Advanced anesthetic monitoring (Proceedings)
Critical patients and patients undergoing prolonged and invasive procedures may need more intensive monitoring due to their increased risk for anesthetic complications. Some of these methods of monitoring may become more common in the near future considering veterinary anesthesia has advanced dramatically over the past decade. This section will cover capnography, arterial blood gases (ABGs), and direct blood pressure monitoring.
Capnography is measuring the level of carbon dioxide (CO2) at the end of expiration (end-tidal). It is this and measuring the amount of carbon dioxide in arterial blood, also called the partial pressure of carbon dioxide, (PaCO2) via blood gas analysis that are the quantitative means of assessing ventilation. Qualitative methods include the auscultation of breath sounds, observing thoracic wall movement and movement of the breathing bag. It is imperative that the anesthetic patient maintains adequate ventilation because carbon dioxide is a waste gas that is carried in blood to the lungs, exchanged with oxygen in the alveoli during inspiration, and exhaled with every breath. Excessive CO2 in the body, hypercapnia, can cause respiratory acidosis and increase the potential for complications.A capnometer non-invasively measures carbon dioxide levels, and gives a reading at the end of expiration (ETCO2). This is when the highest concentration of CO2 should occur for the diluted gases from the trachea and primary bronchi are no longer being sampled. The capnometer will also alarm to apnea or excessive expired CO2. The information gained from its use can improve patient outcome because it allows the anesthetist to asses a patient's ventilation, airway and breathing circuit integrity, and cardiopulmonary function.
There are two different types of capnometers, sidestream and mainstream. These names are used to describe the location of the CO2 sensor. A sidestream gas monitor needs proper scavenging of the sampled waste gas to function properly. It also requires an additional connector that increases the amount of dead space within the breathing circuit. Dead space is created when the endotracheal tube is extended from the patient to its connection to the breathing circuit, and it causes an uneven exchange of gases. Ultimately, it can increase the amount of CO2 the patient breathes. The sample tubing of a sidestream capnometer may obstruct with patient secretions, blood, water, or condensation, requiring replacement when it produces a false apnea alarm. A mainstream capnometer uses infrared light absorption to analyze the patient's respiratory gas. It does not require scavenging of sampled gases, though it can still occlude with patient secretions, blood, water, or condensation.
An increase of PaCO2 can be expected during anesthesia since anesthetic drugs cause respiratory depression. The normal range for PaCO2 is 35-45 mmHg, and a capnometer in proper working order provides ETCO2 values that are usually 5-10mmHg lower than the PaCO2, making the normal range for ETCO2 between 30-40 mmHg. Elevated ETCO2 levels indicate an imbalance between the production and respiratory excretion of carbon dioxide. The first response is to check for patient problems such as airway integrity and breathing quality. This is followed by equipment troubleshooting such as checking valves and seeing if the soda lime is exhausted because these cause patient rebreathing of expired gases, thereby contributing to an elevation of CO2. An ETCO2 of greater than 60mmHg indicates hypoventilation and should prompt either manual or mechanical intermittent positive pressure ventilation as well as a decrease in anesthetic depth. If the patient is already on a ventilator, the tidal volume (the volume of air displaced between inspiration and expiration) and/or respiratory rate may need to be increased. A value of less than 30mmHg indicates hyperventilation, hypocapnia, and/or metabolic alkalosis. The patient's anesthetic depth should be assessed, and less frequent ventilation can continue with a lower tidal volume. The patient may need further treatment if the cause of the alkalosis is metabolic.
Capnographs provide a graphic display of the amount of exhaled carbon dioxide. The display is usually a wave form, and proper evaluation can provide early detection of hypo- or hyperventilation, a problem with the patient's airway or breathing, and leaks or occlusions in the breathing circuit. A normal capnographic wave form will have a sharp rise from zero at the beginning of exhalation to a smooth plateau and then to a sharp drop back to zero at the beginning of inhalation.
Capnography can be useful in both anesthesia and emergency and critical care settings for the capnograph can provide information regarding the effectiveness of cardiopulmonary resuscitation (CPR). Higher CO2 readings obtained during CPR can indicate effective chest compressions indicating good pulmonary blood flow because CO2 is being delivered and cleared by the lungs.