Capnography, King of the ABC's

Atypical hypoventilation, technically called “hypopneic hypoventilation”, occurs most commonly in procedural sedation, analgesia and seizures. This counterintuitive phenomenon causes normal or decreased ETCO2 despite a low RR, hypercapnia and a normal circulatory and metabolic status. Atypical hypoventilation occurs when tidal volume becomes excessively lower than the patient’s already low RR. Even though CO2 is still being delivered to the lungs faster than it is being exhaled and PaCO2 is increasing (hypercapnia), the patient’s low tidal volume creates a decreased capacity for gas exchange. Depending on the severity of the patient’s low tidal volume, ETCO2 may be normal or low, but it will not be high like it is in classic hypoventilation where tidal volumes are adequate. Note that further assessments are required to distinguish a low ETCO2 caused by atypical hypoventilation vs. a circulatory or metabolic issues. Once the paramedic has completed the initial impression, provided BLS interventions of an immediate nature and assessed the waveform, the third step involves analyzing the patient’s V/Q ratio. An understanding of the V/Q ratio is the “gold nugget” of capnography as it can give the deepest insight into the patient’s physiology. However, in order to grasp the V/Q ratio concept, the paramedic must first have a complete understanding of what governs RR and how respiratory, circulatory and metabolic changes affect ETCO2 values. To understand the capnography of CO2 retention, the paramedic needs to understand that ETCO2 is a measurement of pressure and is directly affected by the pCO2 in the pulmonary circulation. Regardless of whether the etiology is chronic or acute, when a patient retains CO2, a “bottleneck traffic jam” ensues in the pulmonary circulation causing “back pressure”, which results in hypercapnia and increases in ETCO2. CO2 retaining patients compensate for their condition by having a higher than normal RR to combat their hypercapnia (as well as their hypoxia) and may also retain bicarbonate to combat metabolic acidosis if the pathology is chronic. CHF waveforms are normal in shape because CO2 diffuses through fluid at the same rate it does through air due to CO2’s high solubility1, 2. Even in the presence of cardiac asthma, the waveform will still be a normal shape1. The reason for this is that the pathology of cardiac asthma is markedly different than that of bronchial constriction. Cardiac asthma is a result of increased hydrostatic pressure in the interstitial space between the pulmonary capillary beds and the alveolar membrane. This increased pressure compresses the alveoli and terminal bronchi leading to lower airway wheezing. Hence the proverbial phrase “Not all that wheezes is asthma”. Therefore, since the pathology of cardiac asthma only affects the terminal airways and is not a result of diffuse bronchial constriction, there is no waveform shape change. If a shark fin waveform is seen in a CHF patient with no history of COPD and/or asthma, the etiology is probably related to acute bronchial constriction triggered from the turbulent airflow of respiratory distress and/or a severe bilateral infection that the paramedic misinterpreted as CHF. Bicarbonate administration should be considered if ETCO2 levels begin to decrease in the presence of good CPR and an appropriate ventilatory rate. If paramedics approach bicarbonate administration from a “cookbook” approach (going through the algorithms with minimal critical thinking), potentially more harm than good can be done as extreme alkalosis is just as detrimental to a patient as extreme acidosis. Instead of giving an ambiguous “amp” of bicarbonate that usually takes the patient from acidosis to extreme alkalosis, use capnography to administer incremental, small doses of bicarbonate titrated to an appropriate ETCO2 level. Additionally, if early ETCO2 levels remain low despite ruling out a V/Q mismatch, an excessive ventilatory rate and/or poor CPR, consider bicarbonate administration earlier than what the cookbook calls for. Likewise, if it is 10 minutes into a cardiac arrest call and ETCO2 levels are fairly normal, do not slam an amp of bicarbonate “just because”. Paramedics should use capnography to determine when and how much bicarbonate is needed.