Abstract
A major challenge of airway management is safe care of the patient with a narrowed airway. Small tracheal tubes offer one solution but pose a problem with ventilation. While inspiration may be achieved by use of a high-pressure source to overcome airway resistance, two problems exist: first, the high-pressure source demands technical excellence and exposes the patient to a high risk of barotrauma; second, conventional (passive) exhalation through a narrow tube is slow and cannot achieve a normal minute ventilation with a tracheal tube of less than 4.5 mm diameter. Recently technical developments have led to the ability to assist expiration and make it, like inspiration, an active process. This technology is used in the Ventrain manual ventilator, the 2.4 mm wide Tritube tracheal tube and the Evone automatic ventilator. These new devices and the applied technology enable solutions for safe management of the narrowed upper airway.
Introduction
When ventilating through (artificial) small-diameter airways, decreasing the diameter increases the resistance to flow exponentially. Therefore, insufflation requires specialised equipment to generate high enough pressures. However, the elastic recoil of the lungs and thorax cannot always generate the required driving pressure to provide the same airflow back through the small-diameter airway.
Traditional insights therefore dictate that the patient’s own airway must be non-obstructed or that expiration should be prolonged leading to a decrease in minute volume and carbon dioxide retention. Importantly, if expiration is incomplete, this will lead to air trapping with the potential for barotrauma, haemodynamic compromise and even death.
Expiratory Ventilation Assistance
Ventrain (Dr Enk, Ventinova Medical, Eindhoven, the Netherlands, Figure 18.1) is a manually operated, flow-controlled ventilator that, when combined with a high-pressure gas source, can generate the pressures needed to overcome the resistance to inspiratory flow of a small-diameter airway but also generate sufficient subatmospheric pressures to facilitate expiration through the same small-diameter airway. Thus, both inspiration and expiration are active processes. This has been coined expiratory ventilation assistance (EVA).
Contrary to traditional means of ventilating through small-bore catheters or cannulae, an open airway to enable passive egress of air is therefore no longer a prerequisite when ventilating with Ventrain. In fact, airway obstruction improves ventilation with Ventrain as gas that is insufflated will not leak out through the upper airway, increasing the efficiency of intrapulmonary gas delivery and pressure build-up (for instance positive end-expiratory pressure (PEEP)).
Originally invented to be used through transtracheally placed cannulae in case of emergencies, Ventrain is now being used in other settings as well, both emergent and elective. In essence, any airway tube with a Luer lock could be used including intubating catheters, airway exchange catheters and bronchial blockers.
Mode of Mechanism
Ventilation with Ventrain works as follows: its gas tubing is connected to a high-pressure source, usually an oxygen flowmeter or an oxygen cylinder with a flow regulator. The tubing at the side port (located at the bottom of Ventrain) is attached to the patient’s small-bore airway tube via a Luer lock connection. An additional connection is provided for sidestream capnometry.
Ventrain has two openings that can be occluded by the clinician’s thumb and index finger. With both apertures open (Figure 18.2A), no relevant flow occurs to or from the patient (equilibration phase). With both apertures closed, flow is directed to the patient (inspiration, Figure 18.2B).
After releasing the upper opening (lifting the thumb) while keeping the index finger on the lower opening, air from the high-pressure source will preferentially flow forwards. Inside the Ventrain the tubing decreases in diameter. This results in an increased speed of airflow which according to Bernoulli’s principle leads to a pressure drop. The resulting subatmospheric pressure causes suction and therefore facilitates gas flow from the patient through the small-bore airway during expiration (EVA, Figure 18.2C).
Flow Control
Ventrain is a flow-controlled ventilation mode. Inspiratory volumes can be calculated from the set flow. For instance: at a flow of 15 L min−1, during inspiration each second 250 mL is insufflated and with an inspiratory to expiratory (I:E) ratio of 1:1 this would theoretically lead to a minute volume of 7.5 L min−1.
In clinical practice minute volume will also depend on other factors including pulmonary compliance, resistance to flow through the cannula, the degree of upper airway obstruction and the number of equilibration manoeuvres. Therefore, minute volumes are usually (slightly) lower than calculated.
The gas flow is set according to the patient’s characteristics. Flows should be individualised, but the following settings can be used as guides: 12–15 L min−1 for the adult, 4–6 L min−1 if used to ventilate a collapsed lung through a bronchial blocker and 2–6 L min−1 for paediatric patients through an airway exchange catheter or intubating catheter. Start with a low flow and increase as necessary.
Typically used I:E ratios in adults are 1:1, but in very small-diameter airways the resistance to flow might be increased to such an extent that the suction pressure will not be enough to accomplish complete expiration as quickly as inspiration is accomplished. It is therefore recommended to use longer expiration times compared to inspiration times when ventilating through paediatric intubating or airway exchange catheters, or through bronchial blockers.
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