Positive Pressure Ventilation

Outcomes have greatly improved with this therapy in foals. A commitment must be made for a prolonged and expensive course of therapy prior to initiating PPV in foals. With central causes of hypoventilation we have avoided PPV by administering I.V. doxapram to foals with hypercapnia and hypoxemia (See Assessment of Oxygen Needs).
Positive pressure ventilation (PPV) is indicated whenever a patient is not ventilating adequately as might occur in central nervous system or neuromuscular disorders, loss of rigidity of the chest wall or open pneumonia. A closed pneumothorax is a specific contraindication to the use of PPV. PPV may be useful in foals with moderate to severe pulmonary parenchymal disease that cannot properly oxygenate blood because of small airway and alveolar collapse in hyaline membrane disease of premature foals.

I. General Principles of Positive Pressure Ventilation

1. The amount of pressure to apply to the proximal airways for ventilation of normal lungs is about 20 cm H2O. Apply just enough pressure to achieve a full and adequate tidal volume; additional pressure is not necessary and may deleteriously affect intrathoracic blood flow. It is often necessary to increase the peak airway pressure above 20 cm H2O for lungs that exhibit reduced compliance due to parenchymal disease.
2. Inspiration should last only as long as is necessary to achieve a full and adequate tidal volume; a longer inspiratory phase is not necessary and may induce inordinate impairment of thoracic blood flow. In normal lungs an inspiratory time of 1.0-1.5 seconds is usually sufficient.
3. The tidal volume is determined by the airway pressure and the inspiratory time. The tidal volume should range between 10 and 20 ml/kg.
4. Minute ventilation is the product of tidal volume and breathing frequency and should be 150-250 ml/kg/min.
5. Once the initial ventilator settings are established the patient should be evaluated clinically and, if available, by laboratory analysis of tidal volume, minute ventilation, and arterial blood oxygen and carbon dioxide to verify an acceptable patient response and the absence of any untoward effects.

1. If the response is less than desirable the ventilator settings should be altered in an appropriate manner.
2. If the patient begins to fight the ventilator or breathe at a rate faster than the ventilator, check for inappropriate ventilator settings, ventilator malfunction, or soda lime exhaustion. Ascertain that pre-existing conditions have not been worsened by the ventilation procedure (e.g., pneumothorax, hypotension).

II. End-Expiratory Pressure

1. Indications

1. Some lungs are so severely diseased that standard ventilator settings are not sufficient to restore acceptable blood oxygenation. Higher airway pressures and longer inspiratory times improve alveolar ventilation and are useful up to the point at which they cause inordinate impairment of venous return and cardiac output. The application of positive airway pressure during the exhalation phase of the breathing cycle enhances transpulmonary pressure and helps prevent small airway and alveolar collapse without impairing intrathoracic blood flow. Airway alveolar units which do not collapse between breaths are better ventilated on the subsequent breath.

2. Methods

1. Expiratory resistance may be applied by attaching a narrow apertured device to the exhalation port of the ventilator or patient circuit. Exhalation is retarded and mean airway pressure is increased, however, airway pressure will still decrease to atmospheric.
2. A positive end-expiratory pressure (PEEP) plateau may be achieved utilizing a corrugated breathing tube leading from the pressure relief value or exhalation port of the ventilator to an underwater position in a partially filled bottle. The depth to which the end of the tube is positioned determines the airway pressure at the end of exhalation. Commercial PEEP valves are also available.
3. Continuous positive airway pressure (CPAP) is a control variable on new ventilators which allows the patient to breathe spontaneously between cycles of the ventilator, while maintaining airway pressure at a preset level during the duration of the spontaneous breath. The inspiratory cycle of the ventilator is independently regulated.

1. Optimal end-expiratory pressure is often in the range of 5-15 cm H2O.

4. Optimal end-expiratory pressure is determined as:

1. Settings which alleviate the clinical signs of hypoxemia while causing minimal diminution of measured blood pressure or palpated pulse quality in a peripheral artery; or
2. Settings which generate an arterial partial pressure of oxygen (PaO2) of at least 60 mmHg with an arterial partial pressure of carbon dioxide PaCO2 of between 30 and 40 mmHg, and an inspired oxygen concentration of less than 50 to 60%, with less than about a 20% decrease in arterial blood pressure or cardiac output; or
3. Settings which generate the highest oxygen availability (cardiac output x arterial oxygen content); or
4. By settings which generate the highest mixed venous PO2. Mixed venous PO2 reflects the overall adequacy of oxygen delivery to the tissues compared to oxygen consumption.

III. Management of a Foal Receiving Continuous Ventilatory Support

1. Minimize trauma to the trachea by the tube: avoid traction or torsion on the tube by the foal or the ventilator circuit.
2. Minimize trauma to the trachea by the cuff: inflate the cuff carefully with the minimal occlusive volume and record the volume.
3. Emergency intubation and tracheal suction equipment should be readily available.
4. Maintain airway humidity and patient hydration.
5. Carefully suction the trachea every 2-3 hours as necessary. If the suctioning procedure fails to collect any secretions consider that airway humidification procedures may not be adequate.
6. Weigh foal daily and avoid overhydration.
7. Consider nasogastric tube or TPN for nutritional support.
8. Re-evaluate ventilator function and the foal's response to the therapy frequently throughout the day.