The Blood Pressure Can of Worms – Part II
This is the second of our four-part series on untangling the blood pressure can of worms. In this article we look at blood pressure monitors, examining direct and indirect methods, techniques and equipment.
In 1915, Sir Frederick Hobday, a British veterinary surgeon, noted that “it is of no avail to have done any operation, however clever, if the patient succumbs to the anesthetic.”
Nearly 100 years later, Hobday’s observation is still relevant.
Over time, however, many factors have caused a shift in the benchmark used to measure a successful anesthetic outcome. Success is no longer characterized by mere survival. The emphasis is now focused on preventing anesthetic morbidity (illness or abnormal condition). The nation’s leading experts agree that the shift has occurred largely due to more effective monitoring — especially keeping a close watch on the patient’s blood pressure, correcting it when necessary, and doing so all the way through recovery. The American College of Veterinary Anesthesiologists (ACVA) declared in their revised Position Statement that “Frequent and continuous monitoring of vital signs in the peri-anesthetic period by trained personnel, and the intelligent use of various monitors are requirements for advancing the quality of anesthesia care of veterinary patients”. Among the physical parameters listed in their guidelines to be monitored frequently and continuously is blood pressure.
The Canadian Anesthesiologist’s Society says that the only indispensable monitor is the presence, at all times, of an anesthetist with appropriate training and experience. Mechanical and electronic monitors are, at best, aids to vigilance. Such devices assist us to ensure the integrity of the vital organs and, in particular, the adequacy of tissue perfusion and oxygenation.
In other words, the safety of your patient is dependent on your awareness and response to potential problems. A thorough understanding of the principles of anesthetic monitoring and awareness of normal and abnormal patient parameters is crucial to providing safe anesthesia. Experts estimate that hypotension (low blood pressure) occurs in 25 to 30 percent of anesthetized small animal patients. During anesthesia, changes in blood pressure are the best first alert warning of impending problems in both cats and dogs. As the anesthetist, you interpret subtle changes in a patient’s parameters and prepare to address any issues immediately, as they arise. Do not wait until a patient is critical to take action and ask for assistance.
Clinically, blood pressure values provide a tremendous amount of useful information. Because of the level of skill required to measure blood pressure by direct means, non-invasive monitoring has become a practical method of estimating arterial blood pressures and pulse rate. There are several user-friendly non-invasive blood pressure monitors available for use in small animal anesthesia. One is the oscillometric method, which involves placing a cuff around a limb or the tail and activating a machine that automatically inflates and deflates the cuff at programmed intervals. Another is the Doppler ultrasonic flow detector and sphygmomanometer. In addition to a cuff, the Doppler ultrasonic crystal is placed over an artery to create an audible signal of blood flow.
Direct Blood Pressure – Arterial Catheterization
Measuring direct arterial blood pressure is the gold standard for blood pressure measurements. It involves placing a catheter into an artery and attaching the catheter to a pressure measuring device. One such device is a transducer. The transducer is connected to an oscilloscope which continuously interprets the pressure wave into systolic, mean and diastolic readings. In addition, most monitors that use a transducer also display a wave form that corresponds with the pulse. This visual display provides important information about the patient and the blood pressure.
It takes an advanced level of technical skill to access direct arterial blood pressure. To access blood pressure information directly, a catheter is placed in any peripheral artery. With sufficient practice, this can be done percutaneously. Consider the possible complications of arterial catheterization when choosing a site. Hematomas, air embolism, thrombosis and infection are the most common (yet rare) complications. Use sterile technique when placing these catheters to reduce complications. Always clearly mark arterial catheters so that nothing but heparinized saline is ever injected into them. Flush the catheter at regular intervals to prevent clotting. Never administer drugs through an arterial catheter.
Unfortunately, the technology required to measure direct arterial blood pressure can add $1,000 or more to the price of a monitor. However, if a transducer system is not available, direct blood pressure can be measured through the arterial catheter employing the same methods used to measure central venous pressure. Also, the creative use of an arterial catheter, a sphygmomanometer, a three-way stopcock and a couple of extension sets can provide an inexpensive, yet accurate mean arterial blood pressure monitor.
Indirect Blood Pressure – Oscillometric Devices
Oscillometric devices work by picking up pulsation under an occlusion cuff placed over an artery. The cuff is connected to a monitor that can be programmed to take measurements at specific intervals of time. These devices deliver systolic, mean and diastolic readings as well as the pulse rate. Most have alarms that can be set to alert when readings are out of an acceptable range. The cuff size should be approximately 40% of the circumference of the limb (or tail) around which it will be placed. Cuff placement with its relation to the position of the heart can also affect the accuracy of readings. For dogs, the best site for cuff placement is on the front leg over the metacarpal area. Alternately, use either the area over the anterior tibial artery just below the hock or the metatarsal area. In the cat, the cuff should be placed over the median artery of the forelimb between the elbow and carpus. In both dogs and cats weighing less than 5 lbs., place the cuff above the elbow, over the brachial artery. Use the “Goldilocks” method when determining how tightly to apply the cuff. The cuff should not be so loose that it can be rotated over the site nor so tight as to obstruct venous return. It should be just right.
Recent studies have shown that, when used properly, oscillometric technology can give reliable and accurate readings in anesthetized dogs and cats. In recent years, oscillometric technology has been re-engineered so that monitors now detect oscillations even in the small vessels of cats and kittens, and the heart rate range has been expanded to 300 bpm.
Indirect Blood Pressure – Doppler Flow Detectors
Doppler flow detectors work by placing an ultrasonic probe (crystal) over an artery. The frequency of the moving arterial blood is converted into sound. Doppler probes may be placed over any artery, but the most useful for measuring blood pressure are the palmar arteries of the forelimb and hindlimb. Shave the area and apply a generous amount of ultrasonic or lubricating gel. Secure the crystal in place snugly (again, using the “Goldilocks” method) with tape. Place an appropriate sized occlusion cuff (40% of the circumference of the limb) on the leg above the crystal. Inflate the cuff using a sphygmomanometer. Allow the cuff to deflate slowly until the sound of blood flow returns. The first audible sound heard represents the systolic blood pressure. In some patients it may be possible to detect a second sound, which is said to be the diastolic pressure. There is some controversy over what the first audible sound represents in cats, though. Some suggest that the first audible sound correlates more closely with mean arterial pressure in feline patients.
Doppler flow detectors are very useful monitors, with their audible sound of blood flow being one of their biggest advantages. They detect changes in flow as well as changes in the regularity of the pulse. These monitors have the advantage of being useful in all sizes of patients, from the tiny exotic species to large equine and zoo animals. And they are comparatively inexpensive.
The Good Anesthetist
As the standard of veterinary care advances and client expectations expand, the importance of vigilant anesthesia monitoring increases as well. Remember when assessing the anesthetized patient that, while absolute numbers are obviously important, subtle changes and trends are often an early indicator that the patient may be getting into trouble.
Many drugs used for sedation and anesthesia depress heart function, and may also cause blood vessels to dilate – all of which contributes to low blood pressure during anesthesia. Add to this the effects of age, disease, and/or surgical procedures, and it’s easy to understand why hypotension a common complication during anesthesia. If blood pressure is too low, vital organs (brain, kidney) may not receive sufficient blood to meet their metabolic needs, and organ damage will occur. In rare cases, death may result. It is your soul responsibility as anesthetist to monitor the physical status of your patient at all times. The most important thing to remember is to ASK QUESTIONS anytime you have a concern about a patient’s status. Don’t ever hesitate to call for help.