ANESTHETICPEER REVIEWED MONITORING Devices to Use & What the Results Mean Jeff Ko, DVM, MS, Diplomate ACVA, and Rebecca Krimins, DVM This article is the second in a he main fundamental aspects of anesthetic monitor- ing are: series discussing the goals of 1. Oxygenation (circulatory and respiratory func- tion) anesthetic monitoring as well 2. Ventilation (respiratory function) T 3. Circulation (circulatory function with an empha- as associated procedures sis on cardiac output). and equipment. In the first These three elements work simultaneously in order to maintain adequate tissue and organ perfusion with oxygenated blood.1-3 article, the authors answered Oxygenation and ventilation are essential for maintaining a high oxygen level in the blood, while cardiac output plays a questions about anesthetic pivotal role in maintaining tissue and organ perfusion with highly monitoring, including why oxygenated blood. In this way, oxygenation, ventilation, and cir- culation each play a critical role in providing oxygen to tissues. itÕs performed, information PHYSIOLOGY FUNDAMENTALS obtained during monitoring, Oxygen Delivery Oxygen delivery is a product of blood oxygenation and cardiac and important components output.1-3 of the anesthetic process. UÊBlood oxygenation is represented by blood oxygen con- tent, which is the total amount of oxygen carried by the blood, including oxygen dissolved in plasma and oxygen bound to hemoglobin.1-3 UÊCardiac output is maintained by a complex interaction involving heart rate, stroke volume, peripheral vascular resistance (afterload), blood volume returning to the heart (preload), and blood viscosity.1-3 March/April 2012 Today’s Veterinary Practice 23 | ANESTHETIC MONITORING: DEVICES TO USE & WHAT THE RESULTS MEAN When heart rate or stroke volume or a combination Table 1. Anesthetic Monitoring Equipment of both is low, cardiac output is reduced. When blood volume returning to the heart is low due to dehydra- Circulation tion or acute blood loss, stroke volume is decreased. Electrocardiography (ECG): If peripheral vascular resistance or blood viscosity • Monitors heart rate and rhythm (ie, vasoconstriction or polycythemia, respectively) • Definitively diagnoses arrhythmias is increased, cardiac output is reduced. In addition, • Monitors progress of cardiac arrhythmia treatment excessively high heart rate (lack of ventricular filling Ultrasonic Doppler blood flow detector: for preload) and profound peripheral vascular dilation • Measures blood flow, pulse rate, and systolic blood (lack of venous return due to peripheral pooling of pressure (BP) when used with sphygmomanometer blood) also reduce cardiac output. Oscillometric BP measurement: Application to Anesthesia • Uses a BP cuff on the limb to obtain systolic, Acute changes in ventilation or oxygenation can diastolic, and mean arterial BP at a set time interval have dramatic effects on oxygen delivery; thus, in but not continuously addition to monitoring circulatory function (see Invasive BP measurement: CIRCULATION, below), it is important to carefully • Uses arterial catheter, BP transducer, and monitor monitor: to obtain continuous beat-to-beat pulse waves • Saturation level of oxygen in hemoglobin via SaO2 • Gold standard for measuring systolic, diastolic, and (measured by blood gas analysis) or SpO2 (mea- mean BP sured by pulse oximetry) Ventilation • Hematocrit (an important indicator of hemo- Respirometer: globin concentration; hemoglobin concentration • Measures respiratory rate and tidal volume (minute equals approximately 1/3 of the hematocrit) volume) • Partial pressure of oxygen in arterial blood Arterial or venous blood gas: (PaO2). When oxygen delivery is inadequate, cells seek • Measures partial pressure of CO2 (PaCO2 or alternative ways to supply energy (ATP). The body PvCO2) then converts to anaerobic metabolism, producing Capnography: lactate as a metabolic by-product and utilizing it as an • Noninvasively measures end-tidal CO 2 alternative energy source.4 Measuring a series of blood concentration lactate concentrations provides a trend for assessing Oxygenation tissue perfusion.1-4 Pulse oximetry: Following is a categorization of anesthetic monitor- • Noninvasively measures saturation of oxygen ing equipment by physiologic function. See Table 1 bound to hemoglobin (SpO2) and Figure 1 for an outline of anesthetic monitoring Arterial blood gas: equipment and its purposes. • Measures partial pressure of oxygen (PaO2) in arterial blood samples CIRCULATION Body Temperature Monitoring circulation during general anesthesia is aided by the use of electrocardiography (ECG) and Rectal thermometer Esophageal temperature probe blood pressure (BP) monitoring as well as patient Infrared thermometer: assessment. • Measures tympanic membrane temperature Electrocardiography Depth of Anesthesia Electrocardiography is easy to perform and should be Anesthetic gas analyzer: used continuously during the perioperative period to • Measures expiratory inhalant concentration obtain quick, real-time information about heart rate (allows anesthetist to estimate depth of anesthesia and rhythm. together with other vital variables mentioned in this table) Placement Bispectral index (BIS) monitor: The majority of ECGs used in veterinary hospitals • Algorithmic analysis of a patient’s electroencephalogram during general anesthesia have 3 leads and attach to the skin surface. • The 3 leads can be placed as either: BIS = bispectral index; BP = blood pressure; CO2 = carbon dioxide; » ECG = electrocardiography; PaCO = partial pressure of carbon dioxide Limb leads: Left and right forelimb and left 2 hindlimb electrodes attached to the limbs as indi- in arterial blood; PaO2 = partial pressure of oxygen in arterial blood; PvCO2 = partial pressure of carbon dioxide in venous blood; SpO2 = cated, with selection of lead II during recording saturation level of oxygen in hemoglobin as measured by pulse oximetry » Base-apex leads: Right forelimb and left 24 Today’s Veterinary Practice March/April 2012 ANESTHETIC MONITORING: DEVICES TO USE & WHAT THE RESULTS MEAN | Table 2. Cardiorespiratory & Physiologic Parameters in the Anesthetized Dog & Cat Variable Reference Interval (Dogs) Reference Interval (Cats) Circulation Heart rate (beats per min) 60–120 120–160 Systolic blood pressure (mm Hg) 90–140 90–140 Diastolic blood pressure (mm Hg) 60–90 60–90 Mean arterial blood pressure (mm Hg) 70–90 70–90 Ventilation Respiratory rate (breaths per min) 8–16 12–24 Tidal volume (mL/breath) 10–15 10–15 Arterial blood pH 7.35–7.45 7.35–7.45 PaCO2 (mm Hg) 35–45 35–45 Bicarbonate (mmol/L) 22–26 22–26 End-tidal CO2 (mm Hg) 35–45 35–45 Oxygenation SpO2 (%) ≥ 95 ≥ 95 PaO2 (mm Hg) ≥ 100 ≥ 100 Other Body temperature (°F) 98–101 98–101 Hematocrit (%) 34–59 28–47 Total protein (mg/dL) 5–8.3 5.9–8.4 Blood glucose (mg/dL) 90–150 90–150 Blood lactate (mmol/L) < 2 < 2 Urine output (mL/kg/H) 1–2 1–2 CO2 = carbon dioxide; PaCO2 = partial pressure of carbon dioxide in the arterial blood; PaO2 = partial pressure of oxygen in the arterial blood; SpO2 = saturation level of oxygen in hemoglobin as measured by pulse oximetry VMD Lisa Wirth, Figure 1. A schematic representation of clinical anesthesia monitoring equipment to ensure proper tissue perfusion with well-oxygenated blood. Continous systemic surveillance can provide an early warning system, prompting immediate intervention. March/April 2012 Today’s Veterinary Practice 25 | ANESTHETIC MONITORING: DEVICES TO USE & WHAT THE RESULTS MEAN hindlimb electrodes attached to the right (pre- Placement ferred) or left (alternative) jugular furrow, and Arterial catheter placement is usually in the dorsal pedal/ left forelimb electrode attached to the opposite metatarsal, femoral, or palmar artery. In dogs with side of the thoracic wall caudal to the heart large ears, such as basset hounds, the catheter may be (Figure 2). Select either lead I (negative deflec- placed in the auricular artery. In cats, arterial catheters tion) or lead III (positive deflection) on the are most frequently placed in the dorsal pedal artery. monitor for recording. • The advantage of using base-apex lead placement Advantages & Disadvantages is that it avoids attachment to the hindquarters, Advantages: minimizing artifact motion during abdominal and • Accurate pressure readings with beat-to-beat infor- hindlimb surgical manipulation. mation • The application of conduction gel and/or medical • Arterial blood sampling for blood gas analysis, alcohol on the ECG electrodes minimizes artifact packed cell volume (PCV), glucose, and lactate and background electrical noise on the monitor. measurement. Disadvantages: • Technical skill required to place the catheter • Expense of equipment required • Frequent maintenance (of pressure transducer, pressure tubing, saline flushing) and calibration • Potential hematoma formation and thrombosis (rare) • Infection at the catheter insertion site and acute blood loss if an inadvertent disconnection occurs between the catheter and artery (rare). Noninvasive Blood Pressure Measurement BP monitoring is more commonly accomplished Figure 2. A base-apex lead is placed on a dog with through noninvasive (indirect) BP monitoring using a right forelimb (RA) and left hindlimb (LL) electrodes Doppler ultrasound or oscillometric method. positioned on the right jugular furrow of the dog, and the left forelimb (LA) electrode placed caudal to Ultrasonic Doppler Method the heart to obtain the best signal. The ECG monitor The ultrasonic Doppler method involves placing