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

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.

Blood gas analysis

      Grimm 2003

The purpose of blood gas analysis in anesthesia is to give an accurate representation of a patient's respiratory function and acid-base status. The acid-base status refers to the mechanisms the body uses to maintain a normal pH (i.e. neither acidotic nor alkalotic). The respirations may appear normal during anesthesia when there is significant respiratory compromise present.

Thorough analysis of blood gases is intricate and complicated. Simply put, the systems that regulate acid-base balance are the renal and respiratory systems, and these systems react to an imbalance. For instance, if a patient has a metabolic alkalosis, the respiratory system will slow respirations to retain carbon dioxide (CO2), therefore increasing the acid level in the body. Conversely, if a patient has a respiratory acidosis, the renal system will retain bicarbonate (HCO3) to buffer excess acid, therefore lowering the acid level in the body. The acidotic patient may also pant to "blow off CO2". The respiratory response adjusts within minutes to changes in the acid-base status as opposed to the renal response, which takes hours. The ability to monitor blood gases gives the anesthetist a tool in helping the compromised patient undergoing anesthesia. Small changes in ventilation can make a big difference in acid-base regulation.

There are two types of blood gas sampling, arterial and venous. This section concentrates on arterial sampling because arterial blood is much more informative regarding respiratory status for it is oxygenated. If an artery is inaccessible, the lingual vein can be used as these values are closer to arterial readings due to the extensive anastomoses with the arteries in the tongue. Arterial blood gas sampling is considered an invasive way to monitor the anesthetic patient.

Basic definitions of the components in blood gas analysis as they apply to respiratory assessment are as follows:

      1. pH is a mathematical representation of Hydrogen ion concentration. The normal range is 7.31-7.41. A value below 7.31 can indicate acidosis, as a value above 7.41 can indicate alkalosis. The pH is inversely proportional to CO2 meaning if the CO2 is elevated, the pH is decreased and vice-versa.

      2. Partial pressure of carbon dioxide (PaCO2) is the major ventilation parameter because changes in ventilation affect this value. As stated in the capnography section, the normal range is 35-45 mmHg. A value of less than 35mmHg indicates hyperventilation, hypocapnia, and/or metabolic alkalosis, and values greater than 45mmHg generally lead to respiratory acidosis as they indicate a ventilation deficit, hypoventilation, and/or hypercapnia.

      3. Partial pressure of oxygen (PaO2) determines the physiologic response to ventilation and oxygen. It is supposed to be approximately five times the inspired oxygen concentration (FiO2). The FiO2 in room air is 21%, while mask delivery, nasal oxygen cannula, or an oxygen chamber is 35-40%, and the FiO2 is 100% in a patient that is intubated with the cuff inflated. The PaO2 in a healthy awake patient breathing room air (FiO2 of 21%) should be between 90-115mmHg, while a reading of less than 80 mmHg indicates hypoxemia. The PaO2 should be nearly 500 mmHg in a patient that is intubated and ventilating adequately as that patient is getting nearly 100% oxygen.

      4. Oxygen saturation (SaO2) measures the percentage of hemoglobin binding sites in the bloodstream that are occupied by oxygen. The value should be greater than 95%, and is equivalent to a PaO2 of 85-100 mmHg. If the patient is hypoxemic with a SaO2 of between 75-90%, the PaO2 is that percentage minus 30.

      5. Bicarbonate level (HCO3) represents the metabolic (or non-respiratory) component/response of acid-base regulation. This regulation is achieved by the renal tubules balancing bicarbonate ions with hydrogen ions. The normal range is 18-26 mEq/L. Metabolic acidosis occurs when the bicarbonate is less than 18 mEq/L, and alkalosis occurs when it is greater than 26 mEq/L. There may be a corresponding increase or decrease in PaCO2 to compensate for imbalances.

      6. Base excess (BE)/Base deficit refers to the total of bases (alkalis) in the blood, and is usually affected by metabolic processes. A negative number is a base deficit and is considered acidotic, while a positive reading is base excess and is considered alkalotic. The normal range is -2 to +2, but cats can have a wider range.

The two most common analyzers used in veterinary medicine are point-of-care and countertop units. Point-of-care units are handheld and use a disposable cartridge for each test, while countertop units usually use a touch-screen to initiate testing.

Arterial sampling

Proper sample collection is imperative as accuracy is paramount in blood gas analysis. Errors in blood gas sampling tend to be due to improper sampling, but there are other causes such as inadequate machine maintenance, and microclots and/or air bubbles in the sample. The steps for proper arterial sampling are as follows:

      1. Take the patient's temperature if the analyzer adjusts for it. Temperature can affect PaO2, PaCO2, and pH.

      2. Attach a 25 gauge needle to a 3 ml syringe and heparinize it by filling the syringe with Heparin, and expelling it back into the bottle so there is just the remainder in the hub. There are also syringes made specifically for blood gas collection.

      3. If the animal does NOT have arterial line, common sites for sample acquisition are the femoral or dorsal pedal arteries.

           a. Locate the site, clip, and prepare with surgical scrub.

           b. Palpate and isolate the pulse with the non-drawing hand's index and middle fingers.

           c. Direct the needle at 45 degree angle in between fingers, and insert the full length of the needle into the site. A flash should be seen upon entering the artery.

           d. Hold the needle steady and slowly draw at least one milliliter (ml) into the syringe. One ml is needed to prevent dilutional effects from the heparin.

      4. If the animal does have an arterial line, the sampling process entails wiping the hub of catheter with Isopropyl alcohol, inserting the needle into the hub and drawing at least one milliliter of blood into the syringe, then flushing the catheter with heparinized saline.

      5. Immediately expel air bubbles from the syringe, and cap the needle with a rubber stopper. Redraw if the blood has a frothy appearance or air bubbles because room air will alter values.

      6. Have the restrainer apply direct digital pressure to the draw site for at least 2 minutes.

      7. Run the sample right away, or place on ice and run within 2 hours.

Direct blood pressure

Direct blood pressure monitoring is the most accurate way to measure blood pressure. It is considered invasive because it requires an arterial catheter, and the catheter is connected to either a commercial pressure transducer or an aneroid manometer.

The placement of an arterial catheter is different than placing a venous catheter. There is also a greater potential for complications, thereby practice and training are needed to master this skill. Even with proper technique, there can be increased difficulty in feeding the catheter, and a higher risk of the catheter "burring" during placement. The dorsal metatarsal artery is the most commonly used for arterial catheterization. First clip the hair over the artery and surgically scrub the area. Palpate the artery with one or two fingers of a sterile gloved hand. Insert an over-the-needle catheter (20 to 24-gauge) at a 30-45° angle at first with the bevel up on top of the artery and then flat against the skin surface and parallel with the artery. Once arterial blood is observed freely flowing through the catheter, the catheter is then advanced to its full length, the stylet is removed, and an injection cap is placed. The catheter is secured in place with suture and gently flushed with heparinized saline. Skin glue can be placed on the catheter hub onto the skin at the entrance site prior to routine bandaging to help stabilize the catheter.

Once the arterial catheter is placed, it is then connected to a monitoring device such as an aneroid manometer with sterile extension tubing, or the commercially available pressure transducers. The measuring device can be a long fluid administration set filled with heparinized saline that is suspended above the patient and used similarly to central venous pressure measurement. The pressure transducer uses stopcocks that are connected to the transducer at one end, and the patient at the other end. The transducer should be initially zeroed at the approximate level of the patient's heart by first making sure that there is no pressure across the transducer, and then by zeroing the transducer as per manufacturer recommendations. Ensure that no air bubbles are present in the tubing before zeroing your system. Once the system is zeroed, the stopcocks are opened to the patient's catheter and transducer so that the generated pressure trace is observed. A small rise known as the dicrotic notch represents a transient increase in aortic pressure. Pressure waves that have a steep upstroke with the dicrotic notch signal a reliable arterial measurement. Flushing the arterial catheter may help if the pressure waves appear dampened on the monitor. The transducer will need to be re-zeroed anytime the position of the patient changes.

Normal arterial pressures are 100 to 160 mmHg systolic, 70 to 90 mmHg diastolic, and 80 to 110 mmHg for the mean. Systolic pressures below 80 mmHg and mean arterial pressures below 60 mmHg result in inadequate perfusion of the kidneys and brain, and immediate response is warranted.

There are some hazards associated with arterial catheters and direct arterial pressure monitoring. They include arterial thrombosis, hematoma formation, infection, bleeding from the catheter site if the line becomes damaged or dislodged, air embolization, and formation of AV fistula or aneurysm. The patient must be monitored closely with the catheter site kept clean. Patency is maintained with periodic flushing with heparinized saline. Do not administer fluids or drugs through the arterial line.

References

Levensaler, Amy. Capnography, Direct Blood Pressure Monitoring (not published). 2008.

McKelvey, Diane, Hollingshead, Wayne. Veterinary Anesthesia and Analgesia, Third Edition. Mosby. 2003.

Waddell, Lori. Evaluation and Interpretation of Blood Gases. Proceedings Western Veterinary Conference 2004.

Willard, Sandra. Tips in Obtaining Blood Gases & Interpreting Results. Proceedings American College Veterinary Internal Medicine Forum 2002.