Anesthetic Monitoring Utilizing Mechanical Devices - Questioning Limitations of Each Device
College of Veterinary Medicine, Western University of Health Sciences, Pomona, CA, USA
Use of mechanical devices during veterinary anaesthetic monitoring has become more common with technological advancement and also with increased awareness by the clinicians the value of obtaining readily quantified physiologic information in anesthetized animals.
The ACVA published a guideline for standards of monitoring during anesthesia and recovery (J Am Vet Med Assoc 1995, ACVA home page update 2009) in which patients are recommended to be monitored by continuous monitoring devices (ECG, BT, pulse oximeter, capnography) along with intermittent monitoring devices (esophageal stethoscope, BP). Adequate monitoring is needed even for brief anesthetic periods, during the transport of patients, and with sedation that might cause cardiovascular or respiratory complications. Pulse oximetry, capnography and non-invasive blood pressure monitoring represent three current non-invasive monitoring techniques available to use in the anesthetized patient. When pulse oximetry and capnography are used together, a beat-to-beat and breath-to-breath non-invasive cardiorespiratory monitoring is provided. Indeed, these two have become a 'standard care' in human patients during general anesthesia in 1998 (www.asahq.org) because inclusion of these two was proven to improve the safety of the patient.
The current discussion will focus on limitations of the mechanical devices including pulse oximeter and capnograph in clinical application.
Circulation (Cardiovascular) Monitoring
The goal of monitoring the circulatory system is to ensure adequate blood flow to tissues during all anesthetic procedures.
It measures electrical activity of cardiac cells. Other circulatory information including blood pressure, stroke volume and cardiac output is not provided by the ECG. It must be noted that ECG may even be monitored in mechanically non-functioning heart and additional means to monitor the cardiac function other than ECG monitoring must be always present. Body position and precise lead placement are not important for monitoring purposes when the primary objective is to describe the electrical pattern in a general manner and monitor for changes that may signal the deterioration of the patient.
Arterial Blood Pressure
It provides information regarding the adequacy of blood flow to the patient's tissue. At a mean arterial blood pressure below 60 mm Hg, organ and tissue perfusion is inadequate. The systolic blood pressure is determined by a combination of peripheral vascular resistance, stroke volume, and intravascular volume whereas diastolic blood pressure primarily arises from peripheral vascular resistance. Clinically, the pulse pressure (systolic-diastolic) allows the anesthetist to estimate the stroke volume. Normal systolic blood pressure range for anesthetized dogs and cats is between 90 mm Hg and 120 mm Hg, diastolic blood pressure ranges between 55 mm Hg and 90 mm Hg.
A Doppler ultrasound probe coupled with a pressure cuff and sphygmomanometer, or an automated oscillometric device (e.g., Dinamap(R), Criticon, Tampa, FL) is widely used in veterinary practices. The advantage of automated oscillometric detector device over Doppler ultrasonic detector is that the oscillometric is automated and requires neither experience nor labor of the operator. The disadvantage of oscillometric detector is decreased accuracy or unreliability when used on a hypotensive or a small patient (less than 5 kg body weight). The advantage of the Doppler technique is that it can be applied to any sized animal (from a rat to a horse) and is relatively inexpensive. The disadvantage of the Doppler technique is the requirement for operator experience and labor intensity.
A catheter is placed into an artery and then connected to a pressure transducer and monitor/recorder. The 'gold standard' against which all other methods are compared. This method provides a continuous, beat to beat assessment of the patient's blood pressure, is more accurate in hypotensive situations, and provides access to an arterial blood sampling site for blood gas analysis. However, it is less practical in the clinical setting, requiring more expensive equipment, demands skills in placement of a catheter, and potentially increases the risk of infection and air embolism.
Cardiac Output (CO)
It is not measured routinely by the practicing veterinarian due to high degree of technical difficulty and expenses involved, but the concept of cardiac output is extremely important in the generation and treatment of cardiovascular disease. CO is diminished by influences which decrease contractility and by inadequate end-diastolic filling volume (decreased preload). Inadequate circulatory blood volume and venous return, especially in conjunction with vasodilating anesthetics may be the most common cause of cardiovascular inadequacy. CO is the main determinant of oxygen delivery when the PaO2 is above 60 mm Hg.
A thermodilution technique is the gold standard of CO measurement, but other less invasive and/or continuous techniques have been adopted in clinical settings including echocardiography, LidCO(TM) and NiCO(TM) despite lack in accuracy.
Central Venous Pressure (CVP)
It is the luminal pressure of the cranial vena cava or right atrium. It monitors the adequacy of venous return, intravascular blood volume and right ventricular function. It should be measured whenever heart failure is suspected, whenever rapid changes in blood volume are expected, or there is a danger of volume overload (e.g., renal failure). Normal CVP is 0 to 10 cm H2O. Measurements in the range of 15 to 20 cm H2O are too high and efforts should be made to determine and treat the cause of the deviation. High normal CVP values do not contraindicate the administration of fluids when other parameters indicate hypovolemia. Verification of a well-placed, unobstructed catheter can be ascertained by observing distinctive pressure wave characteristics on the monitor. Similar to invasive arterial blood pressure monitoring, it is less practical in the clinical setting, requiring more expensive equipment, demands skills in placement of a catheter, and potentially increases the risk of infection and air embolism.
Oxygenation Monitoring Using Pulse Oximetry
Pulse oximetry provides a non-invasive, continuous detection of pulsatile arterial blood in the tissue bed, calculates the percentage of oxyhemoglobin described by the oxyhemoglobin dissociation curve present, and provides the pulse rate of the monitored patient. It has become rapidly standard care in anesthetized human patients after introduction and has gained widespread popularity in veterinary anesthesia. Methemoglobin and carboxyhemoglobin do not contribute to functional oxygen transport, and one should be aware of the impact of these abnormal hemoglobin species in pulse oximetry. It is affected by motion artefact (e.g., shivering), ambient light, poor peripheral blood flow from hypotension and vasoconstriction, and electrical noise from electrocautery. The sites for probe placement include the tongue, ear lip folds, toe pads, axillary or inguinal skin fold, or prepuce/vulva.
Ventilation Monitoring Using Capnography
A capnograph displays a graphic shape (capnogram) of exhaled CO2 gas, end-tidal CO2 concentration and also indicates the respiratory rate of a patient. Normally end-tidal CO2 reflects the partial pressure of CO2 in the alveoli. End-tidal CO2 concentrations between 35 and 45 mm Hg are considered normal in anesthetized animals. The measurement of end-tidal CO2 is useful for determining optimal minute ventilation, hypoventilation, and airway disconnection or airway obstruction. This technique is finding increased use in veterinary anesthesia as it is becoming relatively inexpensive. It provides continuous method to document ventilation adequacy and limits the need for invasive procedures such as arterial blood gas analysis. Any deviations from the normal shape of capnogram should be investigated, and a few examples will be presented in the talk.
The primary goal of monitoring anesthetized animals is to ensure adequate tissue perfusion with oxygenated blood. Understanding limitation of various monitoring devices is critical to allow the veterinary anesthetist to identify problems early, institute treatment promptly, and thus avoid irreversible adverse outcomes.
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