Servo Pressure increases
Servo Pressure decreases
Improving compliance
Worsening compliance
Increased lung volume
Atelectasis
Improving resistance
Worsening resistance
Leak around ETT
Rt. mainstem bronchus intubation
Leak in the system
Tension pneumothorax
Secretions in large airways
46.1.3 Suctioning During HFJV
Routine suctioning should be avoided in most patients to avoid loss of lung volume that invariably occurs with disconnection from the ventilator and even closed suctioning systems. In patients who have significant secretions or where there is clinical suspicion of airway obstruction suctioning can be accomplished in two ways. In relatively stable patients, the ventilator can be placed in standby mode briefly for the duration of the procedure and restarted as soon as completed. The ventilator will take some time to return to the full level of support, which may not be tolerated well in unstable patients. In such patients, it is preferable to suction with the ventilator running. In this case, suction must be applied throughout the procedure (during insertion of the catheter as well as its withdrawal), so that excessive pressure does not build up in the airway. Even so, the ventilator may detect excessive pressure and will then open the dump valve, releasing pressure to the atmosphere. A loud click is heard with a brief pause in cycling when this occurs.
46.1.4 Detecting Inadvertent PEEP
When CO2 retention is unresponsive to an increase in ΔP, especially in a setting where increasing time constants may be present, a decrease in frequency should be considered to eliminate the possibility of inadvertent PEEP due to insufficient expiratory time. In most cases, inadvertent PEEP is detectable by an increase in the monitored PEEP and Paw on the front panel of the ventilator when no increase in PEEP has been made on the conventional ventilator. Normally, VT does not change with changes in frequency, because the Ti remains constant. Therefore, lowering the HFJV rate should result in an increase in PaCO2 . However, if gas trapping is present, lowering frequency will eliminate auto-PEEP and thus lower the effective PEEP, leading to increased ΔP, increased tidal volume, and falling PaCO2.
46.1.5 Weaning HFJV
The goal is to actively wean the patient toward extubation; patients who are stable within the target ranges for lung inflation and blood gases should have their settings weaned on a regular basis. It is important to balance the desire to wean these patients with the need to avoid causing atelectasis by dropping below the critical closing pressure of the lungs.
FiO2 should be weaned first in response to good oxygenation. In most cases, mean airway pressure should not be weaned until the FiO2 is <0.4. In general, stable patients in the first week or two of life, an attempt should be made to lower mean airway pressure and/or amplitude at least every 6–12 h. Patients that are more chronic may not tolerate as aggressive a weaning schedule. The question of whether patients on HFJV should be switched to conventional ventilation at some point or remain on HFJV until extubation has not been well studied. The only clinical trial which directly addressed this issue was the one by Clark, which demonstrated that infants who were remained on HFOV until extubation did better than infants who were changed from HFOV to conventional ventilation after 72 h (Clark et al. 1992). While there are some advantages to having a patient on conventional ventilation (e.g., tidal volumes can be accurately measured, fewer chest radiographs may be needed, easier to hold the infant), there are few compelling physiologic reasons to change infants from HFJV to conventional ventilation during the acute stage of the disease. It has been the author’s practice to wean directly from HFJV to CPAP for many years. Very low birthweight infants can usually be extubated when they have reached PIP of 12–14 cm H2O, mean airway pressure of 6–8 cm H2O, and FiO2 ≤ 0.25–0.30. Older, larger infants can be extubated from somewhat higher settings when they demonstrate good, sustained respiratory effort.
In the following chapter, I will formulate recommendations for the optimal use of HFJV in each of the relevant pathophysiologic conditions previously defined in Part VII of this book, under Clinical Use of HFJV based on a combination of published data and personal experience with HFJV spanning 25 years. The general strategy is applicable to other HFJV devices, but the specific steps and instructions are based on the functionality of the only widely available HFJV device for newborn infants, the Bunnell Life Pulse jet ventilator.
46.2 Management of Patients with Specific Pathophysiologies
46.2.1 Uniform, Atelectatic Lung Disease
Optimization of lung volume is key to achieving a homogeneously aerated lung, ensuring even distribution of lung volume, reducing exposure to high FiO2, preserving surfactant function, and minimizing lung injury (Clark et al. 2000; Froese 1989). Alveolar recruitment maneuvers typically consist of stepwise increases in mean airway pressure. With HFJV, mean airway pressure is controlled primarily by increasing PEEP on the tandem conventional ventilator. PEEP values much higher than those with which many clinicians are comfortable on conventional ventilation are required to achieve adequate alveolar recruitment, because the inspiratory time is extremely short. The need for alveolar recruitment is best judged from a combination of oxygen requirement and chest radiograph.
46.2.1.1 Rescue HFJV in Infants with Atelectatic Lung Disease
When faced with the patient with severe atelectatic lung disease that needs to be transitioned to HFJV in late rescue mode, the clinician must first determine the exact nature of the underlying disease and any coexisting problems, such as PIE or pulmonary hypertension that may require modification of the basic strategy. When transferring from conventional ventilation to HFJV, PEEP should initially be increased by about 2 cm H2O in order to avoid an inadvertent drop in MAP. When lung volume recruitment is indicated (poor lung inflation, high oxygen requirement), it is most easily achieved by a combination of escalating PEEP level with conventional ventilator sighs at a rate of 10/min providing the critical opening pressure to achieve alveolar recruitment. Once the lung volume is optimized, the lungs become more compliant, and an effort must be made to lower MAP and stabilize the patient at a point just above the critical closing pressure on the expiratory limb of the pressure-volume loop.
The basic steps are as follows:
1.
The default jet rate of 420 is appropriate, inspiratory time is 0.02.
2.
Begin with PIP value equal to or slightly lower than previously used on conventional ventilation and adjust PIP to achieve adequate chest wall movement.
3.
Set background IMV PIP 2–4 cm below the jet PIP, so as not to interrupt the cycling of the jet ventilator with a TI of 0.4 s and rate of 2–5/min.
4.
Confirm adequate PaCO2 with ABG and TCCO2.
5.
Increase IMV rate to 10/min.
6.
Increase PEEP by 1–2 cm H2O (depending on how high the FiO2 is) and observe SPO2.
7.
When SPO2 increases to >93 %, lower FiO2 as tolerated.
8.
Continue to increase PEEP every 5–10 min until (a) the FiO2 is below 0.40 or (b) there is no further improvement with a 2 cm increase in PEEP and PEEP is at least 10 cm H2O or the CXR shows hyperinflated lung fields.
9.
Monitor TCCO2 and chest vibration and be prepared to lower jet and IMV PIP quickly, if indicated, to avoid hyperventilation.
10.
Once FiO2 is in target range, drop the IMV rate to 2/min and continue to monitor SPO2 closely for at least 20–30 min.
11.
If SPO2 starts to drift down, the PEEP is not adequate and needs to be increased further (the IMV sighs were contributing to the MAP, lowering the IMV rate reduces MAP).
12.
If the SPO2 remains in the mid-90s, lower PEEP by 1 cm and observe SPO2 for at least 20 min.
13.
Continue to reduce PEEP in small increments every 10–20 min, as long as SPO2 remains in the 90s with FiO2 < 0.40, but do not reduce below 6 cm H2O.
14.
If SPO2 drops too much, indicating loss of recruitment, briefly increase the IMV rate again and repeat the recruitment process until the target FiO2 is achieved, then lower PEEP as before, but stop above the point that resulted in de-recruitment.
15.
HFJV rate and inspiratory time remain unchanged throughout.
If there is evidence of hemodynamic impairment at any time, lowering the PEEP should be considered, especially if the lung fields are well expanded on chest radiograph. However, it is not uncommon for infants with such severe lung disease to require MAP that is high enough to result in some degree of impairment of venous return and volume expansion is often necessary to maintain stable hemodynamics. A chest radiograph should be obtained following the initial recruitment maneuver and at least daily to detect lung overexpansion that may not be apparent clinically.
46.2.1.2 Elective Use of HFJV in Atelectatic Lung Disease
When initiating HFJV soon after birth, the clinician must choose settings based on clinical impression of the severity of lung disease. The pressure needed with a self-inflating bag or t-piece resuscitator in the delivery room is helpful in selecting the initial PIP. Appropriate PEEP will be determined based on initial oxygen requirement and chest radiograph, if available. The same general approach as described above is used, but more gently, as often these infants do not need aggressive lung volume recruitment.
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