Dead space is the portion of the respiratory system where tidal volume doesn’t participate in gas exchange. We often worry about anatomic and physiologic dead space. We often forget equipment dead space, the dead space belongs to any airway equipment used to assist ventilation. Ignoring equipment dead space can lead to significant hypoventilation.
3 Different Types of Dead Space
The 3 different types of dead space consist of anatomic, physiologic, and last, but not least, equipment.
Anatomic Dead Space
Anatomic dead space does not have alveoli, such as the trachea, bronchi, and bronchioles. These fixed parts of the respiratory tract are ventilated but not perfused . Ventilation here is wasted breath. The lungs cannot absorb oxygen or eliminate carbon dioxide in anatomic dead space. 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. To read an article on how anatomic dead space affects ventilation see:
Physiologic Dead Space
In Physiologic dead space, lack of capillary flow at the time of measurement prevents gas exchange. Physiologic dead space changes from minute to minute. Decreased cardiac output or decreased lung perfusion increases pulmonary dead space by diminishing pulmonary capillary blood flow. It makes ventilation less efficient. for more on physiologic dead space see:
Equipment Dead Space
Equipment 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.
Importance of Equipment Dead Space
Your manually administered breath must include enough volume to cover this equipment dead space to avoid hypoventilation. Usually equipment dead space is small compared to the total tidal volume, but not always. The photo shows the end of an anesthesia breathing circuit I recently used. The colored portion shows 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. 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.
The smaller the tidal volume, the larger the impact of equipment dead space. 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.
Hypoventilation Causes CO2 Retention & Hypercarbia
You can miss hypoventilation, even when using an end-tidal CO2 (ETCO2). I recently anesthetized a 35 year old, 70 kg, 5” 9” man for peripheral orthopedic surgery. He was spontaneously ventilating under general anesthesia with sevoforane through a LMA. I used the filter attachment shown in the first image. As the case progressed I noticed the ETCO2 readings and took photos of the monitor screen.
In this initial monitor photo you can see that respiratory rate is 12 and the ETCO2 is 32mmHg. Although both of these numbers are very reassuring, the tidal volume of 121ml shows the patient is hypoventilating.
Now let’s subtract out the additional 50ml of equipment dead space from the filter, elbow attachments, and the LMA tube outside his mouth. Now we’re only delivering 71ml (121ml – 50ml).
A 70 kg patient would normally have a tidal volume of about 7ml/kg or 490ml. He would have an adult anatomic dead space of 2 ml/kg or 150ml. With these numbers the alveolar ventilation is 340 ml (490ml – 150 ml). The effective tidal volume of 71ml is woefully inadequate. 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.
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. That breath, shown here in this second monitor photo, reveals his real ETCO2 to be 62 mmHg. His tidal volume was too small to show hypercarbia. Not enough gas moved back and forth inside the equipment dead space with each breath for the sensor to detect it.
Hypercarbia Can be Dangerous
A little hypercarbia is fairly common during spontaneous ventilation under anesthesia. However, too much can be potentially harmful. The ETCO2 that we measure tends to be about 5 mmHg lower than the patient’s PCO2 at the time. ETCO2 has to be lower than PCO2, otherwise CO2 wouldn’t transfer from the blood to the alveoli to be exhaled.
For every 10 mmHg of change of PCO2, pH tends to change by 0.1. For this patient, a ETCO2 of 62 calculates to a PCO2 of 72 mmHg, which is 32 mmHg above the normal 40. The corresponding pH is about 7.1 (7.4 – 0.3), which is quite acidotic. The higher PCO2 rises, the lower the pH and the more toxic the cellular environment. I aim to keep my spontaneously breathing patients with an ETCO2 no higher than high 40s to low 50s. In order to maintain this range, I often will assist their breathing.
Assist Ventilation When Appropriate
It is common to have to support the spontaneous breathing of a patient to prevent hypoventilation. If the patient is on a ventilator, we compensate by providing continuous positive pressure (CPAP), positive end-expiratory pressure (PEEP) and periodic increased tidal volumes. Use assisted ventilation with bag-valve-mask to do these same things.
For more discussion on how to manually assist your breathing with mask ventilation see:
- Assisting Ventilation With Bag-Valve-Mask
- Difference in Ventilation With A Self Inflating Ventilation Bag vs. a Free Flow Inflating Bag
- Exhaling During Manual Ventilation Is As Important As Inhaling
- Mask Ventilation: Avoiding Hand Fatigue
- Getting A Good Mask Seal Ventilating A Patient
Always watch your patients for potential hypoventilation.
May The Force be With You!
Christine E. Whitten MD, author
Anyone Can Intubate: A Step By Step Guide t Intubation And Airway Management
and
Pediatric Airway Management: A Step By Step Guide
Note: During the pandemic my books are available on amazon.com at cost
