The ABCs of veterinary dentistry: V is for ventilation monitoring

Ventilation is the process of gas exchange in and out of the lungs (i.e. bringing in oxygen and flushing out carbon dioxide). The goal of assisted ventilation is to provide adequate respiratory support, which improves oxygenation and stabilizes the plane of anesthesia.

What does ventilation have to do with veterinary dentistry? Whereas in human dentistry only local anesthesia is used for most procedures, veterinary dental procedures necessarily involve general anesthesia. Paying close attention to respiration, oxygenation and carbon dioxide levels increases the safety of our anesthetic procedures and patient wellbeing.

Respiratory rate

Inhalant and injectable anesthetics, including opioids and alpha-2 agonists, can lead to ventilatory suppression. In the absence of capnography, respiration can be observed subjectively by watching the anesthesia bag, the patient’s chest wall and condensation in the endotracheal tube, as well as by auscultation of breath sounds.

Many patient monitoring systems default respiration readings to impedance respiration, using the indirect method of deriving respiration from the up-and-down movement of the patient’s chest via the electrocardiogram leads. This indirect method is neither accurate nor reliable. Apnea monitors with loud alarms are also helpful in alerting the veterinary team to respiratory arrest.

An elevated respiratory rate may indicate progression from a moderate to light plane of anesthesia. Digital monitoring of respiratory rate and other signs of arousal during dental procedures can help avoid bite trauma to staff, radiography sensors or plates, and even monitoring equipment (Figure 1).

Figure 1. Monitoring respiratory rate and other signs of arousal digitally during dental procedures can help avoid bite trauma to staff, radiography sensors or plates, and even monitoring equipment Here, a respiratory rate of 45 breaths/min is visible on the monitor. (All images courtesy of Dr. Bellows)

Pulse oximetry

The pulse oximeter noninvasively calculates oxygen saturation of hemoglobin using light absorption in tissue. A probe from the oximeter emits red and infrared lights, which are detected by a photodetector that is placed across an arterial bed.

Oxygen saturation in an anesthetized patient should be maintained between 95% and 100%, particularly if the patient is breathing 100% oxygen. Saturation readings of 95% or less indicate hypotension, insufficient oxygen flow, cardiac disease, pulmonary disease, vasoconstriction or shock. A patient with an abnormal reading may have an underlying problem that should be determined and corrected. The most common causes are prolonged attachment in the same area, hypotension and vasoconstriction due to temperature or drugs.

Pulse oximetry readings may be unreliable if the animal shows excessive movement, poor perfusion or pigment in the area where the sensor is placed. Excessive hair between the sensor and the mucous membranes can also prevent accurate readings in animals. One of the most effective placements of the peripheral capillary oxygen saturation probe is on the tongue. Dental procedures by their nature involve movement and instruments in the mouth. Extraoral areas for probe placement include the prepuce, vulva, ear, toe webbing, digits, tail and rectum (the latter may be unreliable because of interference from fecal matter).

End-tidal carbon dioxide

The gold standard for evaluating ventilation is carbon dioxide monitoring. End-tidal carbon dioxide (EtCO₂) is the concentration of carbon dioxide in the exhaled breath during exhalation. EtCO₂ monitoring through capnography is often called the “anesthesia disaster early warning system.” Vitally important, it is the only parameter that thoroughly reflects a patient’s ventilatory status and can signal problems within two breaths. Interpreting the capnograph waveforms (capnograms) can also demonstrate the quality of the patient’s breathing.

The device used to measure carbon dioxide should be a key consideration when choosing equipment to monitor veterinary patients during dental procedures. When selecting a mainstream device, make sure that the probe is solid state (no moving internal parts) to endure the rigorous environment of a busy veterinary practice. When using a sidestream device (Figure 2), pay close attention to the sample rate. Sample rates of 50 mm/min or less are recommended for small dogs and cats). If carbon dioxide monitoring is not currently in your practice’s budget, check whether you current monitor is equipped to upgrade later.

Figure 2. Sidestream carbon dioxide monitor.

The absolute carbon dioxide value is not as important to embrace as the capnograph waveform, which graphically demonstrates carbon dioxide levels during one inspiration and expiration cycle (see Capnogram Parameters). A normal waveform should have a baseline of zero during inspiration (i.e. inspiratory baseline). This is followed by an expiratory upstroke that contains initially little or no carbon dioxide and moves upward until it levels out at a plateau.

Carbon dioxide concentration continues to increase until it reaches its maximum just before the onset of inhalation (i.e. inspiratory downstroke).

The height, frequency, shape, rhythm and baseline position of the waveform should be monitored closely during anesthesia. Carbon dioxide concentration in the sample is reflected by the wave height. Changes in the standard waveform should alert the anesthetist to a problem with the patient, the airway or the anesthetic circuit. Normal readings are in the range of 35 to 45 mm Hg (Figure 3A).

Hypercapnia, a result of hypoventilation (Figure 3B), is when EtCO₂ exceeds 50 mm Hg. Hypercapnia can lead to vasodilation, a decrease in systemic vascular resistance and, subsequently, a reduction in blood pressure. Increased carbon dioxide readings may also occur because of faulty check valves, exhausted soda lime, mild to moderate patient airway obstruction and hypoventilation.

Hypocapnia, a result of hyperventilation (Figure 3C), is when the EtCO₂ is less than 30 mm Hg. Hypocapnia is uncommon in anesthetized animals but may occur as a result of hypothermia, esophageal intubation, extubation, disconnection from the breathing circuit, obstruction of the endotracheal tube or impending cardiopulmonary arrest.

Figure 3A. Capnogram showing normal breathing.

Figure 3B. Capnogram showing hypoventilation.

Figure 3C. Capnogram showing hyperventilation.

Anesthesia ventilators

Because dental procedures commonly take hours, the patient’s head-down positioning can lead to poor ventilation secondary to abdominal contents pushing on the diaphragm. The ventilator delivers a predetermined respiratory rate, volume and pressure during the entire procedure. Monitoring EtCO₂ is essential if mechanical ventilation is used.

Mechanical ventilation is more efficient than spontaneous breathing at delivering even doses of inhalants and less time-consuming than manual ventilation. Anesthetic planes are generally more consistent throughout the procedure.

Some practices use anesthesia ventilators on all patients undergoing dental care, while others use them only when indicated due to possible effects on cardiac output.

Of note, there are drawbacks to mechanical ventilation, and each case should be evaluated individually. Positive-pressure ventilation may impede venous return and negatively affect blood pressure. There is also a risk for accidental over-inflation of the lungs and barotrauma from inappropriate ventilator settings.

“Bucking the ventilator,” characterized by the patient attempting to breath around or against the ventilator, may indicate a light level of anesthesia or high EtCO₂. Check the patient first to assess anesthetic depth and correct as needed to deepen the level. If the carbon dioxide level is too high, adjust the ventilator settings to increase the volume or pressure being delivered to the patient. Monitor closely as depth can change quickly.

It is not always recommended that patients be ventilated to a specific volume; instead, ventilate to a predetermined pressure and EtCO₂. Turn the volume on the ventilator to the lowest setting. Set breaths/min to the desired frequency and increase the volume in small increments until the pressure on the manometer is reaching the desired pressure with each delivered breath. Turn on the ventilator, fill the bellows if necessary (Figure 4) and set the controls in the following manner:

• Set a reasonable respiratory rate based on patient size (typically, between 10 and 15 breaths/min).
• Adjust the inspiratory time to reflect an inspiratory-expiratory ratio close to 1:2. Physiologically, this is the most normal.
• Once you are ready to connect an anesthetized patient, fill the bellows (if necessary) by increasing the flow rate rather than using the oxygen flush valve. The flush valve will dilute out the anesthetic gas concentration and may cause your patient to become light or wake up. Turn the flow rate down to the rate you intend to use.
• Turn on the ventilator and watch the pressure manometer and EtCO₂ readings. Make sure pressure does not exceed the desired positive airway pressure and keep the EtCO₂ setting between 35 and 45 mm Hg.

Figure 4A. Ventilator with bellows being used to assist a cat undergoing full mouth extraction.

Figure 4B. Ventilator without bellows being used to assist anesthesia in a dog.

Having a thorough understanding of basic respiratory physiology is essential to grasp the importance of adequate ventilation and respiration in the anesthetized patient. Monitoring your patient during and after anesthesia is probably the most important consideration to create anesthesia success (Figure 5).

Figure 5. A veterinary assistant monitors a recovering patient’s temperature, pulse oximeter reading and ECG.

Dr. Jan Bellows owns All Pets Dental in Weston, Florida. He is a diplomate of the American Veterinary Dental College and the American Board of Veterinary Practitioners. He can be reached at dentalvet@aol.com.