1999 Aclam forum
Anesthesia monitoring systems
Simon S. Young MA, VetMB, PhD, DVA, DiplECVA. Schering-Plough
Research Institute, Kenilworth, NJ, USA
Why monitor during anesthesia?
The ideal anesthetic agent would produce reversible unconsciousness and analgesia (i.e. absence of response to painful stimuli) with no effect on other homeostatic measurements. Unfortunately, no anesthetic drugs meet this standard and they all affect other body systems to some extent. The most important and serious side effect of anaesthetic drugs is depression of respiration and the cardiovascular system. There are two reasons why these side effects are important:
They occur at "therapeutic" doses, i.e. there is significant depression at the doses of anaesthetic needed to produce unconsciousness.
Severe depression of either respiration or the cardiovascular system is life-threatening.
Point (1) affects the way in which anaesthetic drugs are given. Compared to other drugs, anaesthetic are highly toxic. The therapeutic index of propofol, for example, is 4.5 (Glen and Hunter 1984). This range is comparable to that seen with normal inter-animal variability, which means that what is an effective dose for one animal may be ineffective for another (i.e. they don't go to sleep) and too large for others (marked cardiovascular and respiratory depression). Smith (1993) noted considerable variability in the depth of anesthesia in laboratory animals. The only way that these drugs can be used safely is to administer them on an individual basis and observe (or monitor) their effects in that particular animal.
Since we know that our anesthetic drugs will produce significant side effects, and these side effects are serious for the animal, we must treat the side effects if necessary. Some treatments can be applied "blindly" with little monitoring of their effect. For example, it is recommended that supplemental oxygen is given to all anesthetized animals whether or not they are hypoxaemic. The rationale for such blind treatment is that inhalation of an oxygen-rich mixture will not harm animals if they are not hypoxic and will benefit animals that are. Vender et al. (1995), for example, showed that rats anesthetized with a standard regimen become hypoxaemic and this can be reversed with oxygen supplementation. Even with this apparently innocuous procedure there are problems. Firstly, prolonged administration of oxygen is harmful (Hall and Clarke 1991 p.49) and when oxygen is given for hours or days the inspired concentration must be titrated against arterial oxygen saturation. Secondly, and much more important, administration of oxygen does not guarantee that an animal will not become hypoxaemic. if its airway is obstructed no amount of supplemental oxygen will help. Thus arterial oxygenation should be monitored during anesthesia even if supplemental oxygen is given.
When we consider the other main side effect of anesthesia, cardiovascular depression, the argument for monitoring is even more compelling. Anaesthetic agents in general depress the myocardium and cause peripheral vasodilatation. Both of these effects lead to hypotension, although the degree depends upon the state of the circulation and the health of
the animal (Booke et al. 1996, Goodchild and Serrao 1989). In addition, there will be other causes of hypotension such as blood loss and vascular compromise. Mild hypotension is tolerated by healthy animals but prolonged moderate hypotension (mean arterial pressure < 6OmmHg) produces renal shutdown. if this persists for a long time the animal can go into renal failure. In large animals, a mean arterial pressure of < 60 mmHg is insufficient to perfuse the muscles and leads to post-anaesthetic myopathy (Lindsay et al. 1989). if the blood pressure falls further, vital organ perfusion is affected. Cerebral autoregulation is lost at a mean pressure of 60 mmHg (Aitkenhead and Smith 1990 p.605) and myocardial perfusion is dependent upon an adequate diastolic pressure (Aitkenhead and Smith 1990 p. 55). If perfusion of the heart or brain is compromised the animal will rapidly deteriorate and die.
There are, therefore, good reasons to control blood pressure during anesthesia. In order to control any physiological variable we need to be able to do two things: (1) measure it, and (2) change it if necessary. In the case of blood pressure control, we measure it with a variety of techniques detailed below. We can alter blood pressure in several ways: fluid administration, positive inotropes (such as dobutamine) and reducing the depth of anesthesia will increase blood pressure. Reduction of blood pressure is rarely needed in veterinary anesthesia but is common in human anesthesia and vasodilators (e.g. sodium nitroprusside) are widely used.
The same arguments can be applied to other less critical parameters such as body temperature. General anesthetics interfere with temperature regulation both by depressing the hypothalamic centers and by suppressing heat generation mechanisms such as shivering. Mild to moderate hypothermia does not kill animals but leads to other problems, the most important being prolonged recovery from anesthesia. Again, to control body temperature and maintain it at the normal level we need to have a means of measuring temperature (a thermometer) and controlling it (heating blankets, insulation etc).
One common question with regard to monitoring is "How much monitoring do I need to use?". There is no one standard because the more invasive the procedure, and hence the more likely it is to interfere with normal homeostasis, the greater the need for monitoring. However, several attempts have been made to define minimum standards for both human and veterinary anesthesia. A key point is the importance of monitoring cardiovascular and respiratory function. The recommendations of the Association of Anesthetists of Great Britain and Ireland, for example, state that "Continuous monitoring of ventilation and circulation is essential. This may be performed by use of the human senses augmented, where appropriate, by the use of monitoring equipment". This minimum standard is directly applicable to veterinary anesthesia. In laboratory animal work it may be difficult to achieve even this level of monitoring, in an anesthetized mouse for example. The guidelines of the American Society of Anesthesiologists state that "During all anesthetics, the patient's oxygenation, ventilation, circulation and temperature shall be continually evaluated." and the guidelines emphasize the use of monitoring equipment to augment clinical observations.
Another question asked is "How frequently do I need to monitor the animal?" This should be re-phrased to "How frequently do I need to write down physiological measurements?" because, as discussed above, cardiovascular and respiratory monitoring should be continuous. To be realistic, however, there is a difference between being aware that the heart rate is "about right" and actually counting the number of beats and writing it down. The former is an almost unconscious task that is performed by glancing at the ECG screen and noting that there is the usual number of QRS complexes on the screen. The latter takes time and concentration. Also, other less critical parameters such as temperature do not need to be measured continuously. Writing down the physiological variables on the anaesthetic record every 10 or 15 minutes is a good working standard. 5 minute intervals take up a significant proportion of the anesthetist’s time unless automatic monitors are used but are recommended for blood pressure monitoring in human anesthesia . If readings are taken more than 15 minutes apart the animal can deteriorate between measurements.
There is a temptation to "cut corners" on routine procedures and reduce the amount of monitoring used, in an effort to increase "efficiency" and throughput. This is not advisable. When looking at anesthetic disasters in veterinary medicine, two features are apparent. Firstly, the cases that die unnecessarily are not the complicated surgeries but the routine ones. The complicated (i.e. "interesting") cases are assigned to the most experienced staff and are well monitored. The boring neuterings are relegated to the most junior staff in the back room and forgotten - until something goes wrong. Secondly, anaesthetic disasters are not due to a single factor. The chance of the anesthetic machine suddenly failing and delivering a lethal gas mixture is extremely remote. Anesthetic disasters are due to a combination of events, none of which on its own seems important. The usual member of staff is sick so a new person is assigned to anesthesia. The batteries in the pulse oximeter were not replaced at the end of the previous day (the sick person had to leave early for a doctor's appointment), it fails after 5 minutes and replacement batteries are not to hand. All goes well until the oxygen tank runs out on the anaesthetic machine because the replacement anesthetist did not notice that the gauge (in an unfamiliar place) was reading low. The anesthetist has stepped away from the table for a couple of minutes to get some suture material from the next room. The problem with the oxygen is not seen until the surgeon notices dark blood oozing from the wound edge and calls the anesthetist back. By the time he/she has replaced the empty tank the animal is dead.
This case may seem artificial but most veterinary anaesthetic misadventures involve similar factors. Not everything can be avoided but equipment problems can be reduced by purchasing good quality equipment, maintaining it and have spares on hand. A certain amount of "resource redundancy" is also advisable.
Finally, it is important that staff are adequately trained in basic physiology and anesthesia so that they can recognize problems as they occur. Everyone anesthetizing animals should, for example, know that an oxygen saturation of <70% requires attention and be able to instigate appropriate corrective measures (check the airway, supply 100% oxygen and commence IPPV). This problem is most serious in laboratory animal anesthesia of rodents where the anesthesia is carried out by the principal investigator (PI) who often has no knowledge of, and very little interest in, animal anesthesia.