The Slippery Slope of Weaning from Ventilatory Support
Steve Haskins United States
Weaning an animal off ventilatory support requires special attention since it may be slow to develop the ability to maintain its own ventilation and oxygenation requirements. Severe diffuse pulmonary parenchymal disease heals slowly; gradual withdrawal of support needs to mirror progressive improvement in oxygenating efficiency. The ventilatory muscles undergo disuse atrophy and ventilatory support must be withdrawn slowly so that the animal has time to redevelop muscle strength.
In a sense, the weaning process begins the moment an animal is set up and stable on the ventilator. That is to say, that one is frequently testing the animal during the course of the ventilation procedure by trying slightly less aggressive ventilator settings and then assessing the animal’s response. If there is no deterioration in lung performance, the ventilator settings are decreased a bit more, and so on. If there is unacceptable deterioration in lung performance, the settings are returned to their previous level since it is apparent that the animal is not yet ready to be weaned. After a period of time over which one might expect measurable improvement (but not longer than 24 hours), the process is repeated.
The ability to safely discontinue ventilatory support is always a retrospective assessment. Prospective predictors of “weanability” are not very reliable and must address the reason for which the animal was initially placed on the ventilator (e.g., don’t look at oxygenation parameters if the animal is being ventilated because of tetraplegia).
1. Respiratory center-mediated hypoventilation/apnea is usually associated with other signs of intracranial disease such as coma. These animals do not need to be sedated to be maintained on the ventilator. When these patients regain sufficient consciousness so that they no longer tolerate the ventilation procedure and endotracheal intubation, they probably will have regained sufficient function of their respiratory centers to maintain adequate ventilation. The ventilator settings are progressively decreased to see if the patient can maintain itself.
2. Patients with spinal cord or peripheral neuromuscular disease-mediated hypoventilation should be ventilated via a tracheostomy tube so that they do not have to be sedated/anesthetized. Return of muscle function and strength can then be more easily assessed. Strength of the withdrawal reflex; resistance to appendage flexion; strength of the spontaneous breathing effort; ability to generate a sub-atmospheric pressure greater than 5 to 15 cm H20 (cat to large dog, respectively) when inspiring against a closed airway; and ability to sit up or stand are all signs of possible weanability.
3. Patients with pulmonary parenchymal disease are the most difficult to assess. They may be ready to wean off mechanical ventilation when they require only minimal ventilator settings to generate acceptable arterial oxygenation; oxygenation parameters do not need to be normal, just acceptable, with inspired oxygen concentrations of less than 40%. The ability to maintain a PaCO2 of less than 45 mmHg with a normal-range tidal/minute volume and peak pressures also suggest that the patient may be weanable. The ability to maintain a PaO2 of greater than 80 mmHg with a PEEP < 4 cm H2O and an inspired oxygen of < 40% is also encouraging.
A series of screening questions to assess potential weanability is suggested. A “no” answer to any question suggests that the patient may not be weanable at this time. If the answer to all questions is “yes,” the ventilator settings should be decreased. An assessment of inspiratory muscle strength will help guide the direction of the process. If the patient has the muscle strength to decrease airway pressure more than 5 cm H2O (for a cat), 15 cm H2O (for a small dog), or 15 cm H2O (for a large dog), it is assumed that the patient can tolerate a more aggressive weaning process. If not, a “mild” weaning process, which does not initially demand too much of the patient, should be implemented.
There are many different methods of weaning but they all involve a gradual decrease in support and require a gradual increase in work by the patient. The method that you use is largely dependent upon the capabilities of your ventilator and the patient. Weaning methods include (in approximate order, from least to most work required of the patient; although there is considerable overlap): 1) pressure support (with decreasing amounts of pressure support); 2) intermittent mandatory ventilation (with decreasing mandatory breaths); 3) spontaneous breathing with continuous positive airway pressure; 4) a gradual decrease in set ventilator cycle rate so that the animal has to trigger progressively more breaths on its own (assisted breathing with a gradual increase in the required trigger effort); and 5) removing the patient from the ventilator for progressively longer spontaneous breathing trials. Combinations of these different weaning techniques are commonly used on a single patient.
The endotracheal tube, the breathing circuit, and the ventilator generally increase the resistance to inspiratory flow. Several modes (SIMV and CPAP) require that the animal breathe through the ventilator. Without pressure support, this adds resistance to the inspiratory effort and increases the work of breathing for the patient, and cannot be ignored.
1. Pressure support is a spontaneous breathing mode offered on some ventilators. The patient determines when to start an inspiration, how fast to inspire, and when to terminate the inspiration. The ventilator applies the pre-selected amount of positive pressure to the airway throughout the inspiratory phase. Gas flow rate from the ventilator is initially very high in order to keep up with the patient’s inspired gas flow rate, while maintaining the preset proximal airway pressure. As the patient’s lungs fill, the inspiratory flow rate decreases, and the fresh gas flow from the ventilator decreases proportionately. When the flow rate from the ventilator decreases to 20% of its peak, the ventilator switches to the expiratory phase. This is a very “patient-friendly” mode of ventilation because the patient is in total control of each tidal volume, be it small or large, or fast or slow. As the patient becomes stronger and capable of more of the work of breathing, the amount of pressure support can be decreased.
2. Intermittent mandatory ventilation (IMV) modes provide for a minimal number of controlled breaths from the ventilator, but allow the animal to breath spontaneously through the ventilator the rest of the time. Most IMV modes are synchronized to the patient’s breathing efforts (SIMV) so that a mechanical ventilator breath coincides with a spontaneous patient. As the intermittent controlled breaths are progressively decreased, the patient must breath more on its own. The patient has to breath through the resistance of the ventilator, breathing circuit, and tracheal tube.
3. Continuous positive airway pressure (CPAP) is a spontaneous breathing mode at an elevated airway pressure. Both rate and tidal volume are controlled by the patient while the CPAP keeps small airways and alveoli open between breaths. This maximizes oxygenation and facilitates the distribution of the next breath to the lower airways. The patient has to breath through the resistance of the ventilator, breathing circuit, and tracheal tube.
4. A gradual decrease in the ventilator cycle rate in ventilators with an assist-mode function, requires that the animal to trigger progressively more breaths of its own. Once triggered, however, the ventilator finishes the breath at the preset tidal volume, flow rate, and inspiratory time. The sensitivity control can be carefully adjusted to require progressively more effort from the patient to trigger a breath.
5. Ventilators without any type of weaning modes require that the patient be removed from the ventilatory for short spontaneous breathing trials. The length of time off the ventilator is progressing increased as the patient’s condition and strength allows.
A second series of questions is aimed at determining how well the animal is tolerating the weaning process. A “no” answer to all questions suggests that the animal is tolerating the weaning process and that a further reduction trial in the level of ventilator support is warranted. A “yes” answer to any question suggests weaning failure and that it is probably necessary to return to a more supportive ventilator settings which require less work from the patient.
Getting the animal off the sedative drugs
Sedative drugs cause respiratory depression and muscle weakness. Sedated/anesthetized patients will not breath as well as unsedated patients and therefore are more difficult to wean. Unfortunately, animals must remain on sedative drugs throughout the weaning process, until such time that it is safe to disconnect them from the ventilator and extubate. It is especially important, therefore, during the weaning process, to be sure to utilize the least amount of sedation compatible with patient-ventilator synchrony and retention of the endotracheal tube. It is also desirable to use a short-acting or reversible agent so that the sedative effects terminate as soon as possible following extubation.
Endogenous neurotransmitters will accommodate to long-term sedation and withdrawal of these drugs can be associated with excitation and possibly seizures. This is especially true for pentobarbital, which is the frequently used drug for most ventilation procedures. Recovery from short-term pentobarbital anesthesia is often rough and hyper-reflexive; adjunctive tranquilizers (diazepam, acepromazine) will smooth these out. Recovery from intermediate-length pentobarbital anesthesia (3–4 days) is associated with dysphoria and intolerance to handling; an adjunctive diazepam infusion (0.2–0.5 mg/kg/hr) from the time the pentobarbital is turned off to 24 hours after weaning and extubation should smooth this recovery. Recovery from prolonged anesthesia (1+ weeks) is associated with seizures. These patients should be started on phenobarbital (6 mg/kg q12h) at least four days prior to stopping the pentobarbital infusion. Since one never knows in advance when weaning will occur, it is better to start the phenobarbital early. It will help reduce the dose requirement of the other drugs during the procedure and then it will be there when you decide to start weaning the pentobarbital. Long-term pentobarbital infusions should not be stopped abruptly even with a phenobarbital background. Reduce the dose by ½ each day until the dose is below 1 mg/kg/hr, and then turn it off. An infusion of diazepam should be started if hyperexcitability or seizures occur. If more help is needed, keep the animal anesthetized with a propofol infusion (0.1 mg/kg/min) for 24 hours after the pentobarbital has been turned off (during which time a substantial proportion of the pentobarbital will have been metabolized and eliminated) and then let the animal recover from the propofol).
Neuromuscular blocking agents have been occasionally used in the management of ventilated animal patients, and they are commonly used in human intensive care units. Neuromuscular blocking agents remove all breathing capabilities which might be protective should there be an inter-rim ventilator malfunction or circuit disconnect. Withdrawal of the neuromuscular blocking agent is not always associated with reversal of the neuromuscular blockade. The reason for this is not known but may be associated with a down-regulation of acetylcholine receptors at the neuromuscular junction. In general, neuromuscular blocking agents are not necessary for ventilating animals and, given their potential adverse effects, are not recommended for use by untrained individuals.
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