Making Waves: The Use of Waveform Capnography for Procedural Sedation in the Cardiac Cath Lab

Capnography is an underutilized technology that could potentially save lives. By definition, it is simply the measurement of a patient’s exhaled carbon dioxide represented as a waveform on a monitor, but it is so much more than that. It provides a real-time analysis of a patient’s ventilations and an indirect analysis of a patient’s level of sedation. Capnography has long been used by anesthesia providers in the operating room, but has, in recent years, slowly made its way into other areas outside of the operating room. Capnography is slowly becoming the standard of care in settings such as the emergency department, the gastrointestinal lab, and prehospital services where procedural sedation is commonplace. It has a wide range of applications and shows much potential for use in the cardiac cath lab.

Currently, it is the common practice to measure blood pressure, heart rate, cardiac monitor, and pulse oximetry during procedural sedation in the cath lab.¹ This leaves out a key piece of the puzzle required for safe sedation: ventilations! Most practitioners evaluate respirations by either looking for chest rise and fall, or by using a cardiac monitor lead to pick up on movement in the chest. The problem with these practices is that the patient’s chest movement is difficult to evaluate when covered by a sterile drape and C-arm.¹ It is also difficult to place a cardiac monitor lead over the chest wall without getting in the way of radiographic images.

There are many pitfalls associated with monitoring only pulse oximetry. Oxygenation is the process of delivering oxygen to the cells in our body. Pulse oximetry measures oxygenation. Ventilation is the mechanical process of moving oxygen into the body and expelling carbon dioxide. Capnography measures ventilations. Not only does capnography measure ventilations, but it does so instantly, in real time.² You can see  hypopnea or apnea instantly. With pulse oximetry, it may take thirty to ninety seconds longer to see a decrease in the reading and detect the same event.³ This is especially true with the use of supplemental oxygen via a nasal cannula or face mask. The extra oxygen can mask the effect of the hypopnea or apnea.³ The scary part is that the use of supplemental oxygen is a common practice in the cath lab. To demonstrate this principle in an anecdotal way, I hooked myself up to both a capnography monitor and a pulse oximetry monitor. I breathed regularly for one minute to get a baseline of my respiratory status and then held my breath. The capnography monitor detected apnea immediately and alarmed after twenty seconds of apnea. The pulse oximeter recorded a one point drop after sixty seconds of apnea when I could no longer hold my breath.  Evaluating a patient’s respiratory function with only pulse oximetry is like evaluating a patient’s cardiac function with only a blood pressure and not seeing their electrocardiogram.

The goal of moderate sedation/ analgesia is to reduce the patient’s anxiety and pain during an otherwise uncomfortable procedure. The amount of medication required to move a patient from moderate sedation to deep sedation or even general anesthesia is often not much. As such, over sedation and respiratory compromise are a real concern. Likewise, a practitioner fearing over sedation may under sedate their patient, resulting in a very uncomfortable and traumatic experience for the patient. This is where the benefits of capnography in the cath lab can truly be seen. Ultimately, the use of capnography in the cath lab can lead to a safer, more tolerable experience for the patient.^2,3

While capnography is a wonderful tool to aid the cath lab professional, it does have limitations. Under normal conditions, there is a direct correlation between the amount of carbon dioxide in our arterial blood (PaCO₂) and the amount of carbon dioxide at the end of exhalation (etCO₂). A problem arises when a patient presents with a ventilation perfusion mismatch as can happen with shunting or pulmonary embolism.³ A practitioner has to stay sharp and watch for signs of these clinical conditions, as they can result in false readings. The use of capnography can also be limited by budget concerns or uninformed decision makers. While relatively inexpensive, recent changes to the economy and healthcare have made many departments look for ways to cut spending.

Capnography is now standard procedure in areas such as the emergency room and endoscopy lab. It is slowly being introduced to other areas where procedural sedation is used, including the cardiac cath lab. It is a relatively cost-effective technology with signs of great promise. As we look to the future with ways to provide evidence-based, high-caliber patient care, capnography should definitely be added to the list of equipment utilized in the cardiac cath lab to monitor the patient’s condition during the procedure.

Travis Mackey can be contacted at travismackey_1@hotmail.com.

References

1. Kern MJ. The Cardiac Catheterization Handbook. 4th ed. St. Louis, MO: Mosby;2003:12-16.
2. Krauss B, Hess DR. Capnography for procedural sedation and analgesia in the emergency department. Ann Emerg Med 2007 Aug;50(2):172-181.
3. Cacho G, Pérez-Calle JL, Barbado A, et al. Capnography is superior to pulse oximetry for the detection of respiratory depression during colonoscopy. Rev Esp Enferm Dig 2010 Feb;102(2):86-89.