Instruments that supplement clinical monitoring
As we begin to focus on instruments to aid us in our monitoring task, we also need to ask for justification for their use. Does this monitor offer benefits that justify the cost (amortization of the instrument and cost of consumable supplies and, don’t forget, time needed for application) and the potential hazards inherent with the use of the monitor? Several instruments have been identified as essen-tial minimal monitors always to be used. With others, the clinician must decide whether a cost–benefit assessment justifies its use. Many monitors will be used routinely, others only with special indications. We must also point out that, over time, clinical practice changes with changing assessment of the value of this or that monitor.
The American Society of Anesthesiologists has published Minimal Monitoring Standards for patients undergoing general anesthesia.1 In brief, these standards call for the monitoring of the patient’s oxygenation (inspired gas and saturation of arterial blood (SpO2)), ventilation (capnography and clinical assessment), cir-culation (ECG, arterial blood pressure), and temperature (a thermometer).
Some instruments put numbers on observations (feel a thready pulse and assume arterial hypotension; take a blood pressure and put numbers on the hypotension). Others provide information that our senses fail to detect (ECG and capnography, for example).
The reference point for blood pressure recordings is the heart. For example, when upright, your blood pressure just above the ankle will be much higher than in your upper arm – by the weight of the column of blood between ankle and heart. Conversely, if you worry about cerebral perfusion pressure, remember that the pressure in the upper arm will be higher than that in the head if the patient stands or sits upright. Thus, in a horizontal and recumbent patient, you can monitor blood pressure in the upper or lower arm or just above the ankle (the best place if you have to use the lower extremity) and obtain reasonably accurate readings as long as the cuff is at the level of the heart.
You should be able to take a blood pressure by cuff and stethoscope listening for the Korotkoff sounds. You can also feel a pulse distal to the cuff and register systolic pressure when the distal pulse disappears. Instead of feeling the pulse, you can use a pulse oximeter, which depends on a pulsatile signal to work. Use it while inflating the cuff rather than during deflation. The pulse oximeter averages incoming data and thus takes a little time before reporting a signal, but it stops working rapidly when suddenly deprived of a pulsatile signal, as happens during inflation of the cuff.
The world (at least the Western world) has now taken to oscillometric sphyg-momanometry. The concept is fairly simple. The unit inflates a cuff around the arm (or just above the ankle) and monitors the pressure in the cuff. Well above systolic pressure, the tight cuff transmits no pulsations to the unit. However, as the cuff pressure approaches systolic pressure, the pulsations of the artery begin to cause some oscillation of pressure in the cuff. When the cuff pressure falls just below systolic, the oscillations gain in amplitude, and the clever unit regis-ters systolic pressure. Soon the cuff pressure drops to mean arterial pressure, at which point the oscillations reach their peak amplitude, and the unit recognizes and reports mean arterial pressure. You can imagine that now the oscillations become smaller and smaller and eventually disappear altogether as the cuff pres-sure drops to and below diastolic pressure. Identifying diastolic pressure presents the algorithm in the unit with the greatest challenge; hence diastolic pressures are more likely to be inaccurate, mean arterial pressure most likely to be accurate, and systolic pressure reasonably accurate. Oscillometric blood pressure recordings have become generally adopted in anesthesia where accuracy within +/− 10% is clinically quite acceptable. Oscillometric measurements may become unreliable when arrhythmias or extremely slow heart rates fool the algorithms that govern the systems.