Basic shared ventilator setup and settings
The design of a shared ventilator setup has been published before. Briefly, the inspiratory limb of the ventilation circuit is split with an y-piece, with one subsequent limb going to each patient. The expiratory limbs are joined together again with an y-piece prior to their connection to the ventilator. To prevent flow from one patient to the other, and thus avoid cross-contamination, one-way valves can be added to the expiratory limbs of the circuit.
The only way to safely ventilate patients in a basic shared ventilator setup is by delivering pressure-controlled ventilation. With volume-controlled ventilation, a sudden increase in airway resistance in one patient will result in large increases in delivered tidal volume for the other patient. Inspiratory time should be set, not variable.
Patients should be placed close together with the ventilator in between them, so that no additional deadspace (tubing) needs to be added. The one-way valves also reduce dead space and thereby prevent CO2 rebreathing.
In most modes of ventilation, the patient has the potential to interact with the ventilator. This should be avoided, by deep sedation or neuromuscular blockade. Ideally, inspiratory effort could be monitored to assure absence of spontaneous breathing activity. Another safety measure is that the threshold for the inspiratory trigger should be set as high as possible, so inspiratory effort does not lead to triggering the ventilator. Finally, should some effort still occur, the patient is more protected in the suggested pressure-controlled ventilation mode, avoiding tidal volume ‘steal’ from the other patient.
Supplemental ventilator circuit components to individualize and monitor ventilation
Even though the use of a shared ventilator without circuit modifications could be life-saving in some settings, there are a number of potential modifications to this setup that could make its applicability in clinical practice more feasible.
Monitoring respiratory dynamics becomes both more difficult and more important in a shared ventilator setup. A readily available pressure transducer, commonly used for invasive blood pressure monitoring, can be installed on each ventilator limb, with a fluid/water interface. This can be connected to the patient’s monitor, thus displaying an additional specific curve for airway pressure. This pressure monitor can then also be used to measure intrinsic PEEP during an expiratory hold maneuver. Additionally, spirometry can be added as well, to measure individual tidal volumes, as the ventilator’s output can only display the sum for both patients. Capnometry should be measured and displayed per patient.
- Limiting individual tidal volumes
The delivered tidal volume per connected patient will be a result of their individual lung compliance. This is the reason why ‘pairing’ of patients by lung compliance and size will need to be done prior to connecting a ventilator. However, this complicates the practical applicability of this setup, as patients would need to be shifted around, and possibly re-arranged when one would deteriorate or improve.
To overcome this issue, we can restrict the flow to a patient by partial obstruction of the inspiratory limb of the ventilator’s circuit. (supplemental Fig 1) By this dynamic flow restriction, we can limit the tidal volume that is going to any patient, thus effectively removing the need to pair patients. The restriction needs to be done for the patient with the highest lung compliance.
The tidal volumes per patient can then be measured or calculated, and the pressure on the ventilator set so that the desired tidal volume is achieved for the patient with the least compliant lungs. Subsequently, by restricting the flow to the patient with the more compliant lungs, the tidal volume is limited for that person not to exceed a set level.
- Setting individual PEEP levels
One of the major setbacks of using a single ventilator is that the PEEP level for both patients would need to be the same, whereas titration of PEEP is an important element of ventilator management in ARDS.[13, 14] This problem can be overcome by introducing a PEEP-valve in the circuit. When this valve is placed in-line, the released gas is returned to the ventilator, and the measured expired volume is still accurate. The PEEP valve can be placed on the expiratory limb of the circuit for the patient that needs the highest PEEP of both connected patients, the PEEP level for the other patient can then be set by the ventilator.
The inspiratory oxygen fraction (FiO2) remains a setting that is shared when you split a ventilator. As far as we know, no studies have yet tested the potential to individualize FIO2 in a shared ventilator setup. Our hypothesis was that we could increase FiO2 in a patient's ventilator circuit by injecting a variable flow of 100% oxygen into the inspiratory limb of that patient. Figure 5 demonstrates this concept.
Pitfalls and safety
- Intrinsic PEEP and need for long inspiratory times
As we use pressure-controlled ventilation, it is important to make sure that the set airway pressure results in the desired alveolar pressure. In a dynamic situation, i.e. not during an inspiratory breath hold, an indication that this is actually present in patients is by creating a stable plateau pressure with a no-flow end-inspiratory result. Consequently, longer inspiratory times can be useful to achieve this stabilized distributed tidal volume. In ARDS, this could result in higher levels of intrinsic PEEP, and dynamic airway obstruction and expiratory flow limitation frequently result in air trapping. Our bench setup was designed to see whether we could demonstrate the presence of intrinsic PEEP in the individual patient in a dynamic situation.
- Recommended alarm settings and bias flow and estimation of compressible gas volume in circuit.
Alarm settings are described in the ‘Columbia protocol’, developed in New York. As stated in this protocol, the ventilator may misestimate compressible gas volume in circuit. A mismatch of approximately 80mL is reported for both tidal volumes and minute volume. Therefor ventilation should be checked by blood gas analysis.
- Adding deadspace in acid-base differences between the two patients
When a large difference in acid-base status is present between the two ventilated patients (one patient acidotic and one alcalotic), restoring one will aggravate the other. This can in theory be overcome by adding deadspace for the patient with a respiratory alkalosis, so that increased minute ventilation does not lead to increased alkalosis.
We performed a bench testing with five experiments to: 1) determine lung compliance as function of the ventilator settings 2) determine volume, plateau pressure and PEEP in two lungs without added in-line PEEP or flow restriction 3) illustrate individualization of airway pressures and tidal volume with a flow restrictor, 4a) illustrate that PEEP can be applied and individualized 4b) create and measure intrinsic PEEP 4c) determine PEEP as a function of flow restriction 4d) and create PEEP with an additional in-line PEEP module 5) test our hypothesis that FiO2 can be individualized.
Several ventilator settings were applied (see Table 1). Two Dräger test lungs were used: lung 1 was a standard and another test lung was made less compliant, lung 2. The flow restrictor we used was a diaphragm valve with markings per quarter to adjust the flow. (See supplemental material S1) The balloon was ventilated in a pressure controlled mode, inspiratory pressure was always 20cm H2O above PEEP.
The ventilators used in our experiments are a Puritan Bennett 980 Series and an AS3/ADU anesthesia machine, all with standard 22mm tubes. The use of two different types of ventilators was due to clinical needs.
Pressure transducers were obtained from the transpac disposable pressure transducer and connected to a monitor. We used an additional monitor to display the pressure curves in mmHg from our transducer from circuit A and B. The transducers were equilibrated, prior to each experiment. We used a Datex Ohmeda gas analyzer spirometry module for an AS3/ADU anesthesia machine, with oximetry. To introduce expiratory obstruction in experiment three we used a gradual clamp.
Different bench tests
1) Determining the compliance as function of the ventilator setting
The lung compliance was measured for our test lung as function of the different ventilator settings with the anesthesia ventilator in a normal circuit of 1 test lung.
2) Determining volume, plateau pressure and PEEP in two lungs whitout in-line PEEP or flow restriction.
Using the anesthesia ventilator. (Fig1) the volumes were measured via our spirometry module on the inspiratory limb of circuit A and B and were displayed respectively on the anesthesia machine in mL for A and on the monitor in mL for B. The plateau pressure and total PEEP were displayed on and could be read from the monitor in mmHg, for the different ventilator settings.
3) Measurement of the volumes and plateau pressures as a function of the flow restrictor.
Using the same set-up as in experiment 2, the flow restrictor was added on the inspiratory limb of circuit A and adjusted from totally closed per quarter to fully open. (Fig 2) We measured the volume and plateau pressure as described above using ventilator settings 1 and 5.
4a) illustrate that PEEP can be applied and individualized
Using the same set-up as in experiment 2, we applied the in-line PEEP 7.5 cmH2O in circuit A as a variable. (Fig 3). The total PEEP was displayed on and could be read from the pressure curve on the monitor in mmHg. The experiment was done with ventilator settings 1 and 5 and with ventilator settings 1 and 5 without extrinsic PEEP. We did three runs with the in-line PEEP as a variable: 1) no in-line PEEP, 2) no in-line PEEP and 8cmH2O extrinsic PEEP on the ventilator and 3) with in-line PEEP 7.5cmH2O in circuit A.
4b) Applying intrinsic PEEP and measurement of the total PEEP to determine intrinsic PEEP.
For this experiment we used the Bennett ventilator, with an in-line PEEP 7.5cmH2O on circuit A. (Fig 4a) Obstruction on the expiratory limb from circuit A was simulated with a clamp adjusted from fully open to totally closed, to induce intrinsic PEEP. The total PEEP was measured, by performing an expiratory hold maneuver, as a function of flow obstruction in the expiratory limb. The intrinsic PEEP was calculated in cmH2O as follows: (measured total PEEP in mmHg *1.36)-[(extrinsic PEEP from ventilator in cmH2O)+( 7.5cmH2O in-line PEEP)]. The experiment was done with the different ventilator settings.
4c) Measurement of the total PEEP on the ventilator, in circuit A and B and as function of the flow restriction to determine intrinsic PEEP in both circuits.
We used the same set-up as experiment 2, the flow restrictor was added on the inspiratory limb of circuit A and adjusted from totally closed per quarter to fully open. (Fig 5a) We measured the total PEEP as described above using ventilator settings 1 and 5.
4d) Experiment 4c with an additional in-line PEEP.
We used the same set-up as experiment 4c with an additional in-line PEEP. (Fig 6a)
5) Measurement of the FiO2 % in circuit A and B as a function of added flow in circuit A.
We used the anesthesia ventilator in the standard set-up with test lung 1 on both circuits. (Fig 7a) We added a laterally inserted varying flow (C) in two different positions and measured the volume and FiO2 at points A and B in the circuits of the same name. The additional flow was inserted at two points on the inspiratory limb of circuit A. Point 1, just after the splitting of the inspiratory limbs on circuit A and point 2 just before the entrance of the lung on circuit A. Ventilator setting 1 was used to perform the test with a flow of 15L/min and FiO2 of 0.21. We waited one minute per measurement to record the values.