Modifying IPPB Therapy
To know when and how to modify IPPB therapy, you first must understand the three basic indications for these treatments:
Although IPPB for the last indication (providing short-term ventilatory support) has been largely replaced with noninvasive positive pressure ventilation (e.g., BiPAP), it is still used frequently enough for this purpose to warrant a brief discussion.
From Sills JR. Respiratory Care Certification Guide. 2 ed. St. Louis: Mosby; 1994.
Depending on the IPPB device used, you generally have direct control over the following parameters during IPPB therapy:
In addition, you are responsible for selecting the appropriate airway appliance (mouthpiece, flanged mouthpiece, mask, or 15 mm ET tube adaptor) for the patient.
The rate and depth of breathing are primarily determined by the patient and the interaction of the patient's lung mechanics with the therapist-chosen settings. Changes in these parameters can require both patient coaching and adjustment of the IPPB device, as outlined below.
Airway Interface. Generally, you select a mouthpiece for alert patients; for patients with a trach or ET tube, you select a 15 mm adaptor and short flex tube.
Basic Settings. Regardless of indications, you generally start out new patients with similar initial IPPB settings:
FIO2 Control. Matching the patient's prescribed FIO2 is easier said than done. Pneumatic units can be powered by either compressed air (21% O2) or 50 psig oxygen. When run on oxygen, you alter FIO2 via an air-mix control. When the air mix control is off/closed, these devices deliver 100% O2. When air mix is on/open, pneumatic IPPB units deliver a variable FIO2, usually averaging above 50%. To get precise control over FIO2 with pneumatically-powered IPPB units, you must attache them to an O2 blender and set them to deliver pure source gas (air mix OFF).
The air mix control also affects the amount and pattern of inspiratory flow. In the air-mix modes, flow capability is higher and the pattern of flow (decelerating ramp) more normal. When air-mix is off (pure source gas delivery), available flow is less and often delivered via a less normal square wave pattern (constant flow).
Electrically-powered IPPB units use small air compressors and thus deliver 21% O2. Moderate but inexact FIO2s can be achieved with these units by bleeding 100% O2 into the delivery circuit.
Initial Adjustments. Immediately upon initiation of therapy, you should carefully observe the IPPB unit's manometer for the pattern of pressure changes occurring during breathing (see the graphic representations of these changes in the figure below). The goal is to achieve quick and near-effortless on-triggering, followed by a relatively rapid pressure rise and ending in a short plateau (C in the figure). In pattern A, the large drop in airway pressure below zero before the breath starts indicates improper triggering, usually corrected by increasing the sensitivity. In pattern B airway pressure drops below zero after the breath starts. This 'scalloping' of the pressure waveform is a tell-tale indicator that the patient's flow exceeds that provided by the machine. Normally, if you observe this condition you would either coach the patient to relax and let the machine do the work, or increase the inspiratory flow setting (Bird and PR-2 only).
Scanlan, CL, Wilkins, RL & Stoller, JK (Eds.). Egans Fundamentals of
Respiratory Care (7th Ed.). St. Louis, MO: Mosby, 1999.
If, on the other hand the machine does not cycle when the patient reaches end-inspiration, you most likely have a leak. Assuming prior proper check-out of the device before patient application (to confirm no system leaks), the problem is most likely a leak at the airway interface. In alert patients, simple coaching can overcome this problem ("seal the lips tightly around the mouthpiece!"). Sometimes adding noseclips will help if the leak is mainly through the nose. Mouth leaks in edentulous patients or those less alert can generally be overcome by using a flanged mouthpiece ('Bennett Seal'). For patients with tracheostomy or ET tubes, the primary source of leaks are cuff-related. Proper cuff inflation can solve most of these problems.
If leaks cannot be completely overcome, some IPPB devices provide a control to help. Typically called 'terminal flow' these controls add flow at the end of the breath to help compensate for the leak and allow proper off-cycling.
In unconscious patients, those with significant upper airway muscle weakness or those not able to cooperate, you may sometimes need to use a mask to apply IPPB therapy. You must take great care when applying IPPB therapy by mask, due the risk of gastric aspiration. Always use a clear mask and try to avoid pressures over 15 cm H2O (the esophageal opening pressure).
Once proper triggering, a good pressure pattern and normal off-cycling are established, you should coach the patient to breath slowly (6 to 8/min) and establish an I:E ratio of 1:3 or less (1:4, 1:5).
After establishing a good breathing pattern, you will usually modify the therapy to better achieve the basic goal - either lung expansion or aerosol drug delivery.
Modifications to Achieve Effective Lung Expansion. As a lung expansion therapy, the goal of IPPB therapy is to achieve inspired volumes large enough to prevent or reverse atelectasis. The common goal is a tidal volume of 12-15 mL/kg ideal body weight (IBW). For a 70 kg man, this would mean a tidal volume of between 840 to 1050 mL, or about a liter. To assess goal achievement, you must measure the tidal volumes during therapy. To do so, you must attach a volumeter to the IPPB circuit's expiratory port.
The tidal volume achieved during IPPB is a function of patient effort, patient mechanics and applied pressure. With proper coaching and pressure adjustment, most patients can achieve a tidal volume in the 12-15 mL/kg range. In general, if the volumes are falling short of your goal, you slowly increase the pressure a couple of cm H2O at a time until you approach 12-15 mL/kg. The lower the compliance of the patient's lungs and/or thorax, the greater will be the pressure needed to achieve these volumes.
Modifications to Improve Aerosol Drug Delivery. When used to aid aerosol drug delivery, emphasis is on achieving a good pattern of breathing and properly timing nebulization. As with drug delivery to spontaneously breathing patients, the goal is moderately deep breaths, followed by a breath hold. Volumes need not be measured, but can be judged at the bedside by observation. On the other hand, if a bronchodilator is being administered and the patient is able, pre/post expiratory flow measurements should be made to assess drug action.
To achieve the ideal breathing pattern for aerosol drug deliver usually requires lower flows, especially in patients with airway obstruction. This is because airway obstruction can cause premature off-cycling of IPPB devices when set to deliver high flows. Lower flows also tend to increase the inspiratory time, which can further aid drug deposition in the lungs. However, coaching is required to assure an good breath-hold, since IPPB devices do not provide this feature.
In addition, to avoid wastage, and maximize delivery, you normally only nebulize the drug during inspiration. Otherwise, as much as 1/2 to 2/3 of the drug aerosol will escape out the expiratory port during exhalation. Other modifications to IPPB for aerosol drug delivery are similar to aerosol drug delivery in general (e.g., dosages, frequency of administration, etc.).
Whether trying to aid lung expansion or delivery aerosolized drugs with IPPB, you must always be on guard to prevent hyperventilation. Because some patients tend to breath too rapidly during a IPPB procedure, hyperventilation is a common side effect. Feelings of dizziness, light-headedness, and tingling in the fingers (paresthesia) are all symptoms of hyperventilation. Coaching the patient to maintain low rates of breathing (6 to 8/min) is the best way to avoid this problem.
Modifications to Provide Short-Term Ventilatory Support. Although short-term ventilatory support can be provided by mouthpiece with cooperative patients, it is more common to use an oronasal mask or deliver support directly through an ET tube. When providing short-term ventilatory support via IPPB, you normally select a device capable of automatic time-triggering (Bird Mark 7 or 8, Bennett PR-1 or PR-2 ). This provides the equivalent of assist-control ventilation (assuring back-up ventilation should the patient become apneic). To time-trigger the Bird Mark 7 or 8, you activate the Apnea control. The Bennett PR-1 and PR-2 have a similar Rate control that activates time-triggering. However, the Bennett rate control differs from that on the Bird units in that it also provides for time-cycling of breaths. This makes its function very similar to pressure support ventilation, which is either flow- or time-cycled. In addition, the PR-2 allows independent adjustment of the I:E ratio via an Expiratory Time control. With both the Bird and Bennett units, the inspiratory time is a function of (1) the set pressure limit, (2) the inspiratory flow, and (3) the patient's lung mechanics. You control I-time mainly via the set pressure and flow. In general, I-times are shortest with low pressures, high flows and when the compliance is low and the resistance is high. Long I-times occur under the opposite conditions.
COPD patients present a special case for three reasons: (1) high expiratory flow resistance, (2) potential sensitivity to oxygen-induced hypoventilation, and (3) compensated respiratory acidosis.
High expiratory flow resistance makes COPD patients prone to air trapping during IPPB therapy. To help avoid this problem encouraged these patients to breath at lower than normal rates (< 6-8/min). In addition, you can shorten the inspiratory time and get a lower I:E ratio by increasing the flow. This will give the COPD patient more time to fully exhale. An alternative approach is expiratory retard. Expiratory retard slows expiratory flows and and creates 'back pressure' against exhalation. This keeps the small airways open for a longer period of time and may minimize air trapping/auto-PEEP in patients with COPD. Expiratory retard increases mean airway pressure which may result in cardiovascular depression.
In terms of COPD patient and oxygen-induced hypoventilation, this can be avoided by titrating the patient's FIO2 during therapy to match his or her prescribed FIO2. Often the easiest way to assure this match-up is to apply air-powered IPPB therapy while the patient remains on nasal O2 at the prescribed flow. Pneumatic devices driven by oxygen should be avoided since they provide moderate to high FIO2s.
The last problem is seen when a COPD patient with compensated respiratory acidosis (e.g., pH of 7.37, PaCO2 64 mm Hg, and HCO3 33 mEq/L) receives IPPB. The typically increase in ventilation that can occur with this therapy easily disrupts this compensated state, causing an rapid decline in PCO2 and abrupt alkalosis. You avoid this scenario by coaching the patient to breath slowly.
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