/// QUARTERLY BEAT / DECEMBER 2021
QUARTERLY BEAT / DECEMBER 2021 ///
WEBINAR HIGHLIGHTS
Monitoring the Anesthetized Patient - Part 2
JANE QUANDT , DVM, MS, DACVA, DACVECC Professor, University of Georgia VETgirl, Forum Consultant/Contributor
Need a review of anesthesia and missed Dr. Jane Quandt’s last anesthesia webinar? Get the highlights here on “Monitoring: What monitors tell you and what is considered standard of care.” Dr. Jane Quandt DVM, MS, DACVA, DACVECC reviews the importance of monitoring the anesthetized patient. Check out Part 1 of this article HERE in this 2-part series.
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Oscillometric • Advantages are it is noninvasive, requires little technical experience, reads systolic, mean (measured) and diastolic (calculated) values, automated readings, every minute to every 120 minutes, and some units are battery operated and portable. • Disadvantages they can be unreliable in hypotensive or very small (< 5 kg) patients, • Motion artifact in awake animals can be a problem, readings are affected by an inappropriately sized cuff, large cuffs read low, small cuffs read high. The width of an appropriately sized cuff should be 40% of the circumference of the animal’s leg or tail.
• Advantages of pulse oximetry are they alert to early desaturation, little technical experience is necessary, and is noninvasive. • Disadvantages of pulse oximetry include, readings can be adversely affected by poor perfusion (hypotension, vasoconstriction, hypothermia), anemia (Hct<10%), movement, and improper probe placement, poorly designed (non veterinary) probes, skin pigmentation, cautery interference, and ambient light. Probes may require frequent changes in placement position to prevent vascular compression and reading errors. E. Capnography Capnography, or “respiratory gas monitoring”, measures the partial pressure of carbon dioxide in expired air. This concentration of carbon dioxide at the end of a normal “tidal” volume is the “end tidal C02”, or ETC02. Mainstream capnographs measure respiratory gases directly in the airway, between the ET tube and the circle. They can be bulky and cumbersome. Sidestream capnographs have a small adapter and a sample line which carries the gas sample to a separate analyzer. Four phases make up any capnogram, (waveform): • Phase I is the inspiratory phase, consisting of fresh gas with little CO2 • Phase II is the expiratory upstroke, illustrating the air breathed from the patient’s anatomic dead space (trachea and main bronchi, areas not involved in gas exchange) • Phase III is the expiratory plateau, consisting of high CO2 levels from the alveoli • Phase IV is the inspiratory downstroke, which is the beginning of inhalation
In this part-2 article, learn about key monitoring devices that are a must in your veterinary patients. Monitoring the anesthetized patient is the most important job during anesthesia. Thorough monitoring is the best way to assess the condition of your patient and avert disasters before they occur. Waiting for difficulties to arise not only delays solving the underlying problem(s) but also delays treatment of the obvious abnormalities. Monitoring basic parameters will draw attention to involved body systems. Although the sheet upon which we record information is divided into five-minute increments, monitoring should be constant from the time premedications are given until the patient returns to consciousness and is extubated. An ECG only monitors only the electrical activity of the heart and therefore measures the HEART rate, not the PULSE rate. A NORMAL LOOKING ECG MEANS NOTHING WITHOUT A PALPABLE PULSE! During euthanasia, the mechanical activity and the pulse/arterial wave form is lost long before the electrical activity (ECG). However, ECGs are essential in cardiac arrests in order to distinguish ventricular fibrillation from asystole, remember, a pulse is not generated with either, but the treatment is different! MECHANICAL MONITORING A. Electrocardiogram (ECG or EKG) Needle electrodes (EEG needles) are useful for ECGs in awake animals and during anesthetic induction. Traditional alligator clips are better once the animal is anesthetized, since the clips are less likely to be displaced or fall off. Lead II, R arm and L leg is most commonly used for monitoring. Because the heart sits on a more vertical axis in large animals, a “base apex” lead (lead I) is used to compare potentials between the right jugular furrow and the left cardiac apex. B. Esophageal Stethoscopes This is one of the cheapest, most easily used devices. It provides an accessible way to monitor cardiac function and
ventilation rate. A simple esophageal probe is passed after anesthetic induction, and it is positioned to hear the best sounds of the heart.
C. Blood Pressure Normal blood pressure (BP) values are:
systolic = 100-160 mm Hg mean = 80-120 mm Hg diastolic = 60-100 mm Hg
D. Pulse Oximetry (SpO2) Pulse oximetry indicates the percentage of hemoglobin saturated with oxygen ; the two wavelengths of light absorbed by oxyhemoglobin and reduced hemoglobin are detected. Pulse oximetry offers a noninvasive, continuous assessment of pulsatile arterial flow. Acceptable minimal levels of saturation are 95%-100%. A pulse oximeter will alert the clinician to the early desaturation that often precedes clinical hypoxemia and cyanosis. Cyanosis is not clinically visible until the hypoxemia is severe (i.e. Pa02 << 60 mm Hg); therefore, hypoxemia can be occurring even before cyanosis is noted. Saturations between 90-95% indicate mild hypoxemia and values < 90% indicated serious desaturation (PaO2 < 60 mmHg). Known relationships do exist between Sp02 (pulse oximeter values) and Pa02 (blood gas values):
Blood pressure (BP) is result of cardiac output (CO), vascular resistance (SVR=systemic vascular resistance; PVR=peripheral vascular resistance, same thing) and relative blood volume.
BP = CO x SVR
So, when treating hypotension, consider the possible cause(s). Low BP can result from severe bradycardia (= HR), decreased cardiac contractility (= SV), or vasodilation (= SVR). By knowing what is most likely causing the hypotension, you can effectively and rapidly treat it. Mean blood pressures below 60 mm Hg, assumed to result in inadequate coronary, renal and cerebral perfusion, should be aggressively treated; mean blood pressures should be 70 mm Hg (minimum), and preferably within the normal range (80 120 mm Hg). 1. Indirect blood pressures can be measured by: • Doppler requiring a piezoelectric crystal/ transducer/ amplifier unit, cuff and sphygmomanometer. • Advantages are noninvasive, works in any size patient, from pediatrics to large horses, constant audible indicator of pulsatile blood flow, can be used in awake patients, and is portable and rechargeable. • Disadvantages are it reads systolic only, diastolic can be estimated, with experience, and the mean must be calculated [MAP= [(SAP-DAP) / 3)+DAP] and it requires some technical experience for probe placement.
Sp02 or Sa02 (%)
Pa02 (mm Hg)
Pa02 (mm Hg)
Perfect
100
>100
No cyanosis
Normal
>95
>85
No cyanosis
Serious hypoxemia Severe hypoxemia
Cyanosis NOT visible
<90
<60
<70
<40
Cyanosis visible
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Near death
<60
<30
Cyanosis visible
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