The lack of an accurate and readily measurable parameter to guide resuscitative efforts has long frustrated clinicians and clinical investigators. Pulse quality, pupillary reactivity, serial blood gases, and the coarseness of ventricular fibrillation were the only parameters available until relatively recently. None of these was sufficiently accurate to allow for therapeutic adjustments on an individual case basis. Fortunately, recent technological advances have led to monitoring techniques which allow for much more accurate assessment and guidance of therapy.
Capnometry (the measurement of exhaled end-tidal carbon dioxide levels) has become a valuable and standard monitoring tool in the emergency department. Although used primarily to assess endotracheal tube placement and monitor ventilation, end-tidal carbon dioxide (ET co2) can also be used to assess blood flow. ETco2 is proportional to pulmonary perfusion, which, in turn, reflects systemic perfusion. It is this principle that makes ET co2 monitoring of value in the assessment of perfusion during CPR.
The use of quantitative or semiquantitative ETco2 as an indicator of the effectiveness of artificial perfusion has received considerable attention over the past decade or so. Laboratory models or cardiac arrest demonstrated a statistically significant correlation between ET co2 and coronary perfusion pressure, suggesting that ETco2 could serve as a useful noninvasive method to monitor the effectiveness of resuscitative efforts and to guide therapy. In one clinical study, the initial ET co2 served to predict which patients would regain a pulse during the resuscitation. Those patients with an ET co2 of at least 15 mmHg on arrival in the emergency department (ED) had a greater than 90 percent probability of regaining a pulse, while those patients with an ET co2 of less than 15 mmHg almost never regained a pulse.13 In many of these patients, a sudden, dramatic increase in the ETco2 was noticed well before a pulse could be detected. Unfortunately, ETco2 does not always correlate precisely with coronary perfusion pressure, and the relationship between the two can be affected by therapeutic interventions such as adrenergic therapy. While these studies cannot be used as an endorsement for deciding when to quit or continue CPR, they do lend support to the validity of ET co2 as a reflection of cardiac output. ETco2 is the most accurate noninvasive method of monitoring CPR effectiveness currently available, and its use is encouraged. The most appropriate way to use ET co2 is to maintain minute ventilation relatively constant while adjusting the mechanics of CPR chest compression and titrating adrenergic therapy in an effort to maximize the ETco2.
It has long been recognized that the coarseness of the ventricular fibrillation (VF) waveform has a rough correlation with the duration of cardiac arrest and the prospects for successful defibrillation with ROSC. With the onset of VF, there is a gradually progressive decrease in the peak-to-trough amplitude of the VF waveform in the absence of resuscitative interventions. The recorded amplitude of the VF waveform, however, is subject to such factors as body habitus, electrode location, and contact, and instrumentation. Using mathematical formulas (the fast Fourier transform) and high-speed computers, a digital characterization of the VF waveform can be derived. This method provides a discreet digital number with which to describe the distribution of frequencies present in the waveform. One measure, the median or centroid frequency, has been found to correlate with defibrillation success and to predict the duration of VF. As VF continues without resuscitation, the median frequency gradually deteriorates over time. When effective resuscitation measures are instituted, the median frequency promptly increases. 14 Such interventions as invasive perfusion support or pharmacologic therapy with epinephrine or vasopressin have successfully raised the median frequency of VF and have been associated with enhanced resuscitation success in animal models. The median frequency (or other measures derived from the fast Fourier transform) may eventually find their way to the bedside in the form of a monitor used by the clinician to guide therapy during resuscitation attempts.
Laboratory and clinical studies have demonstrated that both aortic pressure and CPP correlate strongly with coronary blood flow and ROSC. Although placement of arterial pressure catheters is a common occurrence in critical care medicine, arterial catheterization is not routinely performed in cardiac arrest patients. This is partially because of the technical difficulties associated with performing this during CPR. However, arterial pressure monitoring provides a very useful parameter to guide resuscitative efforts. The CPR-diastolic arterial pressure is the major predictor of the actual CPP. Thus, adjusting therapeutic interventions to maximize CPR-diastolic arterial pressure will result in greater CPP and improved chances of survival.
Central venous catheterization in addition to arterial and aortic catheterization allows for the accurate measurement of the aortic-to-right atrial pressure gradient or CPP. Although clearly one of the most useful measurable parameters in human resuscitation, it has been studied by only a few investigators and only as an in-hospital procedure. Thus, out-of-hospital cardiac arrest patients undergoing CPP monitoring have generally been in arrest for an extended period of time. Paradis et al reported that ROSC in the ED correlated with achieving a CPP greater than 15 mmHg among patients with prolonged cardiac arrest. 15 None of the patients with ROSC survived, suggesting that CPP monitoring after prolonged cardiac arrest is likely to be of limited benefit. Thus, efforts to perform invasive monitoring more rapidly upon hospital arrival or even in the prehospital setting should be pursued. The feasibility of performing prehospital invasive hemodynamic monitoring has been demonstrated with the use of a commercially available, lightweight, and portable monitoring system that can be easily transported to the scene of out-of-hospital cardiac arrest.
Rivers et al have reported the use of central venous oximetry in evaluation of cardiac arrest patients. 16 Central venous oxygen saturation yields important information about tissue oxygen delivery/consumption balance and was predictive of ROSC. A central venous oxygen saturation of less than 30% resulted in a zero percent ROSC rate, whereas a value greater than 72% resulted in a 100 percent ROSC rate. Impending ROSC was foreshadowed by an abrupt or gradual increase in central venous oxygen saturation. Interestingly, a supranormal central venous oxygen saturation, termed venous hyperoxia, was frequently seen during the early phase of ROSC, followed by a return to normal levels.
Direct mechanical ventricular assistance (DMVA) was first described by Anstadt et al in 1965 and several laboratory studies have investigated this technique in cardiac arrest models. DMVA utilizes a cup-shaped device that fits around the ventricles and is held in place by a vacuum at the apex of the heart ( Hg.;...20-4). Cyclic positive and negative pressures are transmitted to a flexible diaphragm on the inner surface of the cup, resulting in compression and reexpansion of the ventricles, respectively. The major difference between DMVA and open-chest manual compression of the heart is the active ventricular dilatation, which enhances ventricular filling for the next compression phase. DMVA has been shown to generate higher arterial pressures and greater cardiac output than open-chest manual compression. The clinical utility of this technique in the treatment of cardiac arrest has not been established. The major advantage of DMVA is that it can be sustained for an extended period of time. The major limitation of DMVA is the requirement of a thoracotomy, which largely precludes its use within an effective time frame for most victims of out-of-hospital cardiac arrest.
FIG. 20-4. Direct mechanical ventricular assistance (DMVA). Schematic diagram of DMVA drive system and cup. Note the device actuates the ventricular myocardium into systolic (right) and diastolic (left) configurations. (From Anstadt MP, Anstad GL, Lowe JE: Direct mechanical ventricular actuation: A review. Resuscitation 21:7, 1991, with permission.)
Cardiopulmonary bypass (CPB) (Fig 2.0.-5) is an effective means of providing sustained global perfusion and has been advocated in the treatment of cardiac arrest.
Several laboratory studies have demonstrated improved ROSC and neurologic recovery with CPB compared with standard advanced cardiac life support (ACLS) interventions. There are also several case series describing the successful use of femorofemoral CPB in the treatment of cardiac arrest. The major advantages of CPB are that it can be performed with only a femoral vessel cutdown, artificial perfusion can be sustained for an extended period of time, and perfusion support can be gradually withdrawn. The major disadvantage is the equipment, skill, and time required to perform CPB. A recent report of 10 cardiac arrest victims in whom CPB was initiated in the ED showed that ROSC was achieved in all 10, but there were no long-term survivors.17 The average-time from onset of arrest to onset of CPB support was 32 min. In order to achieve long-term survival, the time interval to CPB support must be much shorter. Unless the technique can be extended to the prehospital setting, it will be of very limited value in the treatment of victims of out-of-hospital cardiac arrest. However, technological advances offer the prospect of developing CPB devices that can be used in the prehospital setting.
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