When the pulse oximeters were issued to us in the 90s, our first thought was that we are required to carry more ‘light weight’ kit. Those first units were not small and anything needing batteries was looked down on. They didn’t have the waveform and only showed the oxygen concentration and the pulse rate. We already had skills to determine our casualty’s perfusion and heart rate. Why did we need more kit? Twenty years later the oximeters are small, lightweight and show quite a bit more about haemodynamic status than the original devices.
Our medical director, COL Winston de Mello, is adamant that all CoROM students understand the full extent of what we can learn from the SPO2. CoROM Remote EMTs and Paramedics start using oximeters from the first time they touch a patient. The oximeter goes on a finger during the Breathing assessment and can be additionally used for the Circulation assessment. It is a valuable tool to quickly see the perfusion status, mean arterial pressure and shock status of the casualty.
6 parts of the waveform
1. Systolic Pressure is the top of the waveform
2. Dicrotic Notch is created by the aortic valve closing
3. Diastolic Pressure is the bottom of the waveform
4. Mean Arterial Pressure is the area located under the entire waveform
5. Pulse Pressure is the difference between the top and the bottom of the waveform
6. Systemic Vascular Resistance can be seen by how wide or how narrow the waveform.
The dicrotic notch is caused by the aortic valve closing which causes a change in pressure that can be seen on the waveform. This marks the end of systole and the start of diastole. How the notch looks on the downward waveform changes with age and cardiac health. The less distinct the notch, the great probability of coronary heart disease.
3 types of waveforms
1. Waveform 1 is normal.
2. Waveform 2 is small and weak indicating decreased stroke volume, increased peripheral resistance and/or hypovolaemia as well as possibly cardiac failure.
3. Waveform 3 is large and bounding indicating decreased peripheral resistance and/or decreased compliance. This could be caused by fever, anaemia, hyperthyroidism, aortic regurgitation, bradycardia, heart block or atherosclerosis.
The area under the waveform shows the Mean Arterial Pressure. Shock can cause a narrow waveform due to vasodilation. During shock, the MAP can go below 65mmHg which is dangerous. Seeing a narrow waveform should be a huge warning sign for your casualty.
Take some time to appreciate all of the information that a good medic can ascertain from just the waveform. This is why the curriculum committee at the College of Remote and Offshore Medicine advocate putting on the pulse oximeter as soon as possible since it gives you both respiratory assessment and circulation assessment from one quick glance.