would
tachycardia should be defibrillated immediately; this is the fundamental suggest that CPR quality in real resuscitation falls far short of the high
principle underlying ICDs, a commonly placed device for patients with quality required for survival—and new technology offers us the ability
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recurrent ventricular tachycardia or history of cardiac arrest. If VF/VT to markedly improve on this in the next few years. This circulatory
18
persists for even 5 minutes without CPR or defibrillation, the chance for phase may be difficult to identify because we usually do not have
a successful outcome falls dramatically. Additionally, data have shown accurate information on time of collapse and thus may not know in
that defibrillation is sensitive to chest compression depth and preshock which phase a patient resides. The circulatory phase depends on very
pause times. In one study, chest compression depth greater than 2 in and good CPR, so prioritizing good compression rate, compression depth,
shortened preshock pauses times (<10 seconds) were associated with a minimal pauses in compression, and proper ventilatory management all
higher percentage of VF removal. 55 become critical priorities.
Given the need for early defibrillation, AEDs have become an impor- The third so-called metabolic phase is the most lethal and challenges
tant tool for paramedics and the lay public. These devices, commonly our basic scientific understanding of ischemia and reperfusion injury.
found in airports and other heavily trafficked public locales, perform Novel therapies, such as advanced cardioprotective pharmacologic
rhythm analysis and provide defibrillatory shocks if needed. In theory, agents, cardiopulmonary bypass, induced hypothermia (see Chap. 26),
no prior experience should be required to operate such a device. AEDs preconditioning pathways, inflammatory mediators, apoptosis signal-
are discussed in more detail below. ing, and hibernation may offer promise in the understanding and

4 minutes 10 minutes
Collapse postarrest postarrest
Time

Therapy Electrical phase of VF Circulatory phase Metabolic phase

Defibrillation Chest compressions New therapies:
Hypothermia?
Apoptosis inhibitors?
Antioxidants?
Defibrillation Cardiac bypass?

FIGURE 25-4. This three-phase model serves as a paradigm shift in the categorization and treatment of cardiac arrest. While some data have been published recently to support the model,
it is still considered theoretical but likely will serve as a tool to think about future therapies.

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