What’s in a Waveform Pulse Oximetry Capnography By: Jane Seifried, B.Sc., ACP Huron County Paramedic Services

Patient Assessment As Paramedics we have various tools to assist in our assessment of a patient in the field. Education and experience develop the most important tool…YOU. Your own history taking as well as your interpretation of visual and physical findings. Electronic equipment such as: cardiac monitoring, 12 Lead acquisition, pulse oximetry (SPO2) and capnography (EtCO2), are also used to further your understanding of a patient’s presentation and to help confirm your findings in a measureable manner. This presentation will be presented in 2 parts. One month, a review of SPO2 findings and the second month, a more detailed look at the value of End Tidal CO2 monitoring for certain patient presentations.

Quick Terminology Review

Oxygenation vs. Ventilation Oxygenation: the ability of Red Blood Cells (RBC) and plasma to bind or absorb oxygen molecules and deliver this oxygen to the rest of the body. Ventilation: the physical exchange of air between the lungs and the environment. It includes inspiration and expiration.

Perfusion vs. Diffusion Perfusion: the circulation of blood through the vascular bed of tissues to allow molecules to be exchanged. In order to have adequate perfusion there must be a high enough BP, enough blood volume and unobstructed arteries Diffusion: the movement of molecules from an area of high concentration to an area of low concentration. This example shows CO2 leaving the cells and O2 entering the cells due to differences in partial pressures.

Aerobic Cellular Metabolism This diagram is a simplified version of cellular metabolism. Through the Kreb’s Cycle of cellular respiration, O2 and Glucose are converted to ATP (energy) with CO2 and Water as byproducts.

Where do these terms fit in? Pulse Oximetry, SpO2 is an indication of: OXYGENATION Capnography, EtCO2 is an indication of: VENTILATION PERFUSION DIFUSSION METABOLISM

Our Zoll X Series Monitor Yellow waveform - SpO2 Plethysmograph (Pleth), with it’s reading of 98% Purple solid waveform - EtCO2 Capnogram, with it’s readings of 36mmHg and 18 breaths/min

Part 1 Pulse Oximetry (SPO2)

Pulse Oximetry (SPO2) Quick non-invasive tool to obtain a reading on a patient’s oxygenation status Ideally it is good to get a reading on room air prior to applying O2. However, never purposely withhold O2 from a patient in severe respiratory distress. Normal SpO2 range is 95-98%, but can go up to 100%. Pt’s with COPD or other respiratory diseases can have a “normal” SpO2 a little lower than this.

Pulse Oximetry (SPO2)

Positions for SpO2 monitor on an infant using the adhesive sensors. Pulse Oximetry (SPO2) Positions for SpO2 monitor on an infant using the adhesive sensors.

Pulse Oximetry (SPO2) What is it looking at? The short answer is… RED BLOOD CELLS (RBCs) RBCs carry 98% of the O2 in our blood, 2% is dissolved in plasma RBCs pick up O2 in the lungs when it binds to the RBCs’ hemoglobin (HgB). Each HgB can carry 4 molecules of O2. The O2 is released to the tissues as RBCs squeeze through the tiny capillaries of the body’s tissues. SpO2 monitoring is used to determine the amount of RBCs that have molecules bound to their HgB. This is done continuously with each arteriole pulsation that occurs with every heart beat.

Pulse Oximetry (SPO2) What is it looking at? The reading is displayed as a percentage of bound HgB versus total HgB after 2 different beams of light have evaluated the bound HgB (Red below) and the unbound HgB (Blue below) in the RBCs after several pulsations.

Pulse Oximetry (SPO2) What is it looking at? continued The Zoll X series SpO2 sensor evaluates the HgB and tissue displacement from arteriole pulsations over 8 seconds. These pulsations are the waveforms that you see as the Plethysmograph (Pleth) while the number is the percentage of bound HgB to total HgB. The ideal Pleth shows a DICROTIC NOTCH in the waveform. However, this may not always be seen.

Pulse Oximetry (SPO2) What is it looking at? continued This is why it is important to make sure that a waveform is seen on the monitor that coincides with the patient’s pulse and/or ECG rhythm. Without a well defined SpO2 waveform, there is no way for the machine to accurately perform a reading. It will try to come up with something but it should NOT be considered accurate.

Pulse Oximetry (SPO2) Image above shows a well defined consistent waveform (blue wave) coinciding with ECG and pulse rate. Can be confident that the pt’s SpO2 reading of 99% is accurate. Image above shows a poor waveform, yet an SpO2 of 100%. This should not be considered accurate. Look for this with your patients in the field and treat according to your physical findings. Do not report an SpO2 value that does not have a proper waveform associated with it.

Pulse Oximetry (SPO2) Are PVCs or PACs Perfusing? If unsure after checking a pt.’s pulse, the SpO2 pleth can help to determine if these ectopic beats are actually perfusing. The cardiac monitor will register these beats in its calculated rate, which if they are not perfusing, does not represent the pt.’s Pulse Rate. In the diagram above, the Blue wave is SpO2 and shows that the PVCs in the Green ECG rhythm are not perfusing, because there is just a little blip in the pleth associated with the PVC. Over the first 12 sec period the monitor would display a HR of 144 bpm, while the Pulse Rate would actually only be 108 bpm.

Pulse Oximetry (SPO2) Limitations Poor waveforms arise from inadequate distal perfusion due to Hypotension or Hypothermia. Artifact from patient tremors, shivering and movement of the SpO2 probe, can affect readings. If the probe keeps slipping off of a patient’s finger, loosely tape it on with IV tape or switch to the “band aid” type probe to see if this helps. Dark nail polish and stray ambient light can give inaccurate values

Pulse Oximetry (SPO2) Limitations Anemia or low HgB due to trauma. These patients may have a normal SpO2 level because what Hgb they do have, has O2 bound to it. They may still be hypoxic though, due to not having enough total O2 circulating in their blood. These pt’s need to have O2 applied if they are in distress. Carbon Monoxide (CO) binds MUCH stronger to HgB than O2, over 200 times stronger in fact! In the setting of CO exposure a patient can have an SpO2 of 98-100% yet may be unconscious due to hypoxia. Administer 100% O2 to anyone with a possible CO exposure. Remember that some O2 can still be transported in the blood’s plasma.

Pulse Oximetry (SPO2) Changes in a patient’s oxygenation status may take several minutes to be picked up by the SpO2 monitor. This is due to the time it takes for the blood to circulate to the periphery after a change has occurred. You can see this if you hold your breath while hooked up to the SpO2 monitor. It will still read 98%, most likely as long as you are able to hold your breath for, even though there has been a significant change in your condition…Apnea. Cited in our Medical Directives: ROSC - titrate oxygenation SpO2 >= 94%. CPAP – condition SpO2 < 90% or accessory muscle use.

Summary In order to have a Reliable SpO2 Reading you MUST have: A good sensor Site Adequate Perfusion to that Site Appropriate Sensor A Visible, well defined, Waveform to assess the Quality of the Reading SpO2 only measures HgB saturation. Not Ventilation, Not Perfusion, Not inspired Oxygen and Not physiologic stress. This is where combining EtCO2 monitoring comes in. Like all monitoring tools, it must be considered in addition to a thorough clinical assessment of a patient’s: LOA, respiratory effort, rate, rhythm, breath sounds, skin condition and relevant history.

References http://www.ems1.com/education/articles/1875427-How-EMTs-can-get- more-from-a-cardiac-monitor/ http://www.amperordirect.com/pc/help-pulse-oximeter/z-what-is-oximeter- plethysmograph.html http://www.lhsc.on.ca/About_Us/Base_Hospital_Program/Education/008Oxime try11-06.pdf Zoll X Series, Operator’s Guide, 2012