The ETCO2 that we measure tends to be about 5 mmHg lower than the patient’s PCO2 at the time. However, too much can be potentially harmful. Hypercarbia Can be DangerousĪ little hypercarbia is fairly common during spontaneous ventilation under anesthesia. Not enough gas moved back and forth inside the equipment dead space with each breath for the sensor to detect it.Īfter a larger tidal volume, the real end-tidal CO2 is revealed to be 62, quite hypercarbic. His tidal volume was too small to show hypercarbia. That breath, shown here in this second monitor photo, reveals his real ETCO2 to be 62 mmHg. Why can’t we see that hypoventilation reflected on the end-tidal CO2 monitor? We can if we give him a larger breath of 499 ml. The article on anatomic dead space shows more details of different tidal volume combinations compared to anatomic dead space and their affect on ventilation and hypercarbia. The effective tidal volume of 71ml is woefully inadequate. With these numbers the alveolar ventilation is 340 ml (490ml – 150 ml). He would have an adult anatomic dead space of 2 ml/kg or 150ml. Now we’re only delivering 71ml (121ml – 50ml).Ī 70 kg patient would normally have a tidal volume of about 7ml/kg or 490ml. Now let’s subtract out the additional 50ml of equipment dead space from the filter, elbow attachments, and the LMA tube outside his mouth. Respiratory rate of 12 and end-tidal CO2 are reassuring, but the tidal volume is too small, masking the real end-tidal CO2 inside the equipment dead space. Although both of these numbers are very reassuring, the tidal volume of 121ml shows the patient is hypoventilating. In this initial monitor photo you can see that respiratory rate is 12 and the ETCO2 is 32mmHg. As the case progressed I noticed the ETCO2 readings and took photos of the monitor screen. I used the filter attachment shown in the first image. He was spontaneously ventilating under general anesthesia with sevoforane through a LMA. I recently anesthetized a 35 year old, 70 kg, 5” 9” man for peripheral orthopedic surgery. You can miss hypoventilation, even when using an end-tidal CO2 (ETCO2). Hypoventilation Causes CO2 Retention & Hypercarbia If I were to ventilate a 7 kg baby, who has a 50 ml tidal volume, with a 50 ml breath using this circuit, no fresh gas containing oxygen will reach the baby’s lungs. The smaller the tidal volume, the larger the impact of equipment dead space. The colored external portion of the LMA, the elbow connector, filter and the end of the breathing circuit are all equipment dead space. A poor mask seal, failure to squeeze the bag effectively, or allowing the patient to breath shallowly could all lead to hypoventilation in this scenario.Įxample of equipment dead space. This particular combination, including the LMA outside the mouth, the end of the breathing circuit, the bacterial/humidification filter, and the elbow adapter added an additional 50 ml of dead space. The colored portion shows equipment dead space. The photo shows the end of an anesthesia breathing circuit I recently used. Usually equipment dead space is small compared to the total tidal volume, but not always. Your manually administered breath must include enough volume to cover this equipment dead space to avoid hypoventilation. for more on physiologic dead space see:Įquipment dead space includes the mask, the part of the endotracheal tube or laryngeal mask airway (LMA) outside the patient’s mouth, even the elbow on the endotracheal tube connecting it to the ventilation bag. Decreased cardiac output or decreased lung perfusion increases pulmonary dead space by diminishing pulmonary capillary blood flow. Physiologic dead space changes from minute to minute. In Physiologic dead space, lack of capillary flow at the time of measurement prevents gas exchange. Anatomic Dead Space Affects Hypoventilation.To read an article on how anatomic dead space affects ventilation see: A third of the normal tidal volume is anatomic dead space, with a volume of about 2ml/kg in an adult and up to 3ml/kg in a baby. The lungs cannot absorb oxygen or eliminate carbon dioxide in anatomic dead space. These fixed parts of the respiratory tract are ventilated but not perfused. Anatomic Dead SpaceĪnatomic dead space does not have alveoli, such as the trachea, bronchi, and bronchioles. The 3 different types of dead space consist of anatomic, physiologic, and last, but not least, equipment. Ignoring equipment dead space can lead to significant hypoventilation. We often forget equipment dead space, the dead space belongs to any airway equipment used to assist ventilation. We often worry about anatomic and physiologic dead space. Dead space is the portion of the respiratory system where tidal volume doesn’t participate in gas exchange.
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