Based on the recently published paper “Impact of Respiratory Rate and Dead Space in the Current Era of Lung Protective Mechanical Ventilation”, we will discuss here briefly the different part of the dead space and how it can be reduced, as well as the relations with respiratory rate, tidal volume and alveolar ventilation (part 2). The alveolar ventilation is defined as the respiratory rate times (tidal volume – dead space volume). Consequently with low tidal volumes and high respiratory rates the weight of the dead space (also referred to as VD/VT) is automatically increased.
What is the dead space during invasive mechanical ventilation?
Often, the instrumental dead space is overlooked during mechanical ventilation. Many studies showed its large impact on the work of breathing during assisted ventilation and on alveolar ventilation during controlled ventilation. Surprisingly, however, in virtually all studies evaluating respiratory mechanics in ARDS or in COVID-19 patients, the instrumental dead space is not provided nor mentioned, even when VD/VT is evaluated! The different parts of the dead space in mechanically ventilated are as follows. From the lungs to the patient, the alveolar dead space may be reduced by limiting overdistention; the dead space of the airways is difficult to modify and is mildly increased by PEEP and bronchodilation; the instrumental dead space is the easiest to modify, may account for almost half of the total dead space and include the humidification filter at the Y-piece, the CO2 sensor, the connections, catheter mount, flex tube, etc. Using a heated humidifier instead of a filter is a very efficient way to reduce this dead space.
Figure 1: The figure represents the different portions of the dead space. Dead space may be divided into instrumental dead space and physiological dead space (including airway and alveolar dead space). Part of the instrumental dead space may be easily limited by reducing the number of useless connections and by using a heated humidifier instead of a heat and moisture exchanger (HME) for gas humidification. Part of the instrumental dead space is not easily reduced: the endotracheal tube (ETT) may be changed for a tracheostomy tube to reduce the dead space by 10-12 ml, or ETT may be cut but the gain is very limited (2-4 ml).
Instrumental dead space may be very high (above 100 ml) when counting HME, catheter mount, connectors and endotracheal tube. Most efficient HME have usually a volume above 50 ml (up to 90 ml), catheter mount may have a volume of 20-60 ml, many connectors may be used in patients (CO2 cuvette, closed suction and other adaptors for inhalation therapies) and may represent an important additional dead space.
The next figures are built, based on the calculations of the dead space detailed in the study. The alveolar dead space is not counted, consequently, the total dead space is underestimated. With protective ventilation and reduced overdistention, the alveolar dead space may be reduced.
All the different parts of the dead space are also taken into account in the VentilO application.
Impact of protective ventilation settings
The use of small tidal volumes (VT) and high respiratory rates (RR) has several consequences in terms of alveolar ventilation and dead space. Firstly, with low tidal volumes, the VD/VT increases as tidal volume decreases if VD remains constant. The weight of dead space increases in proportion of tidal volume decrease. Secondly, the dead space which is the “wasted part of the breath” intervenes more frequently when respiratory rate increases.
Alveolar ventilation (Valv) is the efficient part of the minute ventilation for gas exchange.
Valv = RR x (VT-VD) = RR x VT – RR x VD
Consequently, for a steady minute ventilation, when the respiratory rate or the dead space are increased, alveolar ventilation is reduced. If the respiratory rate is increased in proportion of the decrease in tidal volume, the alveolar ventilation will be reduced and PaCO2 will be increased. For example, if the settings are modified from 6 ml/kg PBW x 25 (150 ml/kg/min of minute ventilation) to 5 ml/kg x 30 (150 ml/ kg/min of minute ventilation), the alveolar ventilation will be decreased. To keep Valv constant here, the respiratory rate should be now 34 (with medium dead space).
Figure 2: these figures represent the impact of instrumental dead space on VD/VT (A) and on alveolar ventilation (B) in critically ill patients with different instrumental dead spaces. The example used for the calculations is the case of a woman of 165 cm; PBW 57 kg. The blue line represents the iso-minute ventilation for 150 ml/kg/min PBW (8.5 L/min with PBW of 57 kg). In the setting ❶, with a VT of 8 ml/kg and a RR of 19/min, VD/VT is 17% with lowest instrumental dead space and 43% with a higher dead space; alveolar ventilation went from 4.9 L/min to 7.1 L/min. When increasing the respiratory rate and decreasing the tidal volume (settings❷ and ❸), the difference is even more striking. In the most severe situation, reflecting a low compliance requiring ultra-protective ventilation (case ❸) with a TV of 4.4 ml/kg and a RR of 34/min, VD/VT is 31% with lowest instrumental dead space and 77% with a higher dead space. In this situation, the alveolar ventilation goes from 6.0 to 2.0 L/min.
Impact of dead space reduction
The previous figures showed the impact of the dead space on alveolar ventilation (CO2 clearance). The next figure shows that with a constant alveolar ventilation, it is possible to reduce the respiratory rate (with constant tidal volume), or to decrease the tidal volume (with constant respiratory rate) very substantially.
Figure 3: these figures represent the impact of different instrumental dead space on the respiratory rate (A) and the tidal volume (B) to maintain alveolar ventilation constant in critically ill patients. The example used for the calculations is the case of a woman of 165 cm; PBW 57 kg. The target alveolar ventilation is 4.7 L/min which is the alveolar ventilation in this patient with a minute ventilation of 150 ml/kg/min PBW and with a medium instrumental dead space (HME 50 ml, catheter mount and connections 20 ml). The different lines represents the iso-alveolar ventilation lines (4.7 L/min) and the different combinations of respiratory rate and tidal volumes to attain this alveolar ventilation.
Figure 3A: this figure shows the potential reduction of respiratory rate when reducing the instrumental dead space to keep constant alveolar ventilation (4.7 L/min) for a constant tidal volume. For example, for a tidal volume of 6 ml/kg PBW (blue lines), the respiratory rate required is 32/min with highest dead space and 18/min with the lowest dead space. For a tidal volume of 5 ml/kg PBW (orange lines), the respiratory rate required is very high (above 40/min) with the highest dead space, 34/min with intermediate dead space and 23/min with the lowest dead space.
Figure 3B: this figure shows the potential tidal volume reduction when reducing the instrumental dead space to keep constant alveolar ventilation (4.7L/min) for a constant respiratory rate. For example, for a respiratory rate of 20/min (blue lines), the tidal volume required is 7.5 ml/kg PBW with the highest dead space and 5.5 ml/kg with the lowest dead space. This will translate in a gain in plateau pressure or driving pressure related to the pulmonary compliance. For a respiratory rate of 28/min (orange lines), the tidal volume required is 6.3 ml/kg with the highest dead space, and 4.2 ml/kg with the lowest dead space.
Ultra-protective ventilation with tidal volumes around 4 ml/kg cannot be achieved with excessive dead space as previously shown.
Lellouche F, Delorme M, Brochard L.Impact of Respiratory Rate and Dead Space in the Current Era of Lung Protective Mechanical Ventilation. Chest. 2020 Jul;158(1):45-47. doi: 10.1016/j.chest.2020.02.033. Epub 2020 Jul 2. PMID: 32654726