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1. When is mechanical ventilation indicated?
Mechanical ventilation is indicated for: (1) animals that are unable to maintain Ra02> 50 mmHg. Art., despite breathing oxygen (through a mask, nasal catheters
or in an oxygen tent); (2) animals unable to maintain PaCO2 <50 mm Hg. Art., despite the end of the action of respiratory depressants or thoracocentesis (if clinically indicated). This is called the “50/50” rule. Other indications for mechanical ventilation include: 1) clinical deterioration, which threatens the development of respiratory failure; 2) cardiac arrest and breathing.
2. What devices are used for mechanical ventilation?
For artificial ventilation, both simple devices - Ambu bag attached to the endotracheal tube, and complex - computer respiratory centers for long-term respiratory support are used. In general, the latter provide better control over ventilation parameters (oxygen content, humidity, tidal volume, inspiratory pressure) than a manually controlled respirator.
3. What types of ventilation are commonly used in veterinary medicine?
For volume-limited ventilation, the respirator supplies a certain amount
air into the patient’s airways regardless of the pressure required
for this. With a sudden increase in airway pressure that occurs:
as a rule, during obstruction, an alarm is triggered. During ventilation with pressure limitation, the respirator delivers air up to a pre-set pressure value for inspiration, regardless of volume. Due to the tendency to a decrease in tidal volume due to obstruction of the airways or changes in the pulmonary complex, the PaCO2 value, as well as the tidal volume (by spirometry), need to be monitored more often. According to some reports, the use of pressure-limited ventilation is preferable in small animals (<4 kg).
4. What are the most commonly used ventilation modes? Auxiliary control mode. The breathing apparatus is set for a certain number of breaths in 1 min. The respirator takes a breath when the patient generates negative inspiratory pressure; if the patient does not breathe, the device delivers the gas mixture at the set frequency. An animal with frequent breathing (e.g. shortness of breath) may develop hyperventilation. In this case, use a different ventilation mode.
With synchronized intermittent forced ventilation, the breathing apparatus produces a predetermined number of breaths in 1 min. Respiration can be initiated by negative inspiratory pressure, but if the patient's respiratory rate exceeds that in the device, the latter does not support breathing. This mode is used when the animal is disconnected from the breathing apparatus, since the number of breaths set by the respirator can be slowly reduced.
For spontaneous ventilation, many types of respirators are used; in this case, the breathing apparatus works as an anesthetic device. In veterinary medicine, this ventilation mode is rarely used, but they resort to it to supply the animal with the established oxygen concentration or during constant monitoring of the patient after it is disconnected from mechanical ventilation (IVL).
5. When is intubation recommended for an animal, and when is a tracheostomy recommended?
The choice of method for ensuring airway patency depends both on the underlying disease and on which method the doctor prefers. The advantages of oral methods of providing access to the respiratory tract include simplicity, speed of their implementation and minor tissue injury. The advantages of a tracheostomy are less need for additional funds and less immobilization, as well as maintaining the ability of the animal to ingest food and water through the mouth; disadvantages - surgical intervention in an animal with weakened immunity and the possible need for more careful observation than with general anesthesia (due to the risk of occlusion or displacement of the tracheal tube).
In general, it is advisable for a patient who has to be on mechanical ventilation for more than 36-48 hours to consider performing a tracheotomy.
6. What drug regimens are designed for sedation and anesthesia?
The ideal drug causes minimal inhibition of the cardiovascular system, is easily titrated and economical. Obviously, such a drug does not exist. Pentobarbital (2-16 mg / kg, intravenously every 4-6 hours) was successfully used in many dogs. The principal advantage of pentobarbital is its duration and relatively low cost; disadvantages - a long recovery phase and a lack of withdrawal drugs. Oxymorphone (0.05-0.1 mg / kg, intravenously as needed) is often prescribed in combination with diazepam (0.250.5 mg / kg, intravenously as needed). Opiates usually do not affect the cardiovascular system, but are expensive and require frequent administration. Also use infusion at a constant speed (before the onset of effect) of fentanyl or propofol. Occasionally, muscle relaxants, in particular atracurium (0.2 mg / kg, intravenously), are prescribed to facilitate mechanical ventilation. It is very important to introduce muscle relaxants in combination with good painkillers.
7. What is PEEP?
Positive end-expiratory pressure (PEEP) improves blood oxygenation in patients with hypoxia that persists despite a high concentration of respirable oxygen and normal or decreased PaC02. PEEP prevents full expiration and thereby increases the functional residual capacity, prevents the early closure of small airways and increases the size and number of functioning alveoli, improving the ventilation-perfusion ratio. PEEP also reduces venous return to the heart and cardiac output.
8. What problems are associated with mechanical ventilation?
Mechanical ventilation is not without risk. The main clinical problems are barotrauma and infection. Barotrauma is the result of excessive positive pressure in certain areas of the lungs, which causes their rupture and the formation of pneumothorax (or pneumomediastinum). One of the most common causes of a drop in blood oxygen saturation in a previously stable patient on mechanical ventilation is the development of severe pneumothorax. Pneumothorax is possible in animals with severe lung disease, but its occurrence should not be considered as a significant deterioration in the condition of the animal.
Infection is another serious problem with mechanical ventilation. The infection often spreads to the lungs of the infected upper respiratory tract and oropharynx, as the physiological defense mechanisms of the upper respiratory tract are shunted. In addition, patients on mechanical ventilation have a state of immunodeficiency, and they are also immobilized, which also increases the risk of infection. It is necessary to make every effort to maintain cleanliness and sterility and conduct regular microbiological inoculations (every 24-48 hours). Their results, taking into account the clinical symptoms, determine antibiotic therapy.
Possible consequences of mechanical ventilation include decreased venous return, oxygen intoxication, damage or irritation of the upper respiratory tract, and impaired musculoskeletal function associated with the patient’s prolonged lying position.
9. What is the prognosis for animals on mechanical ventilation?
The prognosis for ventilated animals depends on the underlying disease. So, in a 15-year-old dog with recurrent aspiration pneumonia due to megaesophagus with rapidly progressive respiratory failure, the prognosis is difficult, and in a young dog with a traumatic violation of the chest frame and lung contusion, the prognosis is favorable. According to the author’s experience, the aim of treating such animals should be to ensure survival rates above 30-40%, followed by a good quality of life.
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