Ventilation through small‐bore airways in children by implementing active expiration

Abstract Management of narrowed airways can be challenging, especially in the smallest patients. This educational review focusses on active expiration through small‐bore airways with the Ventrain (Ventinova Medical, Eindhoven, The Netherlands). Manual ventilation with the Ventrain establishes inspiratory and expiratory flow control: By setting an appropriate flow, the volume of gas insufflated over time can be controlled and expiration through a small‐bore airway is expedited by jet‐flow generated suction, coined “expiratory ventilation assistance” (EVA). This overcomes the inherent risks of emergency jet ventilation especially in pediatric airway emergencies. Active expiration by EVA has been clinically introduced to turn a “straw in the airway” into a lifesaver allowing not only for quick and reliable reoxygenation but also adequate ventilation. As well as managing airway emergencies, ventilating through small‐bore airways by applying EVA implements new options for pediatric airway management in elective interventional procedures. Safe application of EVA demands a thorough understanding of the required equipment, the principle and function of the Ventrain, technical prerequisites, clinical safety measures, and, most importantly, appropriate training.


| INTRODUC TI ON
Narrowed or shared airways (e.g., in ear-nose-throat surgery) can be encountered in pediatric anesthesia for several reasons including subglottic cysts, hemangiomas, or stenosis both in elective and in emergency settings. 1  Ventilation through these artificial airways requires specialized equipment as a decrease in their diameter increases resistance to flow exponentially. As a result, the necessary pressure gradient for both inspiration and expiration needs to be increased. The required high inspiratory pressure can be provided by gas (cylinder or wall) outlet pressure, but the pressures generated by the passive recoil of the lungs and chest wall are likely to be insufficient for exhalation in a timely manner. Consequently, these children will be either hypoventilated as expiratory time needs to be considerably prolonged, or air trapping will easily occur because of impeded expiration.
Use of the Ventrain can overcome these problems: It is a manually operated, flow-controlled ejector ventilator and has been developed to assist the expiratory egress of gas by jet-flow generated suction. 3 Combined with a high-pressure oxygen / air source, it can generate both the high pressure needed to overcome the resistance to inspiratory flow of a small-bore airway and sufficient subatmospheric pressure to facilitate expiration through the same small-bore cannula or catheter, while inspiratory tidal volume can easily be estimated from the set flow and inspiratory time. This process has been coined expiratory ventilation assistance (EVA). 4 Introduced and initially evaluated in combination with a 2 mm inner diameter (ID) cricothyroidotomy cannula for adults, 3,5 the Ventrain has been applied in both emergent and elective clinical airway management. Generally, it can be used on any airway with a Luer lock, such as ICs, AECs, working channels of flexible or rigid bronchoscopes and (with some limitations) bronchial blockers.
Below, we will explain the mode of action of Ventrain, proper handling and operational requirements, efficiency of EVA, the correct use of EVA with mandatory safety measures, and the advantages over other means of ventilation through small-bore airways.
Finally, we will give some recommendations for using Ventrain / EVA in pediatric patients.

| Mode of action
The Ventrain [ Figure 1] is a single-use hand-held device that is directly connected to the outlet of a high-pressure gas source (e.g., flow regulator of an oxygen cylinder, wall-mounted flowmeter) by a 2 m long tubing. Located at the bottom of the Ventrain is a 20 cm long tubing with a distal T-piece, which is attached to the patient's small-bore airway via a Luer lock connector. Inside the shell is a purpose-built ejector.
On its way from the high-pressure oxygen source through the ejector, the gas passes a 0.75 mm ID nozzle [ Figure 2]. As a result, the velocity and therefore the dynamic pressure (~ kinetic energy) of the flowing gas increase. This leads to a decrease in static pressure (~ potential energy) of the gas surrounding the jet released from the nozzle (Bernoulli's principle). During expiration, this resulting subatmospheric pressure is transmitted via the 20 cm long tubing to the patient's smallbore airway and actively supports egress of respiratory gas. 3

| Handling and operation
After setting the desired flow, the Ventrain is manually operated by intermittently occluding and releasing one or two apertures [ Figure 2]. The bottom aperture which is controlled by the operator's index finger acts as an on-off-switch. As long as this opening is released, the Ventrain is functionally disconnected (no airflow occurs to the patient, and, by excessively aspirating air from the environment, the subatmospheric pressure generated by the ejector is almost completely abolished). The top opening which is the exhaust of the ejector and controlled by the operator's thumb is the switch between inspiration and expiration.
If simultaneously closed [ Figure 2B], the flow as set at the oxygen source is directed to the patient, whereas immediately after release of the top opening the ejector will aspirate respiratory gas from the patient via the small-bore airway [ Figure 2C]. Thus, inspiration demands closure of both openings at the same time, whereas active expiration is initiated by only releasing the top opening while keeping the bottom opening occluded. Releasing both openings switches the Ventrain off and allows (almost complete) pressure equilibration of the intrapulmonary with the environmental pressure [ Figure 2A]. In other words, switching to equilibration mode immediately after insufflation of gas initiates a slow, passive expiration. Only occluding the top opening with the bottom opening released will result in some leakage noise without any relevant flow to the patient (=the device is then still functionally switched off).

| Flow control
Contrary to hand-held or automated (mechanical) jet ventilators, where inspiratory pressure is set but inspiratory volume can hardly be estimated, Ventrain is a flow-controlled device, and inspiratory tidal volumes can therefore easily be estimated from the set flow and inspiratory time: If the flow is set to 6 L/min (=6000 ml in 60 s), then during inspiration 100 ml will be insufflated per second. Similarly, a flow of 2 L/min will result in an inspiratory tidal volume of 33 ml/s [ Table 1].
With increased flow, suction pressure becomes more subatmospheric meaning also suction capacity (expiratory volume per time unit) will increase. When ventilating through a 7.5 mm long, 2 mm ID cricothyroidotomy cannula (for adults) in an ideal setting of no air F I G U R E 1 Ventrain leak, at flows of 9 L/min and lower, suction capacity will be slightly higher than inspiratory flow, whereas at higher flows the opposite applies. 3 When ventilating adults through a cricothyroidotomy cannula, an inspiration / expiration (I:E) ratio approximating 1:1 is used. Most small-bore airways used in pediatric patients (like an IC / AEC) will have a higher resistance to flow due to their decreased diameter and increased length compared to an adult-sized cricothyroidotomy cannula. As most cylinder or wall oxygen / air sources deliver flow at a maximum pressure equal to or above 3.5 bar (50.8 psi), inspiratory flow will remain constant regardless of the higher resistance to flow.
However, suction pressure generated within the Ventrain ejector is determined by the set flow only and will not increase at higher resistance to flow. Therefore, when ventilating through long small-bore catheters in high-compliant lung models for benchmark testing, suction capacity will decrease and an I:E ratio of 1:1.5 up to 1:2 may be needed to prevent hyperinflation. However, the (very) lowcompliant chests of newborns, babies, and toddlers help for the expiratory egress of respiratory gas to some extent. In addition, in clinical practice at least some air leakage will occur around smallbore catheters in most cases, so inspiratory tidal volume can be expected to be lower than estimated from the set flow. As leakage also supports expiratory egress of gas, an I:E ratio of 1:1 may still prove to be safe in the majority of pediatric patients during ventilation with Ventrain.
Even though inspiratory tidal volume can easily be calculated in a sealed airway, actual delivered volume into the patient's lungs will be less if leakage occurs. Increasing the flow or increasing inspiratory time may then be considered. Expiratory tidal volume can only be estimated as it is dependent on many factors (e.g., set flow, resulting subatmospheric pressure, resistance to flow, and leakage).
The minute volume can be calculated from the flow and the I:E ratio: In a sealed airway, a flow of 12 L/min at an I:E ratio of 1:1 will deliver a minute volume of 6 L/min. If an I:E ratio of 1:2 is needed, it will be 4 L/min (one third of 12 L/min).

| Efficiency of reoxygenation and ventilation
Traditionally, insufflation or injection of gas through a small-bore air-

| Pressure of the gas source
The auxiliary oxygen outlet of an anesthesia machine or a built-in flowmeter should not be used for ventilation with Ventrain as most cannot generate the required pressure and may therefore deliver lower flows than set and displayed. 8,9 In addition, any high-volume tubing (e.g., the anesthesia circuit) or humidifier reservoirs connected in-line must be avoided, because they will act as a buffer and become pressurized (which is potentially harmful).

| Requirements for flowmeters and flow regulators
The flow may only be set within the range of a flowmeter. A setting outside the scale can result in uncontrolled, excessively high flow.
Because Ventrain is a high-pressure device, the gas source must be able to handle back pressure, which means to deliver a stable flow against (high) resistance to flow. Column flowmeters with a floating ball or cylinder inside (=rotameter) attached to a wall outlet can do this, but their flow display can become wrong if they are not pressure-compensated (i.e., the column is calibrated for uncompressed gas). In these flowmeters, after connection to the Ventrain, the gas inside the column will be compressed because of the high pressure proximally of the ejector nozzle, leading to a drop of the floating ball or cylinder. Even though the flow is still as set before connection of Ventrain, an erroneously lower flow will be indicated.
This should be ignored and not entrap the clinician to increase the flow as then a dangerously higher flow may be delivered to the patient. To change the flow, the Ventrain must be disconnected first.
After reconnection, the floating ball or cylinder can be expected to drop again. One must bear in mind that oxygen from a cylinder or wall outlet is dry. Therefore, only short-term use is advisable. For intermittent humidification of the mucous membranes during longer lasting ventilation with Ventrain, one may consider to slowly inject small increments of 0.1-0.2 ml of saline solution with a 1 ml syringe into the insufflated gas via the side port of the distal T-piece (=the capnometry port, see below).

| SAFE T Y CON S IDER ATI ON S AND MONITORING WHEN US ING THE VENTR AIN
Training is essential to safely use the Ventrain and to efficiently apply If relatively more gas is insufflated over time, the intrapulmonary pressure will rise. If relatively more gas is aspirated, the intrapulmonary pressure will drop. Interestingly, subatmospheric intrapulmonary F I G U R E 3 Tritube with inflated cuff (with courtesy of Ventinova Medical, Eindhoven, The Netherlands) pressure does apparently not affect the lung tissue (e.g., intrapulmonary edema and/or bleeding) as has been shown in artificial coughing experiments in guinea pigs. 10   Ventrain and the small-bore airway. Then, the manometer (or its pressure measurement line) is connected to the side port of the three-way stopcock [ Figure 4A]. Reliable intrapulmonary pressure measurements can only be obtained in a static (=no flow) situation, because only then the pressure equalizes (=principle of communicating tubes).

| Intermittent intrapulmonary pressure measurement
So, at the end of inspiration or expiration, after switching to equilibration mode (by releasing both apertures of Ventrain), the three-way stopcock is turned so that it exclusively connects the manometer to the small-bore airway (with the Ventrain shut off). Now, an immediate reading of intraalveolar pressure can be obtained [ Figure 4B].
This way inappropriately high or low intrapulmonary pressure can be quickly corrected by adjusting the I:E ratio and/or flow.
After each measurement, the three-way stopcock is turned again to exclusively connect the Ventrain to the small-bore airway (with the manometer shut off).  In case of leakage, the reading will be less reliable and may thus only be used as a relative measure. Importantly, capnometry can help in confirming the small-bore airway is placed intratracheally.

| COMPARISON OF E VA TO OTHER OX YG ENATI ON / VENTIL ATI ON TECHNI Q U E S
Use of anesthesia machines / circuits with large-bore ventilation tubing in combination with small-bore airways equal to and below a 2.0 mm ID may give the impression of adequately ventilating patients, even though they are severely hypoventilated. This is because during inspiration high inspiratory pressure mainly compresses the gas column in the anesthesia circuit, instead of insufflating air into the patient. Therefore, specialized equipment should be used when ventilating through small-bore airways.
Jet ventilation (i.e., injection of gas at high pressure through a Intermittent intrapulmonary pressure measurement (e.g., with the help of a three-way stopcock and a manometer, see above) during manual jet ventilation only provides limited safety as a quick reduction of intrapulmonary pressure to restore circulation is impossible in case of an insufficiently open upper airway and lack of active expiration.
Another pitfall of emergency jet ventilation is the principle of pressure control: The injection pressure must be set and then, following release of the trigger, an unforeseeable flow into the patient's lungs results. Here, it becomes obvious why it is more appealing to rely on flow control rather than pressure control, because this allows for estimating the maximum gas volume insufflated over time (Table 1). Alternatively, the flow and inspiratory time can be set according to (normal) weight of the patient to create a tidal volume of 6-10 ml/ kg (refer to [ Table 1]). Calculation of tidal volume may not be easy in a stressful emergency setting. Instead of memorizing Table 1 for future reference, one may keep in mind that a flow of 6 L/min will deliver 100 ml/s. Other flows can then easily be calculated from this, for instance 2 L/min will provide 33 ml/s.
If leakage occurs (which will happen in most instances), it may be necessary to increase the flow and/or inspiratory time as actual delivered tidal volume will be smaller than calculated tidal volume. Commence intermittent measurement of intrapulmonary pressure (see above) to avoid significant imbalance in inspiratory and expiratory volumes. Table 2 provides a quick reference guide on use of the Ventrain in pediatric patients.

| Experience so far in pediatric patients
Ventrain's instructions for use state that it can be used for "all patients, however, for patients with body mass <40 kg (e.g., children,

ACK N OWLED G M ENT
We would like to thank A. Mertens for his assistance in preparing the illustrations.