Monitoring Modalities in Critical Care

Monitoring is crucial for successful management of critically ill patients. The main emphasis is on the cardiovascular, respiratory and neurological systems. Monitoring is eased through the use of monitoring equipment; however the importance of regular and repeated physical examinations can not be over-emphasised. As critical patients are conscious pulse oximeters and capnographs are not useful as there are high levels of interference due to movement. The following equipment may prove useful for monitoring of critically ill patients.

Electrocardiogram

The electrocardiogram is an indication of the electrical activity of the heart recorded by a number of electrodes. It provides information about the electrical but not mechanical activity of the heart and should therefore always be used in conjunction with some form of pulse monitoring. An ECG is advisable in all cases with cardiac disease, or if arrhythmia is suspected as it is the only way to diagnose arrhythmias and allow appropriate management.

Blood pressure monitoring

Arterial blood pressure monitoring
This is useful to monitor trends and response to treatment. Hypoperfusion is common in critical patients and if accompanied by persistent hypotension may prompt the use of vasopressors. Hypotension should be avoided as it can have deleterious effects on neurological and renal function. Non-invasive blood pressure monitoring is a useful technique to be familiar with and Doppler systolic blood pressures have been shown to correlate well with invasive blood pressure over a wide range although it correlates best with mean arterial blood pressure in cats. Doppler method is only useful for systolic blood pressure, diastolic readings are inaccurate. The oscillometric technique also correlates well although tends to be slightly lower than invasive pressures. Good technique should be used to get the most accurate results. The cuff used should be the correct width, which is 40% of the circumference of the limb. Too narrow a cuff results in overestimation of the blood pressure, whereas too wide a cuff results in underestimation. The oscillometric technique is the only non-invasive method capable of measuring systolic, diastolic and mean arterial blood pressure. An appropriately sized cuff is placed on a limb and attached to the machine. The cuff is alternately inflated and slowly deflated. During deflation sensors in the machine detects alterations in cuff pressure. Heart rate is calculated from oscillation frequency. This method can be automated but is prone to movement error and can also be inaccurate at very low heart rates and when arrhythmias are present.
Invasive blood pressure monitoring is infinitely superior to non-invasive methods and has maintained accuracy at low blood pressures and in the presence of arrhythmias. Invasive arterial blood pressure monitoring is more technically demanding requiring placement of a catheter into the metatarsal or femoral artery. This is an easily learnt technique in dogs, however is challenging in cats. The catheter is connected to a transducer via a saline filled tube. The set up is then zeroed at the level of the right atrium. The majority of transducers in use today connect to an electronic monitor which then displays a waveform, systolic, mean and diastolic blood pressures. Other systems are available that do not require an electronic interface but provide much less information. Invasive blood pressure monitoring is useful in critically ill patients as it provides accurate second to second information on blood pressure. Although this technique is the most accurate method of measuring arterial blood pressure there are still potential sources of error that the operator should be familiar with. [...]