Photoplethysmography
Reference: Chapter 5 of Pulse Oximetry 2nd Edition by John TB Moyle
1. The Importance of the Plethysmographic Trace
Every pulse oximeter should display not just the arterial oxygen saturation (SpO2) and heart rate, but also a plethysmographic trace. This trace serves as a critical tool for evaluating the oximeter's performance. Only when the trace closely mimics an arterial pressure waveform, complete with a dicrotic notch, should the displayed SpO2 be considered reliable.
The trace can also flag any mechanical or electrical distortions and show whether the signal's pulsating component has sufficient strength. However, a normal-looking trace doesn't guarantee accurate SpO2. Factors like abnormal hemoglobin or other substances in the blood could interfere with readings.
Handling Trace Amplitude in Software
It's essential to understand how the oximeter's software processes the amplitude of the trace. Some manufacturers display the trace exactly as received, while others normalize the amplitude. In the latter case, the normalized trace is what's analyzed to determine SpO2. Importantly, its amplitude won't change unless the signal's AC component becomes too weak. Some brands, like Ohmeda, even include a bar graph to show the gain applied to the plethysmographic trace.
2. Understanding Photoplethysmography
Photoplethysmography measures changes in light absorption in body tissues, typically using a pulse oximeter. The light variations detected by the photodetector are influenced by several factors:
Key Factors Affecting Light Variation
- Blood volume under the probe
- Orientation and concentration of red blood cells
- Blood flow speed
- Distance between light source and detector
- Arterial and venous blood flow
Studies have found a strong correlation between photoplethysmography and another method, strain gauge plethysmography, especially in measuring pulsatile blood flow. These methods can potentially offer non-invasive insights into cardiac function and blood flow characteristics. When the probe is placed on the fingertip or earlobe, it primarily measures blood flow in the skin, which is affected by various factors that we'll discuss later.
Blood Flow in Skin
In men, skin blood flow can range from 20 ml/min in cold conditions to 8 L/min in heat. This is largely due to the skin's role in regulating body temperature. Various reflex mechanisms like baroreceptors and chemoreceptors also influence this flow. Medications can also have a direct or indirect impact.
The skin has numerous arteriovenous shunts, especially in the extremities, which play a significant role in thermoregulation. These shunts contribute to variations in blood flow and thus influence the photoplethysmograph readings.
Factors Affecting Skin Blood Flow
Neurogenic Factors - Sympathetic fibers affecting arterioles and venules - Cholinergic pathways to sweat glands causing vasodilation
Reflex Constriction - Local or general cooling - Low blood pressure - Pain - Fear - Deep breathing
Reflex Dilation - Heating - Chemoreceptor stimulation - Coronary blockage - Acute high blood pressure
Humoral Control - α-Adrenergic agents causing constriction - Serotonin causing constriction - Prostaglandin F2α causing constriction - Vasopressin and angiotensin also causing constriction
Other Influences - Reduced oxygen or increased carbon dioxide levels - Local effects of high carbon dioxide levels - Autoregulation, although this plays a minor role
The reflex mechanims are shown in the figure below:
3. Evaluating Heart Rhythm
The plethysmogram quickly reveals the heart's rhythm and any fluctuations in its consistency. It can also highlight issues with the refilling of the heart chambers during irregular beats like bigeminy. If there's a sudden drop in heart performance or blood volume, the graph will show reduced peak heights. This only occurs if the graph accurately depicts pulse volume without any adjustments. Low blood volume might also cause rhythmic fluctuations in the graph's peaks, which move in sync with breathing patterns. This behavior resembles what we see in intra-arterial pressure monitoring, where peak variations become more pronounced during positive-pressure ventilation if the patient has low blood volume.
4. Measuring Systolic Blood Pressure
Using a pulse oximeter in conjunction with a blood pressure cuff can provide an effective way to measure systolic blood pressure. When deflating the inflated cuff, observe when the pulse oximeter trace reappears. The pressure reading at that moment is a reliable measure of systolic blood pressure. This method is comparable to using Doppler ultrasound to identify blood flow beneath the cuff.
In a study by Chawla et al., the pulse oximeter method was compared with two other techniques: Korotkoff sounds and an automatic blood pressure monitor based on the oscillometric principle. The study found strong agreement among all three methods in 100 healthy volunteers. Additionally, this method was identified as particularly useful for measuring systolic pressure in cases of Takayasu’s syndrome, a condition where traditional methods often fall short.
Allen's test, which checks the health of the palmar arterial arch, is made easier by using a pulse oximeter. Simply attach the oximeter to the index finger and watch for changes as the radial artery is compressed.
5. Evaluating Blood Vessel Behavior
Soon, we might be able to continuously track how peripheral blood vessels respond to various conditions. Shelly and team compared the waveforms from pulse oximetry and direct arterial pressure monitoring to study these changes. Specifically, they looked at "compliance," which measures how much the volume changes for each unit of pressure applied. They found that vasoconstrictors like noradrenaline and adrenaline reduced compliance as expected. Their case study even provided enough data to create a dose-response curve, showing the relationship between drug dose and vascular response. This could become an important tool for managing patients in critical condition.
6. Choosing Between Finger and Ear
When collecting photoplethysmograms, readings from the finger offer more useful clinical information than those from the ear. Specifically, variations in pulse pressure throughout anesthesia stages are more pronounced when recorded from the finger.
7. Understanding Waveforms
In an unprocessed photoplethysmograph trace, you'll typically see two distinct waveforms: a faster arterial waveform and a slower respiratory waveform. The latter's amplitude is influenced by central venous pressure and intrathoracic pressure. During assisted breathing via intermittent positive-pressure ventilation, a lower central venous pressure correlates with a higher amplitude in the respiratory waveform.
8. Additional Points to Consider
If a photoplethysmograph trace vanishes during anesthesia, immediate investigation is required. The most common reason is probe displacement, but it could also indicate cardiac arrest or end artery compression leading to poor blood flow in the extremities. While a pulse oximeter might be used to evaluate the effectiveness of CPR, its readings can be misleading and should be viewed cautiously.
Other specialized applications of the pulse oximeter's photoplethysmograph include monitoring blood flow in reattached limbs and fingers. Sterile probes can also be employed to evaluate blood flow in internal organs during surgery.