Vol. XIV: A Guide to Short Term Mechanical Ventilation

Ventilation of the lung is defined as the mass movement of gas into and out of the lung. It is essential that the patient has sufficient alveolar ventilation, in other words, that the volume of inspired gas is sufficient to maintain normal partial pressures of arterial oxygen and carbon dioxide. Mechanical ventilation may be provided manually by compressing the reservoir bag intermittently. However, in some cases, a mechanical ventilator is preferred. Because positive pressure is required to force gas into the lungs, this is usually referred to as intermittent positive pressure ventilation (IPPV). This guide is written to use with the Hallowell 2002Pro ventilator. Hallowell Ventilators are pressure controlled and time cycled. Pressure controlled and limed cycled are the most commonly used in veterinary medicine today.

The new Hallowell 2002Pro has a single multiturn flow valve instead of the coarse and fine tune flow valves in previous models. The multiturn flow valve or volume knob can regulate inspiratory flow from 0 – 100 1pm. This new option allows the anesthetist to adjust the volume easier and more precisely with small patients. Mechanical ventilation uses positive pressure to assist or replace normal breathing by the anesthetist. This may involve a machine (ventilator) or manually. The purpose is to provide respiratory support to an animal that may be compromised due to the administration of analgesics, patient positioning, respiratory disease, or thoracic cavity compromise. Short term mechanical ventilation is used with 100% oxygen for no longer than a 12 to 24-hour period.

Key Words:

• Tidal Volume (TV) – amount of gas delivered during one inspiratory phase of ventilation. TV= 10 -15 ml/kg body weight.
• Respiratory Rate (RR) – number of breaths the patient receives in one minute. RR= 8 -12 breaths per minute in an anesthetized patient.
• Minute Volume (MV) – amount of gas the patient inhales during one-minute MV = TV x RR. Normal minute volume should be 150-250 mVkg/min 4.
• Peak lnspiratory Pressure (PIP) – the amount of pressure that the delivered gas puts on the airway. Normal pressure= 15 – 20 cm H20.
• Intermittent positive pressure ventilation (IPPV) – Artificial inspirations either manually or mechanically delivered to the patient’s airway.
• Hypocapnia – Decreased amount of CO2 in the blood. ETCO, levels 35 mm Hg or lower.
• Hypercapnia – lncreased CO, in the blood. ETC02 levels 60 mm Hg or higher.
• Ventilation – perfusion inequality (V/Q mismatch)-A defect were ventilation and perfusion are not matched.
• Compliance – The ability of the lung to fill or expand.
• Atelectasis – collapse of the alveoli.
• Functional residual capacity (FRC) – is the amount of air left in the lungs after expiration.
• Positive End Expiratory Pressure (PEEP) – A pressure remaining in the airway at the end of expiration.
• Capnography – End tidal CO, monitoring.
• “Bucking” – This occurs when the patient is breathing against or around the ventilator.

Risk of Using a Ventilator

Barotrauma can happen if the pressure is set too high causing pneumothorax or pneumomediastinum. Hyperventilation or hypoventilation can occur from over ventilating or under ventilating the patient respectively. Venous return could be decreased from mechanical ventilating causing a decrease in cardiac output resulting in decreased blood pressure and tissue prefusion.

Ventilating the Patient

Use the pediatric bellows for patients up to 30 pounds. The pediatric bellows and housing is 0 – 300 ml. For patients 25 – 220 pounds use the large bellows. The large bellows and housing is 300 – 1600ml. For patients 200 – 440 pounds use the 1600 – 3000 bellows and housing.

1. Plugin ventilator electric power cord.
2. Attach oxygen to ventilator using the 02 demand Wye valve splitter.
3. Close pop-off valve.
4. Replace reservoir bag with ventilator output tubing.
5. Attach scavenger tubing to ventilator exhaust port.
6. Insert Airway Pressure Sampling Tee and tubing in between anesthetic machine and system tubing.
7. Set Maximum Working Pressure Limit to 30 cmH,O.
8. Turn Volume dial to the decrease side of the range.
9. Turn on ventilator and set Rate to 10-12 BPM.
10. Increase or decrease Volume as needed. Start with a 10 ml/kg (5 ml/#) tidal volume.
11. Monitor pressure manometer. Set pressure between 12 – 20 cmH,O
    Do not exceed 20 cmH,O max 12. Monitor patient end tidal CO., ETCO, Should be maintained between 35-45 mmHg.

Turning the Ventilator Off

It is not recommended to just turn the ventilator off. The patient should be weaned off allowing the patients CO, to build up to kick start the respiratory response. The ventilator respiratory rate and flow rate should be decreased. A rise in the ETCO, should happen. Once the patient starts spontaneous respirations the ventilator can be turned off. Make sure to disconnect the ventilator, add the reservoir bag, and open the pop off valve. Careful close monitoring of the patient should be continued until the patient’s respiratory rate returns to normal and the endotracheal tube is removed.

It is recommended lo have your ventilator tested for quality assurance once a year. Vetamac can provide this service onsite along with anesthesia machine service.

Works Sited: Anesthesia tor Veterinary Technicians, Bryant.