Arterial Blood Pressure and Coronary Perfusion Pressure

Successful resuscitation of the arrested heart depends on generating adequate CPP during CPR, which has been directly correlated with myocardial blood flow.²⁰ Animal and human studies indicate that a minimum CPP of 15 mm Hg is necessary to achieve ROSC if initial defibrillation attempts have failed.^20,21 Unfortunately, CPP monitoring is rarely feasible in ED resuscitations of cardiac arrest patients, because it requires both an indwelling arterial pressure catheter and a central venous catheter, both transduced properly to provide simultaneous readings.

Invasive arterial blood pressure monitoring alone can be helpful in guiding resuscitation and should be used when an indwelling arterial pressure catheter is already in place. When adequate personnel are available, it is often feasible to cannulate the femoral artery during CPR, especially with ultrasound guidance. Human studies have shown that radial or femoral arterial relaxation pressures reliably reflect aortic relaxation pressures during CPR.22,23 Researchers have also reported that a CPP of 15 mm Hg or more was required to achieve ROSC and that no patient with an aortic relaxation pressure of less than 17 mm Hg achieved ROSC.^21,24 Titrating resuscitation efforts to arterial relaxation (diastolic) pressure is less reliable than CPP because improper CPR (e.g., leaning on chest during CPR diastole and hyperventilation) can cause undetected elevations in right atrial pressure, reducing coronary perfusion. Despite this limitation, it is reasonable to titrate resuscitation efforts to achieve an arterial relaxation (diastolic) pressure of 40 mm Hg or more when invasive arterial pressure monitoring is available. Invasive arterial pressure monitoring during CPR may also be useful to facilitate distinguishing PEA that does or does not result in mechanical heart contraction, to detect ROSC, and to assist in serial arterial blood gas monitoring. Although arterial and central venous catheters are most often placed in the post–cardiac arrest phase of care, a significant number of patients initially achieving ROSC will rearrest in the ED, making these modalities helpful during the patient’s subsequent resuscitation.

End-tidal Carbon Dioxide

The partial pressure of CO₂ in exhaled air at the end of expiration (PETCO₂) can be a reliable indicator of cardiac output during CPR. This is most reliably measured through waveform capnography after endotracheal intubation. PETCO₂ depends on CO₂ production, alveolar ventilation, and pulmonary blood flow (i.e., cardiac output) and correlates well with CPP and cerebral perfusion pressure during CPR.24,25 Therefore when minute ventilation is held constant (a desirable but often unmet goal) and no exogenous CO₂ is introduced (e.g., sodium bicarbonate [NaHCO₃] administration), only increased cardiac output during CPR and ROSC significantly increases PETCO₂. Resuscitation after cardiac arrest is likely to fail if PETCO₂ values are less than 10 mm Hg,^26,27 and such values should prompt the clinician to enhance the quality of CPR (either rate or force of compression) or consider more invasive maneuvers such as extracorporeal membrane oxygenation (ECMO) if the situation warrants and a good neurologic outcome is believed to be possible.²⁸

PETCO₂ monitoring also can aid in the diagnosis and treatment of PEA. Patients in a state of PEA with mechanical heart activity may have pulsatile flow that simply cannot be detected by palpation of a pulse. In such circumstances, PETCO₂ levels may be elevated even without compressions. Use of cardiac ultrasound in such cases can prove to be helpful in identifying corresponding cardiac activity. In such cases, volume expansion or the use of vasopressors and inotropes should be considered. PETCO₂ monitoring also is useful in rapidly detecting success of tension pneumothorax decompression, pericardiocentesis for pericardial tamponade, and fluid resuscitation for hypovolemia. ROSC causes immediate and significant increases in PETCO₂. Therefore PETCO₂ monitoring can detect ROSC at any time during the chest compression cycle, providing valuable guidance for pharmacologic therapies and minimizing the need for a pulse check when organized rhythms are detected (Fig. 9-1).

Finally, PETCO₂ monitoring is valuable in patients after ROSC to monitor endotracheal tube placement (waveform capnography recommended), titrate minute ventilation to avoid hyperventilation, and detect sudden hemodynamic deterioration.

Central Venous Oxygen Saturation

ScvO₂, when available, provides an additional method to monitor adequacy of resuscitative measures^. The mixed venous blood oxygen saturation in the pulmonary artery () represents the oxygen remaining in the blood after systemic extraction. Studies have shown a close correlation between ScvO₂ and during CPR.²⁹ Because oxygen consumption remains relatively constant during CPR, as do arterial oxygen saturation (SaO₂) and hemoglobin, changes in ScvO₂ reflect changes in oxygen delivery by means of changes in cardiac output.

Although used primarily in the intensive care unit (ICU) setting, multilumen oximetric ScvO₂ catheters are placed in the same manner as regular central venous catheters and can be used to monitor ScvO₂ continuously in real time. ScvO₂ values normally range from 60 to 80%. During cardiac arrest and CPR, these values range from 25 to 35%, indicating greatly enhanced oxygen extraction of tissues owing to the inadequacy of oxygen delivery during CPR. Failure to achieve an ScvO₂ of 40% or greater has a negative predictive value for ROSC of almost 100%.²⁹ ScvO₂ also helps to rapidly detect ROSC without interruption of chest compressions, as ROSC will result in a rapid increase in Scvo₂ as a result of a dramatic increase in oxygen delivery to tissues. ScvO₂ monitoring is also useful in the post–cardiac arrest period for hemodynamic optimization and for recognition of any sudden deterioration in the patient’s clinical condition.

Echocardiography

The main usefulness of echocardiography is diagnostic, especially in patients with PEA. Echocardiography distinguishes organized pulseless cardiac activity that does or does not result in mechanical heart contraction. It also may be helpful in diagnosing mechanical causes of PEA, such as tension pneumothorax, pericardial tamponade, and pulmonary embolism.³⁰ Echocardiography also is useful in guiding pericardiocentesis. In the postarrest period, echocardiography could prove to be valuable in determining the need for postarrest cardiac intervention or mechanical assistance of the failing heart.³¹

Laboratory Testing

Intermittent arterial and venous blood sampling for gas or chemistry analysis is of limited usefulness during CPR. Typical blood gas findings during CPR demonstrate venous respiratory acidosis and arterial respiratory alkalosis. SaO₂ is usually greater than 94% during CPR and of little value in titrating resuscitation therapy except in the case of massive pulmonary embolism or unrecognized esophageal intubation. Although ScvO₂ indicates adequacy of CPR, a single measurement may not be as useful as continuous oximetric ScvO₂ monitoring.

Other laboratory studies during CPR serve more to confirm a diagnosis than to guide therapy because results are usually available too late to make a difference. Serum electrolytes may be ordered to rule out hyperkalemia, hypokalemia, hypomagnesemia, hypercalcemia, and hypocalcemia; however, empirical therapy should be initiated immediately if a high clinical suspicion exists. Hemoglobin levels may indicate hemorrhage, but initial hemoglobin may be normal even in acute exsanguinating hemorrhage owing to a lack of rapid vascular and interstitial compartment equilibration.