As a new grad (2010), a preceptor once mentioned that the most under-appreciated value in respiratory care was alveolar minute ventilation (VAlv). This is because the effectiveness of the set frequency and tidal volume comes down to the total deadspace. Anatomical deadspace can be estimated but can change drastically with disease and can be difficult to measure. Therefore, we need to be mindful of the deadspace we can control…mechanical deadspace. Let’s take a look at some numbers to demonstrate the impact mechanical deadspace can have on VAlv delivered to a patient receiving lung-protective ventilation.
Patient – 6’1” Male
PBW – 79.9 kg (round to 80 for math sake)
Tidal volume (VT) – 6 mL/kg PBW
Frequency (f) – 25 breaths/min
Anatomical deadspace (VDanat) – 1 mL/lb PBW or 176 mL
HME – 30 mL
15 cm flexible tubing – 55 mL
VAlv = f x (VT – VDanat – VDmech)
VAlv = 25 x (.480 – .176 – .030 – .055)
VAlv = 25 x (.219)
VAlv = 5.5 L/min
If we remove the mechanical deadspace
VAlv = 25 x (.304)
VAlv = 7.6 L/min
If this patient was ventilated with 4 mL/kg, the deadspace ratio would further increase and lower VAlv. These numbers do not take into account any other mechanical deadspace that may be included such as end-tidal monitoring, any additional connectors, catheter mounts, etc. We also know that the conditions associated with ARDS may increase deadspace, again, lowering VAlv.
Lellouche et al. (2020) presented a graphical illustration of the impact of mechanical deadspace on VAlv for different settings that provided the same minute ventilation (VE) considering a male, 175 cm tall and 150 mL/kg PBW/min.
Using 6 mL/kg PBW and a frequency of 25 breaths/min (VE = 10.3 L/min), VAlv increases from 5.3 L/min to 8.1 L/min by reducing mechanical deadspace to a minimum. When the settings are changed to 4.7 mL/kg PBW and 32 breaths/min (VE = 10.3 L/min), VAlv increased by twofold from 3.8 L/min to 7.5 L/min by reducing mechanical deadspace to a minimum.
In a small study, Hinkson et al. (2006) demonstrated that by removing both the HME and flexible tubing, the deadspace ratio decreased by approximately 11%, PaCO2 decreased by approximately 11 mm Hg and pH increased from 7.30 to 7.38.
- We have the most control of mechanical deadspace
- At various settings, the addition of mechanical deadspace can dramatically change alveolar ventilation
- Reduce mechanical deadspace where possible and increase alveolar ventilation for the win!
- Lellouche, F., et al. (2020). “Impact of respiratory rate and dead space in the current era of lung-protective mechanical ventilation.” Chest, 158(1), 45–47. https://doi.org/10.1016/j.chest.2020.02.033.
- Hinkson, C.R., et al. (2006). “The effects of apparatus dead space on PaCO2 in patients receiving lung-protective ventilation” Respiratory Care, 51(10), 1140-1144.