Difference in end-tidal CO2 between asphyxia cardiac arrest and ventricular fibrillation/pulseless ventricular tachycardia cardiac arrest in the prehospital setting

Introduction There has been increased interest in the use of capnometry in recent years. During cardiopulmonary resuscitation (CPR), the partial pressure of end-tidal carbon dioxide (PetCO2) correlates with cardiac output and, consequently, it has a prognostic value in CPR. This study was undertaken to compare the initial PetCO2 and the PetCO2 after 1 min during CPR in asphyxial cardiac arrest versus primary cardiac arrest. Methods The prospective observational study included two groups of patients: cardiac arrest due to asphyxia with initial rhythm asystole or pulseless electrical activity, and cardiac arrest due to acute myocardial infarction or malignant arrhythmias with initial rhythm ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). The PetCO2 was measured for both groups immediately after intubation and then repeatedly every minute, both for patients with and without return of spontaneous circulation (ROSC). Results We analyzed 44 patients with asphyxial cardiac arrest and 141 patients with primary cardiac arrest. The first group showed no significant difference in the initial value of the PetCO2, even when we compared those with and without ROSC. There was a significant difference in the PetCO2 after 1 min of CPR between those patients with ROSC and those without ROSC. The mean value for all patients was significantly higher in the group with asphyxial arrest. In the group with VF/VT arrest there was a significant difference in the initial PetCO2 between patients without and with ROSC. In all patients with ROSC the initial PetCO2 was higher than 10 mmHg. Conclusions The initial PetCO2 is significantly higher in asphyxial arrest than in VT/VF cardiac arrest. Regarding asphyxial arrest there is also no difference in values of initial PetCO2 between patients with and without ROSC. On the contrary, there is a significant difference in values of the initial PetCO2 in the VF/VT cardiac arrest between patients with and without ROSC. This difference could prove to be useful as one of the methods in prehospital diagnostic procedures and attendance of cardiac arrest. For this reason we should always include other clinical and laboratory tests.


Introduction
Monitoring of end-tidal CO 2 has become a standard in the prehospital setting to ensure proper placement and function of the endotracheal tube and to help monitor the adequacy of ventilation [1]. In addition, it has been noted that cardiac arrest causes an abrupt fall in end-tidal CO 2 levels to values near zero [2,3]. During cardiac arrest the partial pressure of end-tidal carbon dioxide (PetCO 2 ) falls to very low levels, reflecting the very low cardiac output achieved with cardiopulmonary resuscitation (CPR). It has been shown that the PetCO 2 achieved during advanced cardiac life support reliably predicts an outcome of cardiac arrest [2][3][4][5][6][7][8][9][10][11][12]. Higher levels of the PetCO 2 indicate better cardiac output, higher coronary perfusion pressure and a greater likelihood of successful resuscitation [13,14]. After the onset of cardiac arrest caused by ventricular fibrillation (VF), the PetCO 2 abruptly decreases to nearly zero and then begins to increase after the onset of effective CPR. Further increase is detected upon return of spontaneous circulation (ROSC) to normal or above-normal levels [2, 3,9,12].
In an experimental animal model of asphyxial arrest during CPR, PetCO 2 levels were initially high (after the onset of arrest), then decreased to subnormal levels and then increased again to near-normal levels [15,16]. During a respiratory arrest, the cardiac output of pulmonary blood flow continues for some period of time prior to cardiac standstill. The CO 2 produced in the tissue during this period will continue to be delivered to the lungs, thereby increasing alveolar CO 2 (two-compartment hydraulic model of CO 2 kinetics). However, it is also important to recognize that it is not only the cessation of cardiac output alone that causes the fall of PetCO 2 , but the cessation in conjuction with the washout of alveolar gas. This means that, in the absence of alveolar gas washout, CO 2 will remain in the lungs and probably that, as alveolar oxygen is being utilized, more CO 2 will be delivered.
On the basis of such a concept we built a hypothesis maintaining that the initial PetCO 2 should be higher in an asphyxial arrest model than in a VF/pulseless ventricular tachycardia (VT) cardiac arrest model. In the asphyxial cardiac arrest model there should also be no difference in patients with and without ROSC regarding the initial PetCO 2 , since the initial PetCO 2 in this case reflects CO 2 cumulated in the alveolar compartment. This would suggest that the initial values of end-tidal carbon dioxide in asphyxial arrest do not have a prognostic value for ROSC as they do in VF/VT cardiac arrest.
If our results confirm both hypotheses, then this difference could be helpful in determining the mechanism of arrest in the prehospital setting.

Methods
This prospective observational study was conducted at the Center of Emergency Medicine, Maribor. The study included two groups of patients. The first group represented patients who suffered from heart arrest due to asphyxia. The causes of asphyxia included a foreign body in the airway, aspiration, suicide by hanging, drowning, edema or tumor of the airway, intoxication and acute asthma attack. The definitive cause of arrest has been confirmed in the hospital with further diag-nostic and/or pathological report (autopsy). The initial rhythm was either asystole or pulseless electrical activity (all patients from this group with VT/VF as the initial rhythm were excluded). Patients with severe hypothermia (core temperature < 30°C) were also excluded.
The second group included the patients with primary cardiac arrest (acute myocardial infarction or malignant arrhythmias). The initial rhythm was VF/VT (all patients from this group presenting with asystole or pulseless electrical activity were excluded). The definitive diagnosis (cause of arrest) was confirmed in the hospital (further diagnostic and/or pathological/autopsy report). The inclusion/exclusion criteria for asphyxia and VF/VT group are presented in Table 1.
The resuscitation procedures were performed by an emergency team (emergency medical doctor and two emergency . Measurements for both groups were made immediately after intubation (first measurement) and then repeatedly every minute continuously. Endotracheal intubations were performed after two initial breaths with a valved bag at the beginning of CPR. Further ventilation was performed by mechanical ventilator (6-8 ml/kg at 10-12 breaths/min; Medumat Standard Weinmann, Weinmann, Namburg, Germany). The CO 2 cuvette was located in a connector between the mechanical ventilator and the endotracheal tube (it was applied to the endotracheal tube before intubation). Two patients were not intubated by the orotracheal technique because of complete obstruction of the upper airway, visualized by laringoscopy. In these two cases cricotireideotomy was performed using the traceoquick method (Tracheoquick Emergency Coniostomy Set; Willy Rüsch AG, Kernen, Germany). The procedure was performed in accordance with the instructions of the manufacturer, and both patients were successfully resuscitated and ventilated by mechanical ventilator.
The initial (first measurement after intubation), average (mean of all values obtained during a single resuscitation effort) and final (measurement at admission to hospital or discontinued CPR) PetCO 2 was detected for both groups. We performed the same procedure for the patients with ROSC and for those without ROSC.
ROSC is defined as the return of spontaneous heartbeat or as palpable periferial arterial pulse and measurable systolic arterial pressure. As is seen from the Utstein style template, we distinguish intermittent ROSC, which is short in duration and a temporary event, from ROSC with hospitalization of a patient. In the present article, ROSC represents hospitalized patients.
The paired Student t test was used to compare initial and subsequent PetCO 2 values for each subject. For other parameters, both groups (asphyxial arrest group and VF/VT cardiac arrest group) were compared by Student's t test and the chisquared test. Continuous variables are described as the mean ± standard deviation. P < 0.05 was considered significant.

Results
From February 1998 to October 2002 we analyzed 141 patients with primary cardiac arrest (initial rhythm VF/VT) and 44 patients with cardiac arrest due to asphyxia (initial rhythm asystole or pulseless electrical activity). The study environment, the prehospital environment and the characteristics of cardiac arrest and noncardiac arrest are displayed in Fig. 1a,b (Utstein style). The causes of asphyxial cardiac Available online http://ccforum.com/content/7/6/R? Figure 1 (a) Cardiac arrests placed into the Utstein template. *It was not possible to determine the number of resuscitations not attempted because records for patients who were pronounced dead at the scene were not available. **Return of spontaneous circulation (ROSC

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arrest were solid foreign body in the airway (seven cases), aspiration (seven cases), edema or tumor of the upper airway (five cases), hanging (five cases), acute asthma attack (six cases), drowning (six cases) and intoxications with respiratory arrest (eight cases). Demographic and clinical characteristics for both groups are presented in Table 2.
The values of the PetCO 2 are presented in

Discussion
In the present study we confirmed that the PetCO 2 was markedly elevated during the first minute of CPR in asphyxial cardiac arrest. This study therefore confirmed the results of the studies that used animal models in which cardiopulmonary arrest was induced by asphyxia. In the present study the PetCO 2 values during CPR were initially high, then decreased to subnormal levels and then increased again to near-normal levels in patients with ROSC. This pattern of PetCO 2 changes is different from the pattern observed in   Table 3 The cardiac arrest caused by VF, since cardiac arrest from VF results in an abrupt cessation of cardiac output and pulmonary blood flow. Bhende and colleagues [15], Berg and colleagues [16] and von Planta and colleagues [20] concluded that, during the period of asphyxia, continued cardiac output prior to cardiac arrest permits continued delivery of CO 2 to the lungs, which (in the absence of exhalation) results in higher alveolar CO 2 . This is reflected as increased PetCO 2 when ventilation is resumed.

mean values for all patients of the initial, final, average and after 1 min of cardiopulmonary resuscitation (CPR) partial pressures of end-tidal carbon dioxide (PetCO 2 ) for arrest due to asphyxia and for ventricular fibrillation/pulseless ventricular tachycardia (VF/VT) cardiac arrest
Understanding the physiology of CO 2 production, delivery to the lungs and excretion are important in order to appropriately interpret PetCO 2 monitoring during CPR. The disposition of CO 2 can also be represented in a hydraulic model [21]. The large peripheral tissue compartment drains through a conduit (cardiac output) into the small central pulmonary compartment. The tissues produce CO 2 , which empties into the peripheral tissue compartment. Carbon dioxide then flows by gravity (cardiac output) from the higher level tissue to the lower level pulmonary compartment. Alveolar ventilation, which equals expired ventilation minus ventilation of the anatomical dead space, and the effects of high ratio ventilation/perfusion matching eliminate CO 2 from the lung. In this model the cardiac output affects the distribution and total amount of CO 2 in the body and can help to understand the meaning of the PetCO 2 during CPR.
The present study discovered that we can trace the same pattern of PetCO 2 changes in the asphyxial arrest as were described in the animal models in the first minute after arrest [15,16], even after a longer period of time due to the access time. The inability to measure the PetCO 2 immediately after cardiac arrest was the main disability of this study.
We also concluded that the high initial values of the PetCO 2 in asphyxial arrest do not have a prognostic value for the appearance of ROSC as they do in the VT/VF cardiac arrest. On the contrary, the values after 1 min of CPR and also the final values of the PetCO 2 do have the prognostic value for ROSC. These data, like those from Berg and colleagues [16], suggest that the PetCO 2 during the initial phase of CPR of asphyxial arrest (1 min after intubation and cardiac massage) reflects alveolar CO 2 prior to CPR. In the asphyxial model, cellular respiration results in continued oxygen consumption and CO 2 production. The high pressure of CO 2 in the alveolar compartment is reflected in the high PetCO 2 during the initial phase of CPR.
The fast decline of the high values of the PetCO 2 can therefore only be interpreted by ventilation of the alveolar compartment, which then rapidly decreases the PetCO 2 . However, in the next phase and with the beginning of CPR we can again detect the rise of the PetCO 2 . This rise is achieved by successful cardiac massage, which washes the acumulated CO 2 out of the peripheral compartment [11,[21][22][23].
The acquaintance with this pattern of changes can be helpful in differentiation of cardiac arrest causes and in identification of mechanisms that led to cardiac arrest. This is very useful in the prehospital setting and can lead the course of action as hypoxia is a potentially reversible cause of cardiac arrest. The issue is potentially important when deciding upon the most effective sequence of resuscitation intervention. There is growing evidence that indicates positive pressure ventilation may be postponed for several minutes in instances of arrythmic arrest whereas it might be life-saving in instances of asphyxial arrest. The emergency medical doctor can therefore be orientated with greater reassurance towards the measures that are useful in asphyxial arrest [24][25][26][27]. However, one has to be aware that the initial values of the PetCO 2 in asphyxial arrest do not have the prognostic value for the outcome of CPR that they do have in VF/VT arrest [4][5][6][8][9][10].

Conclusions
The initial values of the PetCO 2 in asphyxial cardiac arrest are significantly higher than in VF/VT cardiac arrest. In asphyxial arrest there is also no significant difference in inital values of the PetCO 2 in patients with and without ROSC. In asphyxial arrest the initial values of the PetCO 2 therefore cannot be used as a prognostic factor of outcome of CPR, as they can be used in VF/VT cardiac arrest. This difference, together with other criteria, can therefore be useful for differentiation between the causes of cardiac arrest in the prehospital setting. For standard use of this difference in the PetCO 2 in the prehospital setting we suggest additional clinical research.