Patient Monitoring during Anesthesia
What monitors does an anesthesiologist use?
What does the anesthesiologist monitor, and why?
As a licensed physician anesthesiologist, I must discuss the risks and benefits of having anesthesia for surgery. Most of the time the risk is pretty low, and even when it is higher due to disease or more involved surgery, I've never had a patient decide to have surgery without it!
Because anesthesia drugs change how the body works, there if of course, risk. But, by using continuous monitoring of vital signs and anesthesia levels, the risk is greatly reduced or minimized.
What Does an EKG Tell Us?
The electrocardiogram (EKG, ECG) gives information about the heart. Some of the information is clear, and some may just provide clues to the state of health of the heart.
- Heart rate
- Heart rhythm
- Size of the heart (especially thickening of the muscle of heart's wall)
- Normal and abnormal conduction through the heart
- Clues to abnormal blood levels of electrolytes (potassium, calcium, sodium, etc)
- Blockages and lack of oxygen to parts of the heart
- Clues to other conditions (fluid around the heart, even lung disease)
EKG Monitor, aka ECG Monitor or Electrocardiogram
Across the top of the monitor shown above is the EKG strip and heart rate display. The line is in orange and shows the heart rate, rhythm and other information to the anesthesiologist. The number in yellow/orange to the right (58) is the heart rate.
Either three or five electrode stickers are placed in certain spots on the chest. These read the electrical conduction through the heart. A great deal of information can obtained from the pattern displayed on the screen. The most direct information provided are the heart rate and rhythm (regular or irregular). Other conditions, such as thickening of the heart muscle from high blood pressure can be reflected in the electrocardiogram, as well. Changes in the levels of blood electrolytes might also have a characteristic appearance on the EKG. High potassium is an example of electrolyte abnormality that causes visible changes to the pattern on the EKG.
If blockages exist in the coronary arteries that supply blood to the heart, the EKG pattern will show changes. Having a heart attack under anesthesia is a rare but potentially serious complication. There are usually risk factors for coronary artery disease present and myocardial infarction is more likely during or after emergency surgery, than elective surgery.
The anesthesiologist must evaluate any change in heart rate or rhythm. Sometimes, the treatment is just a change in the type or depth of anesthesia. Sometimes, heart rate and rhythm can change due to something being done during surgery (did you know that pushing on your eyeball can cause a drop in heart rate?) and sometimes, it is due to an intrinsic problem with the heart or other preexisting health problem.
Blood Pressure Monitor
Blood pressure is displayed in the upper right corner of the monitor in pink/purple. The systolic (top number) represents pressure in the blood vessels while the heart contracts. The bottom number is the diastolic blood pressure, the pressure exerted on the blood vessel walls when the heart relaxes. The number in parentheses is the MAP--mean arterial pressure--and provides even more information to the anesthesiologist.
Blood pressure can be measured invasively or non-invasively.
- Invasive blood pressures are measured with a catheter placed inside an artery--called an arterial line--usually in the wrist. More frequent (continuous) and more accurate measurements can be taken invasively. Usually, this level of monitoring is NOT needed for routine surgery or for healthy patients, even for more involved or lengthy surgery.
- Non-invasive blood pressure are the usual blood pressure measurements taken with a cuff that squeezes around the arm. During anesthesia, your blood pressure will be taken by an automated cuff, every two to five minutes while you sleep to ensure that any changes are detected as quickly as possible.
Anesthetics, including most general anesthesia drugs as well as epidurals and spinals, tend to cause blood pressure to drop. Some people tolerate these drops better than others and it is always important to be able to get rapid and frequent blood pressure measurements under anesthesia.
Pulse Ox Monitor (Pulse Oximeter)
Measurement of the oxygen saturation of the blood improved the safety of anesthesia, perhaps more than any other innovation in history. Because anesthetics intentionally or as a necessary side effect, cause breathing to slow or stop, the anesthesiologist becomes responsible for this obviously vital function. Having a direct measure of the adequacy of ventilation and oxygenation is immensely valuable during any anesthetic.
Hemoglobin is the oxygen-carrying molecule in red blood cells. The pulse oximeter is the device used to measure how saturated the hemoglobin is with oxygen. Varying wavelengths of light are used to detect the oxygen in the arterial blood. A value between 95 and 100 percent is normal. This number is called the "sat" or the "pulse ox".
The pulse oximeter also displays the heart rate, in addition to the oxygen saturation of the blood.
Changes in pulse ox readings are carefully evaluated and are aggressively treated to maintain this number in the normal range. Patients who smoke, have preexisting lung disease or have asthma may be more likely to need specific intervention to maintain oxygen levels.
Whether breathing is assisted by a ventilator, or a patient breathes on his own during an anesthetic, the rate, pattern and depth of respiration can be monitored. Oxygen (along with anesthesia gas during general anesthesia) is breathed in during the "inspiration" phase of breathing. Carbon dioxide and other byproducts of metabolism are breathed out during the phase of respiration called expiration.
During expiration, the CO2 can be measured and graphed to give visual representation of breathing. The amount of carbon dioxide at the end of expiration can also be displayed as a number. If this number falls out of the normal range, it can be a sign of alteration of respiratory function. This may be a normal reaction to the type and depth of anesthesia, or may represent dysfunction of the respiratory or pulmonary system. This is just one of the many variables that the anesthesiologist must monitor, diagnose/evaluate and respond to during the course of anesthesia.
The ventilator itself, also has monitors to display the volume of gas being delivered to the patient. There are also pressure monitors to show ensure that the lungs are not over- or under- expanded during ventilator-assisted breathing.
The amount of anesthesia gas being delivered is also measured and displayed on the anesthesiologists' monitor. Problems (such as a leak) with the delivery system can be determined if these numbers don't display properly.
The adequacy of anesthesia is determined by using all of the information from the monitors, together.
Brain Activity Monitor- BIS monitor
A BIS (bispectral index) monitor uses a modified EEG (brain activity graph) signal or average of signals, represented as a number, to show the depth of anesthesia. It can also provide some information about the oxygen supply to the brain.
When these devices were introduced, they were marketed as the solution to the "awake under anesthesia" complication. Unfortunately, studies have shown that they do NOT prevent awareness under anesthesia (a very rare complication of anesthesia). While the number displayed does give a fair representation of the level of consciousness of the patient, there is probably too long of a lag between when consciousness changes and when the number is displayed. In other words, if an anesthesiologist is relying ONLY on this monitor, awareness can already have happened before the number shows it.
Heart rate, breathing rate, blood pressure and other signs are more reliable indicators of changes in the level of consciousness and careful attention to these parameters, with adjustment of anesthesia, is more useful for preventing awareness.
Other Monitors Used by Anesthesiologists.
These are the basic monitors used by anesthesiologists. Some of them are required to be used every time and some at the discretion of the anesthesiologist.
In addition, temperature monitoring should be done on nearly every unconscious patient because of the rapid changes in temperature that can occur under anesthesia.
Specialized monitors for obtaining more detailed and complicated information (mostly about the heart) can also be used when useful or needed for certain types of surgeries.