2.8 Physiological Monitoring & CPR Quality Metrics

2.8 Physiological Monitoring & CPR Quality Metrics

Historically, the quality of cardiopulmonary resuscitation (CPR) was assessed purely through clinical observation—watching chest rise, feeling for a femoral pulse during compressions, and estimating compression depth and rate. Modern resuscitation science has fundamentally shifted toward quantitative, data-driven approaches. The AHA ACLS guidelines strongly advocate for the use of advanced physiological monitoring and real-time feedback devices to continuously assess the efficacy of resuscitative efforts, guide interventions, and reliably detect the return of spontaneous circulation (ROSC).

Waveform Capnography

Continuous quantitative waveform capnography is arguably the most important monitoring tool in advanced resuscitation. It measures the partial pressure of carbon dioxide (CO2) in exhaled air at the end of exhalation, known as End-Tidal CO2 (ETCO2).

The physiology underpinning capnography during cardiac arrest is elegant. Under normal circumstances, cells produce CO2 via metabolism. This CO2 is carried by venous blood to the lungs, where it diffuses into the alveoli and is exhaled. During cardiac arrest, cellular metabolism continues, but blood flow stops. Therefore, no CO2 is delivered to the lungs, and ETCO2 drops to zero. When chest compressions are initiated, artificial cardiac output is generated, delivering CO2 to the lungs. Thus, during an arrest, ETCO2 is essentially a non-invasive surrogate for cardiac output and pulmonary blood flow.

Clinical Applications of Capnography

1. Confirming Airway Placement: Capnography is the gold standard for confirming the proper placement of an endotracheal tube. A consistent, repeating CO2 waveform confirms that the tube is in the trachea and not the esophagus.
2. Monitoring CPR Quality: High-quality chest compressions generate higher cardiac output, delivering more CO2 to the lungs. The ACLS guidelines state that providers should try to achieve an ETCO2 of at least 10 to 20 mmHg. If the ETCO2 falls below 10 mmHg, it indicates that the CPR quality needs improvement (e.g., pushing harder, pushing faster, ensuring full recoil) or that provider fatigue has set in, prompting a compressor switch.
3. Detecting ROSC: When the heart spontaneously begins beating again (ROSC), there is a massive surge in cardiac output compared to what chest compressions can provide. This leads to a sudden "washout" of accumulated CO2 from the venous system into the lungs. An abrupt and sustained increase in ETCO2—typically shooting up to a normal range of 35 to 45 mmHg—is one of the earliest and most reliable indicators of ROSC. It often precedes a palpable pulse and allows providers to recognize ROSC without interrupting compressions for a pulse check.
4. Prognostication: A persistently low ETCO2 (<10 mmHg) after 20 minutes of high-quality CPR and advanced life support interventions is associated with an extremely low probability of survival and may be used as one factor in the difficult decision to terminate resuscitative efforts.

Invasive Arterial Blood Pressure Monitoring

If a patient already has an invasive arterial line in place at the time of cardiac arrest (common in Intensive Care Units or operating rooms), it provides invaluable real-time hemodynamic data. An arterial line directly measures the pressure generated by chest compressions in the arterial system.

The most critical value to monitor during CPR is the arterial relaxation (diastolic) pressure. This pressure represents the driving force for coronary perfusion. The coronary arteries primarily fill during the relaxation phase of CPR (when pressure is taken off the chest). The ACLS guidelines recommend attempting to improve CPR quality or utilizing vasopressors to maintain a target diastolic pressure of greater than 20 mmHg. If the diastolic pressure drops below 20 mmHg, coronary perfusion is severely compromised, significantly reducing the likelihood of achieving ROSC.

Central Venous Oxygen Saturation (ScvO2)

In patients with a central venous catheter in the superior vena cava, central venous oxygen saturation (ScvO2) can be monitored. ScvO2 reflects the balance between oxygen delivery and oxygen consumption in the tissues. During cardiac arrest, oxygen delivery is abysmal, leading to maximum oxygen extraction by the tissues and a subsequent precipitous drop in ScvO2.

Continuous measurement of ScvO2 can help guide resuscitation. The AHA suggests targeting an ScvO2 of greater than 30%. If the ScvO2 remains below 30%, it indicates inadequate tissue perfusion, prompting the resuscitation team to evaluate and optimize the quality of chest compressions and the overall resuscitation strategy.

CPR Feedback Devices

Beyond physiological metrics, the physical mechanics of CPR can now be closely monitored using real-time audiovisual feedback devices. These devices typically employ accelerometers and pressure sensors placed between the provider's hands and the patient's sternum, or they are integrated directly into modern defibrillator pads.

These devices provide instantaneous feedback on the fundamental components of high-quality CPR:

• Compression Rate: Ensuring the rate stays strictly between 100 and 120 compressions per minute.
• Compression Depth: Confirming a depth of at least 2 inches (5 cm), but no more than 2.4 inches (6 cm).
• Chest Recoil: Detecting if providers are "leaning" on the chest, preventing full re-expansion and thereby inhibiting venous return to the heart.
• Chest Compression Fraction (CCF): The proportion of the total arrest time that compressions are being performed. The goal is a CCF of >80%, minimizing peri-shock pauses and other interruptions.

The integration of these feedback devices prevents the degradation of CPR quality over time and allows the team leader to provide specific, data-driven coaching during the code, ultimately translating to higher rates of survival.