Protective mechanical ventilation should be implemented after intubation in ARDS and in non ARDS patients, and in COVID-19 patients with pneumonia.
However, protective ventilation does not just mean tidal volume reduction and we would like to focus here on the respiratory rate and the instrumental dead space.
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 how to set the initial respiratory rate in mechanically ventilated patients and the determinants of alveolar ventilation (CO2 elimination) other than tidal volume: the respiratory rate will be discussed in part 1 and dead space in part 2 (published next week). We will also discuss the impact of these settings for the specific situation of COVID-19 patients.
How to set the respiratory rate?
It is easy to provide charts for the tidal volume based on gender and height and predicted body weight (PBW). However, there is no clear recommendation and clues for the initial setting of the respiratory rate and therefore minute ventilation. This may be more a habit or intuitions and by experience: RR is set around 10 in the operating room, around 20 in newly intubated critically ill patients, and sometimes increased to 25 or 30 in ARDS patients. A classical recommendation for minute ventilation is 100 ml/kg/min PBW (for instance used to set the ASV mode and recommended since 1955)= in order to get a reasonable level of PaCO2, it is not appropriate for most critically ill patients.
We have developed a free educational SmartPhone application, VentilO, to facilitate the initial settings of protective mechanical ventilation.
We show on Figure 1 the values of minute ventilation, tidal volume and respiratory rate (when available) in several studies conducted in patients managed in the operating room (9 studies) and in critically ill patients in the ICU (25 studies). The dots represent isopleth curves illustrating the levels of 100 and 150 ml/kg/min PBW of minute ventilation.
Ventilator settings in the operating room or in critically ill patients (in the emergency department and in the ICU) are very different, leading to minute ventilation around 100 ml/kg/min PBW in the operating room (OR) and at least 150 ml/kg/min PBW in critically ill patients. These are two different worlds in terms of patient’s needs.
Figure 1: This figure is a schematic representation of the findings based on the analysis of more than 30 studies providing respiratory rate and tidal volumes in different settings (ICU and OR) for more than 40,000 patients. Usual respiratory rates and tidal volumes are represented for surgical patients (planned surgery and one lung surgery with protective ventilation) and critically ill patients (non ARDS, ARDS with protective and ultraprotective ventilation, and CARDS (COVID-19 ARDS) patients). The targeted minute ventilations are very different as well as tidal volume and respiratory rate based on the category of patients.
In the operating room for planned surgery, the patient’s metabolism is usually low, and the body temperature is frequently below or equal to 36°C. The CO2 production is therefore low and minute ventilation around 100 ml/kg/min PBW as shown by Radford in 1955, is still adequate. In addition, the recommended target tidal volume has progressively been reduced and is now around 8 ml/kg PBW or below, consequently respiratory rate should be set between 12 and 16/min. In patients with one-lung ventilation (targeted tidal volume 4-6 ml/kg PBW), the respiratory rate should be set between 16 and 22/min.
In critically ill patients, the metabolism is high and body temperature may be elevated. The CO2 production is high, the dead space is high (including instrumental dead space) and ventilation needs to allow CO2 elimination are higher than normal. In our study, we showed that in mechanically ventilated patients in ICU, minute ventilation was around 150 ml/kg/min PBW (25 studies conducted in ICUs). The respiratory rate must be frequently above 20 breath/minute in critically ill patients. It is logical to use high respiratory rate after intubation of septic patients (with pneumonia or other cause of SIRS) breathing above 30/min before intubation. Some patients are ventilated after intubation with both reduced tidal volume and low respiratory rates (15 or below), which can result in severe acidosis.
To summarize, in critically ill patients, the minute ventilation necessary to maintain PaCO2 within a reasonable range is frequently at or above 150 ml/min/kg PBW. In non-ARDS patients if 8 ml/kg PBW is targeted, a respiratory rate around 20/min may be adequate; with a tidal volume of 6-8 ml/kg PBW, respiratory rate around 25/min maybe necessary (eg COVID patients). In severe ARDS with low compliance, requiring tidal volumes of 6 ml/kg PBW or lower to maintain plateau pressure below 30 cmH2O, respiratory rate should be set at 25-30 or even higher. In CARDS patients (COVID-19 ARDS) the same principles apply.
The right settings from the beginning in patients with CARDS (COVID-19 ARDS)
Five recent studies provided respiratory settings of COVID-19 patients and confirmed that minute ventilation was at least 150 ml/kg/min PBW (mean of 171±17 ml/kg/min PBW). This was very significantly higher when using a filter at the Y-piece (see part 2 which will be published next week).
Table: Respiratory rate, tidal volume, minute ventilation and arterial blood gases in COVID-19 patients in five studies describing the respiratory mechanics of this population.
* The author was contacted and provided additional data: an instrumental dead space of 49.5 ml (HME 36 ml, connectors 13.5 ml) was used in the patients. In addition, a catheter mount of 38 ml was routinely used. Added to the endotracheal tube (around 20 ml), to the physiological dead space related to airways (accounting for 1.1 ml/kg PBW for intra-thoracic dead space ~75 ml) and to the alveolar dead space that is not counted here. In this example, the total dead space is around 185 ml and the dead space ventilation is around 6.1 L/min to which the alveolar dead space should be added. The relatively high dead space associated with a quite high respiratory rate may explain the high minute ventilation required.
** In this study, data from patients managed in two different hospitals with different protocols for humidification in COVID-19 patients were compared. In one ICU, the dead space was minimized to only closed-suction system connector (instrumental dead space estimated to 5-10 ml), in the other unit the dead space was the following: a HME was used (45 ml), with closed-suction connector and CO2 cuvette (both 5-10 ml). These corresponds to low dead space (0ml) and moderate dead space (50 ml) in the VentilO application. In the HH unit, VentilO recommended a mean respiratory rate of 24/min for a tidal volume of 6 ml/kg PBW; while in the HME unit, Ventilo recommended a mean respiratory rate of 27/min for a tidal volume of 6 ml/kg PBW.
François Lellouche, MD, PhD
Centre de recherche de l’Institut Universitaire de Cardiologie et de Pneumologie de Québec
Département d’Anesthésiologie et de Soins Intensifs. Faculté de médecine, Université Laval
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