Airway management and ventilation (Proceedings)

There are many factors that play a role in airway management and ventilation. One should consider reasons for airway support as well as types of materials that are available. Some patients have a need for ventilation support, therefore it is important to understand the terminology as well as equipment available for the veterinary patient.

Endotracheal Tubes and Intubation

The main reason for intubation is administration for administration of inhalation anesthesia, reduced waste gas pollution and protection of the airway. Intubation reduces the risk of saliva or regurgitated stomach contents being inhaled. An endotracheal tube also allows for the administration of oxygen as well as intermittent positive pressure when required. Many designs of endotracheal tube are available and the choice will depend on your patient. A large diameter tube offers the advantage of reducing airway resistance and will therefore facilitate breathing and ventilation. Pre-measure the length of the tube as a short tube might get dislodge during a procedure, while a long tube might increase dead space as well as creating lung trauma and respiratory complications. To increase airway protection, use as often as possible, cuffed tubes. When inflating the cuff, be sure to check the pressure to insure a good seal so to avoid leak, but also to avoid tracheal damage with too much pressure. Always use clean, dry tubes. There are various materials available to aid intubation such as a laryngoscope, mouth gags, lubricating gel, local anesthetic spray, stylets and endoscopes.

Intubation Complications

Some complications associated with intubation are damage to the larynx (more common in small species such as cat, ferret and rabbit). This can come from an over-inflated cuff, an oversized diameter or frequent manipulation of the tube. You may also see hematoma formation, edema, and some species will laryngospasm such as the cat, ferret and rabbit if larynx is stimulated frequently. Use local anesthetics to avoid this. Mucosal sloughing may result due to any airway damage. Endobronchial intubation is another concern if tube was not measured properly. Aspiration can also occur if patient extubates pre-maturely, or if there is a leak in the cuff and or endotracheal tube. This fluid/ reflux can derive from the esophagus or stomach. It may also include fluid from the larynx or oral cavity such as blood or excessive saliva. One must make sure there is little resistance in the tube. The inside of the tube should be clean and free of fluid; avoid kinking the tube at all times. Use elbow adapters if angling is necessary or use a wire-reinforced tube for procedures that involve much head and neck manipulation.

Respiratory Inadequacy

Depression of the respiratory centers in the brain can cause inadequate ventilation. This can be drug-induced. If the inadequacy is due to respiratory depression from anesthetic gas, reduce inhalant and consider ventilatory support. There are metabolic disorders that can affect your patient's ventilation such as an acidosis. A metabolic acidosis can alter the respiratory centers as well. Another cause for altered ventilation would be a physical change in your patient such as thoracic pain. Patients who are unable to expand their lungs or thorax to a normal ability may need ventilation support. To provide this, one must perform intermittent positive pressure ventilation (IPPV).

IPPV

Intermittent positive pressure ventilation consists in opening the airways of a patient by blowing a flow of gas into the lungs. This will create a positive pressure. Exhalation is usually passive. The magnitude, rate & duration of flow are determined by the operator. The flow can be a pre-set volume & pressure variable or pressure limited & volume variable. IPPV can benefit many patients, especially those with respiratory inadequacy

Ventilation Considerations

Controlled or assisted ventilation, if applied improperly, might create more harm than good and knowledge of the equipment, familiarity with the techniques as well as reason for anesthesia are necessary to achieve an optimal result. Knowledge of the normal cardiopulmonary physiology as well as the ability to interpret arterial blood gases are a must. Controlled ventilation, where the breathing rate as well as volume is determined by the anesthetist, should be considered for patients who do not ventilate and or do not oxygenate adequately. It can also help maintain an adequate anesthetic plane. Capnography will help determine ventilation adequacy but arterial blood gases are the best. Settings such as the positive inspiratory pressure (PIP), ventilation rate, or tidal volume of the ventilator will have to set up accordingly to your patient's needs and/or underlying disease. A patient with low arterial blood pressure will have to be corrected if possible before using a ventilator as its use may alter cardiovascular hemostasis. Check to make sure the endotracheal tube is cuffed properly. Positive end expiratory pressure (PEEP) may be helpful in keeping the alveoli open if oxygenation needs to be improved.

Ventilation Terminology

MV- Mechanical Ventilation- A mechanical ventilator is a machine that generates a controlled flow of gas into a patient's airways.

IPPV-Intermittent Positive PressureVentilation Positive pressure that is maintained only during inspiration (manual or mechanical).

PEEP- Positive End Expiratory Pressure used to open small airways after lung trauma or pulmonary edema (PEEP: 1-5 cm H2O)

PIP- Peak Inspiratory Pressure Airway pressure in lungs (PIP: 15-20cm H2O)

VT- Tidal Volume - A full breath inspired and expired measured in milliliters (10-20ml/kg)

VE - Minute Ventilation- Tidal Volume x Respiratory Rate (RR) (200ml X 10 BPM= 2 Liter)

I: E Ratio- Time of Inspiration: Expiration (1:2 - 1:3)

IT- Inspiratory time- Time in seconds to inflate the lungs. (1sec)

Ventilatory rate- Number of cycles per minute

PaO2- Partial pressure of oxygen in arterial blood

PaCO2- Partial pressure of carbon dioxide in arterial blood

Classification of Breath Termination

The classification determines the cycling of ventilators. This tells the ventilator how to switch from inspiration to expiration. It can be based on time,volume or pressure.

Modes of operation- How to control the ventilated breath!!

With volume controlled the preset tidal volume remains constant. A patient's lung compliance and resistance may change over time. The advantage of volume control is you will deliver guaranteed minute ventilation. This may be important in a situation where lung compliance may be influenced by the type of surgery such as a pulmonary lobectomy.

A pressure controlled ventilator will deliver a fixed pressure throughout the breath. In this setting, the tidal volume will vary with lung compliance and resistance. Pressure control ventilators do not guarantee tidal volume or minute ventilation, and therefore requires more monitoring by the operator. The pressure may need to be increased in the patient that has "stiff" lungs (lungs with lower compliance) or in open-chest procedures.

Assisted ventilation- Patient will trigger the ventilatory device by initializing an inspiratory effort

Controlled ventilation- Operator dictates respiratory rate (RR). Ventilator does not respond to patient's inspiratory efforts.

Assisted- controlled ventilation- Operator can control a RR. Patient can also trigger ventilation

Mechanical Ventilator Anatomy

Exhaust valve- During exhalation, it opens to allow driving gas inside housing (outside bellows) to be exhausted to atmosphere. This gas is usually oxygen.

Spill valve- A means for scavenging exhaled patient gases from inside bellows.

Ventilator hose connection- Connects breathing system to ventilator (where reservoir bag is located)

Bellows- Rubber bag inside the housing that delivers the breath; essentially becoming the rebreathing bag. Many ventilators allow for spontaneous breathing.

Optimal Ventilatory Strategy

To achieve good ventilation strategy, try to achieve a minute ventilation to assure a PaCO2 (arterial carbon dioxide partial pressure) of 35-45mmHg. This can be measured using an arterial blood gas or estimated using a capnograph as EtCO2 (end tidal CO2). This can give you a close estimation of the PaCO2. Maintain proper oxygenation to your patient. You can monitor your patient's oxygenation with pulse oximetery and or arterial blood gases. An oxygen saturation of >95% or a PaO2 of >90 mmHg is sufficient. Moderate PEEP will prevent alveolar collapse, which may improve PaO2. When weaning the patient from the ventilator, lower the rate of ventilation, decrease the patient's anesthetic depth, and reverse neuromuscular blocking drugs (NMB's) if required. Monitoring ETCO2 and or PaCO2 is recommended during the weaning process.

IPPV versus Spontaneous Ventilation

With spontaneous inspiration the intra-thoracic pressure decreases and becomes more negative. During IPPV the intra-thoracic pressure increases and becomes positive resulting in physiological changes. A positive intra-thoracic pressure will compress the right atrium & vena cava and consequently venous return will decrease with the cardiac output (CO) and possibly the patient's arterial blood pressure. Increasing intra-thoracic pressure can also cause a vagal response by stimulating the parasympathetic nervous system resulting in a decrease in heart rate (HR) and possibly in a decrease in CO.

CO = SV x HR SV = stroke volume

MAP = CO x SVR SVR = systemic vascular resistance

MAP= mean arterial blood pressure

The degree of a decrease in CO depends on lung and chest wall compliance and the level of intra-thoracic pressure (airway pressure). An increase of PIP or the use of PEEP may also result in a decrease in arterial blood pressure (ABP). Damages of the lung may also be a consequence of a change in PIP or PEEP.

Respiratory Assist Device

Ambu Bags are nice for emergency situations (rapid ventilation) or for supplementing oxygen. They are useful for weaning patients from the ventilator as well as be used to assist ventilation during transport.

References

Greene, Stephen A: Veterinary Anesthesia and Pain Management Secrets, Philadelphia, Hanley & Belfus, Inc., 2002

McKelvey D, Hollingshead KW: Veterinary Anesthesia and Analgesia, 3rd ed. St. Louis, Mosby, 2003

Meyer & Harvey: Veterinary Laboratory Medicine Interpretation & Diagnosis, 2nd ed. Philadelphia, W.B. Saunders, 1998

Muir W, Hubbell J, Bednarski R: Handbook of Veterinary Anesthesia, 4th ed. St. Louis, Mosby, 2007

Paddleford: Manual of Small Animal Anesthesia, 2nd ed. Philadelphia, W.B Saunders, 1999

Seymour C, Gleed R: Manual of Small Animal Anaesthesia and Analgesia, Shurdington, UK, BSAVA, 1999

Thurmon J, Tranquilli W, Benson G: Essentials of Small Animal Anesthesia & Analgesia, Philadelphia, Lippincott Williams & Wilkins, 1999

Tranquilli W, Thurmon J, Grimm K: Lumb, Jones', Veterinary Anesthesia and Analgesia 4th ed. Iowa, Blackwell Publish, 2007