Capnography: Clinical Issues (Textbook)
I recently bought this book, which as far as I can tell is the only close to up-to-date Capnography text book available.
While the book is it not directed at EMS, it does have some chapters dealing with prehospital use of capnography. I will be slowly making my way through the book and will try to notate some of the hightlights below:
Chapter One – Clinical Perspectives
Emphasizes that a capnogram is a “snapshot in time,” a brief episode in a phase of a patient’s disease.” Every change will induce other changes as the body tries to maintain homeostasis. Capnography can only tell part of the story.
Capnography represents pulmonary perfusion so a drop in cardiac output will be reflected in a drop in pulmonary perfusion and thus show up as evidence captured by capnography.
Intubation of right mainstem bronchus will produce lower C02 values because only one lung is being perfused.
Chapter Two – Capnography and Respiratory Assessment outside of the Operating Room
We evaluate respiratory status by watching chest rise, respirations, use of accessory muscles and listening to breath sounds. The problem is the patient the patient needs to cooperate.
Capnography provides continuous readings, regardless of patient cooperation.
While the ultimate test of ventilation is the arterial blood gas, it is an invasive procedure that provides only a momentary value.
End tidal CO2 is an established standard of care for patient monitoring, according to the American Society of Anesthesiologists.
The American Heart Association considers capnography the standard of care for determining ET position.
Colorimetric CO2 indicators are too sensitive to even a low amount of CO2 so they may give false readings.
In stable patients with normal body temperatures, arterial blood gases and end tidal CO2 should be the same.
Capnography is the superior method of measuring respirations or respiratory disruptions compared to visualization or pulse oximetery.
Chapter Three- Airway Management: Prehospital Setting
Unrecognized misplaced intubations (UMI) have occurred at an alarming rate in the prehospital environment. 2/3 of misplaced tubes are in the esopagus, one third in the hypopharanx.
Pulse oximetry may take a few minutes to detect desaturation, especially when the patient has been preoxygenated. End tidal CO2 can detect problem immediately so action can be taken before the patient becomes hypoxemic.
The EDD (esophageal detector device) is unreliable in patients with morbid obesity, pulmonary edema, or ET obstruction by blood or vomit. The EDD does not provide continuous airway confirmation. It is also contraindicated in children under five and pregnant patients.
When patients in arrest have a return of spontaneous circulation, it causes an instantaneous increase in ETCO2, prompting rescuers to stop CPR.
If a wave form is present, the tube is correct.
While ingestion of carbonated beverages may briefly produce ETCO2 readings, the wave form is vastly different from a normal waveform and the ETCO2 falls rapidly to zero.
In a study by Silvestri (see Clinical Studies) continuous monitoring reduced UMI to 0 from 9% for colorimetric and 25% from no capnography.
Chapter Four: Airway Management in the Intensive Care Setting
Endobroncial intubation is a major cause of desaturation.
Capnography can be used to place enteric tubes.
Chapter Five – Airway Management in the Operating Room
Head and neck movement can move an ET tube 5 cm. Problem is worse in infants.
Capnography can be used for blind nasal intubation.
Capnography works with the LMA in spontaneously breathing patients.
Chapter Seven: Monitoring During Mechanical Ventilation
Changes in the metabolic rate change CO2 elimination.
Increased metabolism can be caused by fever, sepsis, pain and seizures.
Decreased metabolism can be caused by hypothermia and sedation.
ETCO2 changes should alert caregiver to changes in the cardiovascular system.
Hypovolemia will cause a decrease in ETCO2
Pulse oximetry provides inadequate monitoring of patient’s respiratory status when the patient is receiving oxygenation.
Increasing ETCO2 may indicate muscle fatigue and need to assist ventilations.
Chapter Eight: Capnography in Transport
Respiratory change is common during transport.
Capnography can be used to place a nasal ET tube. The ETCO2 will increase as the tube approaches the chords and decrease if it starts going down the esophagus.
A number of patients with PEA may actually have cardiac activity. Capnography can distinguish between PEA and low cardiac output states.
Because ETCO2 measures cardiac output, rescuer fatigue during CPR will show up as decreasing ETCO2.
Cardiac arrest survivors had an average ETCO2 of 18 20 minutes into an arrest, non survivors averaged 6. In another study, survivors averaged 19, and non-survivors 5.
But survivors with a low initial ETCO2 are not uncommon.
Bicarb causes a temporary <2 minute rise in ETCO2, high-dose epi can cause a decrease.
High 02 concentrations can lower ETCO2 by 10%.
Chapter Nine: Capnography as a Guide to Ventilation in the Field
Head Injured Patients
End tidal C02 is very useful in monitoring intubated head injured patients. The chapter discusses the problem of prehospital hyperventilation due to the excitement of the situation (which often leads medics to unknowingly over ventilate). Since hyperventilation leads to hypocapnia, which can exacerbate cerebral ischemia, the chapter recommends a target end tidal C02 value of 35. It discusses permissive hypercapnia which may lead to increased cerebral perfusion and improved outcomes. It also says ischemia can occur quite quickly and the brain is very vulnerable right after it has sustained injury.
Chapter Twelve: Capnography During Sedation
Ventilatory depression is common during sedation and often is a greater risk to the patient than the procedure being performed.
While some have advocated withholding supplemental oxygen during sedation to enable a pulse oximeter to more quickly identify hypoventilation, it makes more sense to provide oxygen and monitor for hypoventilation with capnography.
The American Dental Association uses capnography as its standard of care for patients undergoing deep sedation.
The following can, in some cases, hinder accurate readings of ETCO2: secretions, partial obstruction, low tidal volumes, rapid breathing, large diameter of nasal prongs, mouth breathing, dilution by supplemntal oxygen.
Chapter Fifteen - Therapeutic Use of Ambulatory Capnography
Respiratory Training with Capnography can raise a patient's ETCO2 and reduce their vulnerability to panic attacks. This can also apply to asthma patients to help improve their breathing techniques. Decreased PetCO2 can lead to a number of "autonomic, endocrine and metabolic disturbances, which contribute to the pathophysiology of asthma."
In treating asthma and hyperventilation patients, the basic theraputic priniciple is to "keep the PaCO2 high; if neccessary, make it high, and above all, prevent it from being low."
Chapter Twenty - Cardiopulmonary Resuscitation
Animal and human studies have shown a great correlation between ETCO2 and cardiac output during low flow states and during CPR.
When someone tires doing CPR the ETCO2 goes down, when a fresh resucer takes over it usually goes back up.
Patient's not rescusitated show a gradual dimminution of ETCO2 during CPR.
Routine monitoring of ETCO2 during cardiac arrest is preferable to palpating for carotid pulses in monitoring how well CPR is working.
While ETCO2 appears to mirror cardiac output, it may not be applicable to cerebral blood flow.
The administration of Sodium Bicarbonate will show a transient increase in ETCO2 that returns to baseline within five minutes.
ETCO2 may decrease following administration of epinephrine, possibly because of redistribution of blood flow duw to vasoconstiction and increased afterload. This decrease may actually be a good sign because it reveals the presence of vasomotor response.
No patient with an ETCO2 reading of less than 10 after 20 minutes of CPR was rescusitated in any of the studies done.
The higher the initial ETCO2 the better the chance for resiuscitation, but patients with low initial readings have survived.
A rise in ETCO2 is almost always noted prior to having a palpable pulse when a patient has ROSC.
Chapter Twenty-One Embolism
Pulmonary embolism increases alveolar deadspace. This deadspace can possibly be differentiated from deadspace due to COPD due to a difference in the upslope of the capnogram.