Anesthesia monitoring (Proceedings)

The overall goal of anesthesia is survival and optimum recovery from surgery. In order to accomplish this goal, the surgery patient must be continually monitored for changes, especially deterioration in respiration, cardiac function and tissue perfusion regardless of the specific surgery. Cardiovascular (ECG, blood pressure) and respiratory (pulse oximetry, capnography) monitoring are employed to detect patient changes and to direct corrective actions. Technicians who monitor patients during anesthesia are expected to have the mechanical ability to understand and operate these monitoring systems, the creativity to adapt a single piece of equipment to serve both a 3 pound Chihuahua and a 200 pound mastiff, and the knowledge to interpret and respond to results as swiftly as they are collected.

In addition to expensive monitoring equipment, the technician should be well equipped to use his/her senses to detect other indicators of good anesthetic health

     • Mucous membrane color and capillary refill time

     • Urine output

     • Heart rate and pulse equality

     • Respiratory rate and effort

Heart rate and pulse quality/ECG

The ECG is an important monitoring tool for the surgery patient, not just for the conduction abnormalities it detects, but for changes in heart rate which can be indicative of serious events [such as the bradycardia (slow heart rate) which often precedes cardiac arrest]. Coupled with other monitoring devices, the changes in heart rate may give vital diagnostic information. For example, when tachycardia (rapid heart rate) occurs concurrently with hypotension (decreased blood pressure) the patient may be suffering from hypovolemia (lack of sufficient circulating fluids) while patients with tachycardia occurring concurrently with hypertension (increased blood pressure) may be experiencing extreme pain. Auscultation of heart rate and simultaneous palpation of peripheral pulses yields more information that either alone. Pulse quality, and synchronicity with heart rate are major indicators of perfusion.

Arterial blood pressure physiology and measurement

Since most post operative complications are directly related to hypotension during or after surgery, blood pressure is considered a vital sign to monitor throughout all surgical procedures. Considering how many factors predispose the patient to hypotension, some premedicants, gas inhalants, presurgical fasting and loss of body fluids, the chances are great that the technician will have to respond appropriately at some point during the anesthetic period

Physiology

Blood pressure is the measurement of tension within the arterial vessel walls at varies points in a cardiac cycle. Systolic blood pressure represents the maximal pressure created by ventricular contraction. Diastolic blood pressure represents the minimal pressure at rest just prior to the next contraction. Mean arterial pressure (MAP or MABP) is the average driving pressure over the entire cardiac cycle and is the major determinant of perfusion to most organs MAP is not a mathematical average of systolic and diastolic pressures because the diastolic phase is approximately twice as long as the systolic phase. Therefore, MAP is derived by the following formula:

Normal BP values:

Systolic: 120-180mmHg

Diastolic: 60-100mmHg

Mean: 80-100mmHg

Measurement

    Clinical assessment

Palpation of pulses can give significant information about ABP. Pulses will often wane progressively, starting at the periphery, as ABP drops; therefore, the absence of femoral pulses is likely to be a more serious sign than absence of metatarsal pulses if femoral pulses can still be felt. Abnormally high ABP may be evidenced by very strong or bounding pulses even in the most remote periphery.

At MAP < 60mmHg renal arterial blood flow is restricted and the kidneys are no longer perfused. Patients will often become oliguric and then anuric. Urine production is then an indicator that the MAP is at least 60mmHg. In hypovolemic patients urine output may be less than expected but production of some urine (0.5-1ml/lb/hr minimum) confirms renal perfusion. High blood pressure does not seem to affect urine output.

Weakness, lethargy or inability to rise might be manifestations of low ABP. Poor mucous membrane color and prolonged capillary refill time may indicate insufficient blood flow. Likewise, cold, poorly colored extremities may result from hypotension. Of course, patients with these signs should be monitored for signs of dehydration (hypovolemia) by checking membrane moistness, packed cell volume, total solids, skin turgor etc. Some patients may exhibit diminished bleeding at operative sites.

Mechanical methods

Blood pressure can be measured by direct or indirect means. Generally, direct methods are more accurate, particularly in hypotensive patients, but because direct measurement requires invasive arterial catheterization, indirect methods, though less accurate are more often used. In either case, clinical judgment should be integrated with blood pressure to provide the most complete assessment.

Indirect measurement

Indirect blood pressure is measured by inflating an occlusive cuff over an artery and recording the cuff pressure necessary to inhibit and restore blood flow distal to the occlusion. There are two basic types of indirect blood pressure monitors; Doppler ultrasonic blood flow and Dinamap oscillation detection.

Doppler Ultrasonic blood flow relies on audio transmission of blood flow and manual correlation to concurrent cuff pressure. First, blood flow to the artery is occluded with an inflated cuff. The cuff is then slowly deflated until blood flow resumes. The motion of the arterial wall as blood flow is restored is transmitted through a transducer into audible tone. A sphygmomanometer is used to read systolic pressure. Diastolic pressure is difficult to ascertain by this method as it is detected by a subtle shift or muffling of audible tone.

Dinamap automated oscillation devices operate by incremental deflation of their cuffs until blood flow is detected. An occlusive cuff is inflated over an artery. As the cuff deflates, arterial wall motion is detected by a microprocessor located in the cuff. Systolic pressure corresponds to the point when the oscillations in the arterial wall begin to increase. MAP corresponds to the peak in oscillations. Diastolic pressure is recorded when the oscillations cease to decrease upon further cuff deflation. Systolic, diastolic and MAP readings are digitally displayed.

Cuff size is a major determinant of results in both techniques. Cuff width should approximate 40% of limb circumference. Cuffs that are too small will yield erroneously high results due to the increased pressure needed to occlude blood flow. Conversely, cuffs that are too large will yield erroneous low blood pressure readings. Studies have shown greater accuracy of results using hind limbs over fore limbs. Some studies show a preference for tail base placement.

Indirect measurement is a simple noninvasive technique to obtain blood pressure readings in most patients who require intermittent monitoring.

Pulse oximetry

Insuring appropriate deliver of oxygen to tissues is paramount to successful outcome. Oxygen molecules are delivered to tissues via hemoglobin, the primary transport system. A pulse oximeter provides immediate and continual estimations of oxyhemoglobin saturation (SpO2) by transmission of varying light through the skin. Light absorption during arterial pulses is differentiated from absorption due to bone and venous blood.

Oxygen saturation is not the same as arterial pO2 (as measured on a blood gas) but is related to the extent that normal SpO2 is thought to imply normal pO2 and decreased SpO2 correlates with a low pO2. Specifically, due to the oxygen dissociation curve; at arterial pO2 of 60-70 mm Hg, hemoglobin is expected be fully saturated although normal pO2 is>85 mm Hg.

SpO2 measurement gives important information about delivery of oxygen to tissue and may be used to assess the effectiveness of supplemental oxygen therapy. Oximetry is also used as an indicator for peripheral perfusion in the anesthetized patient as well as to monitor patients who are being mechanically ventilated. The limitations of pulse oximetry include inability to obtain readings or inaccurate readings in patients who have serious perfusion problems, severe hypothermia or gross anemia. Unfortunately these are often the patients who most require oxygenation assessments. Because of these limitations arterial blood gas measurement remains essential in critical patients although frequency of sampling can be greatly reduced by pulse oximetry.

Pulse oximetry has the added advantage of being non-invasive and requires less expensive equipment than blood gas analysis.

Capnography

Expired or end tidal CO2 can be continuously monitored in the intubated patient by attaching a sensor device to the endotracheal tube. End tidal CO2 correlates well with CO2 measured on a blood gas (PaCO2) and provides an accurate non-invasive measure of alveolar ventilation. Capnography is useful for assessing and regulating breathing rate particularly in the surgery patient where hypoventilation is common. Since carbon dioxide is the main determinate of appropriate rate of ventilation, capnography is rapidly becoming one of the most valuable monitors for veterinary surgical patients. CO2 should be maintained between 35-45mmHg although <25mmHg is recommended for patients with head trauma or brain surgery to reduce the chance of cerebral edema. Technicians should respond to capnography readings that should increased CO2 (hypercapnea) by providing positive pressure ventilation until normal CO2 is reestablished. Conversely, hypocapnea should direct the technician to slow down positive ventilations. If hypocapnea occurs when the patient is breathing on it's on, this may be an indication of tachypnea due to pain and may require gas inhalant be increased.

Table 1

Combined, pulse oximetry and capnography give the most complete picture of respiratory function available by non-invasive means. They should not be thought of as substitute for arterial blood gas analyses in patients with respiratory disorders; however their usefulness for continual monitoring and detection of changes in patient status during surgery is unquestionable.