Secondary Logo

Journal Logo

Special Article: Special Article

Photoplethysmography: Beyond the Calculation of Arterial Oxygen Saturation and Heart Rate

Shelley, Kirk H. MD, PhD

Author Information
doi: 10.1213/01.ane.0000269512.82836.c9
  • Free

The photoplethysmograph (PPG) waveform was studied and used clinically long before the discovery of its utility in the calculation of arterial oxygen saturation (1,2). That discovery had such a profound impact on clinical monitoring that the other potential uses of the waveform quickly faded from the attention of clinicians. This neglect of the waveform was accentuated by its absence from the early stand-alone pulse oximeter devices. The pulse was indicated by either a bouncing bar or flashing heart symbol.

My personal introduction to this waveform was during my anesthesiology residency in the late 1980s. Already trained as a critical care provider (internal medicine), I found the opportunity to observe clinical waveforms during surgery fascinating. During my residency, a new clinical monitoring system was purchased and thus I observed for the first time the plethysmographic waveform. One fateful day, I asked one of the senior faculty “What does the waveform mean?” pointing to the plethysmograph displayed below the electrocardiogram. His answer … “That means your pulse oximeter is working.” Undeterred I first looked in a standard anesthesiology textbook, and then in a textbook of monitoring. I could find no mention of this “new” waveform.

The rest of this article will outline what I have found during my investigations, both of the literature and experimentally. I have been fortunate that my interest occurred during a period of remarkable growth in computational power (critical to both waveform analysis and literature searches) and an improved understanding of digital signal processing. My ultimate conclusion is that we have only just begun to tap the potential of this remarkable waveform (3). This article was created as part of the International Symposium on Innovations and Advancements in Monitoring Oxygenation and Ventilation (ISIAMOV) 2007 Supplement to Anesthesia & Analgesia. The topic (Photoplethysmography: Beyond the Calculation of Arterial Oxygen Saturation and Heart Rate) matches the material presented at the symposium that took place at Duke University, Durham, NC during March 2007.

SOURCE OF THE WAVEFORM

Beer’s law of light describes the elements that contribute to the pulse oximeter waveform.

Where,

Atotal = absorption at a given wavelength

En = extinction coefficient (absorbency)

Cn = concentration

Ln = path length

Conceptually, it is most useful to view the pulse oximeter waveform as measuring the change in blood volume (more specifically path length), during a cardiac cycle, in the region being studied (typically the fingertip or earlobe). The general consensus is that the waveform comes from the site of maximum pulsation within the arteriolar vessels where pulsatile energy is converted to smooth flow just before the level of the capillaries (4,5). Even this fundamental understanding is not without controversy, as demonstrated in an article by Kim et al. (6) in which they hypothesized that the source of the signal is from “open arteriovenous anastomoses in the cutaneous circulation.”

The PPG is a remarkably simple device consisting of a light source (most commonly an LED) and light detector (photo diode). The detector can be placed either directly across from the light source for transmission plethysmography or next to the light source for reflective plethysmography. The plethysmographic waveform that is displayed on the commercial pulse oximeter is a highly processed and filtered signal. As pointed out in a recent editorial (7), when attempting to use clinical monitoring devices as research devices one must learn how to cope with proprietary filters and algorithms. It is hoped that, encouraged by articles like this one, equipment manufacturers will consider adding features that will allow their devices a broader range of use (Table 1). At the present time the literature consists largely of an anecdotal mass of what are essentially case reports, combined with a few small prospective and retrospective studies attempting to extract new clinical information from the PPG. Key to unlocking the potential of this waveform is unfettered access to the raw signal, combined with standardization of its presentation and methods of analysis. In the long run, we need to learn how to consistently quantify the characteristics of the PPG in such a way as to allow the results from research efforts be translated into clinically useful devices.

T1-6
Table 1:
Desirable Characteristics for a Pulse Oximeter Used for Waveform Analysis

Of the two or more wavelengths measured by the pulse oximeter, traditionally only the infrared signal (approximately 940 nm) is presented. The information from this wavelength is displayed because it is more stable over time, especially when compared to the red signal (660 nm), which is more susceptible to changes in the oxygen saturation. In addition, only the pulsatile component or AC portion is displayed. The static component or DC (created mostly by the absorption of light by surrounding tissue) is eliminated by an auto-centering routine used to ensure the waveform remains on the display screen. With changes in the degree of venous congestion, the waveform can be noted to drift partly off the screen and then return via the auto-centering algorithm.

All clinical pulse oximeters that display a plethysmographic waveform do so with an auto-gain function designed to maximize the size of the waveform displayed. Some manufacturers include an option to turn off this automatic resizing function. Without this option it would be impossible to analyze the amplitude of the pulse oximeter waveform, an important parameter to measure, when analyzing the waveform. When examining the PPG amplitude change over time, the region of the body being measured is important. In the finger, where the walls of the cutaneous vessels are richly innervated by a-adrenoceptors, the sensitivity to changes in the sympathetic system are greater than when compared to other areas of the body such as the earlobe (8).

At this time, no calibration procedure is known to standardize the PPG amplitude for comparing one patient waveform to another. Note, this is an important issue and an excellent research opportunity. The signal is therefore not given a unit designation. Similar to central venous pressure measurement, the value of the plethysmograph comes from an analysis over time, as opposed to any absolute number. The term “plethysmograph” is derived from the Greek root “plethysmos” meaning “to increase.” There is a close correlation (r = 0.9) between the PPG and the more traditional strain gauge plethysmograph (4).

AMPLITUDE ANALYSIS

One of the more useful plethysmographic features is the waveform amplitude (Table 2). Amplitude changes can be concealed by the auto-gain function found on most pulse oximeters. By turning off the auto-gain, certain observations can be made. For example, over a remarkably wide range of cardiac output, the amplitude of the plethysmograph signal is directly proportional to the vascular distensibility (9). If the vascular compliance is low, for example during episodes of increased sympathetic tone, the pulse oximeter waveform amplitude is also low. With vasodilatation, the pulse oximeter waveform amplitude is increased. One should never confuse a large pulse amplitude with the presence of high arterial blood pressure nor vice versa. It is not unusual for the pulse oximeter waveform amplitude to decrease during significant increases in arterial blood pressure that are due to increase sympathetic tone.

T2-6
Table 2:
Factors Affecting Pulse Oximeter Waveform Amplitude

Once a baseline measurement has been established, the pulse oximeter amplitude can be followed as a gauge of sympathetic tone (10–12). An intriguing potential use of the plethysmograph may be as a indicator of MAC-BAR (13), the dose of anesthetic required to block adrenergic response in 50% of individuals who have a surgical skin incision. The degree of sympathetic responsiveness a patient retains during an anesthetic might have important clinical implications (Fig. 1). This may be particularly true in patients with a compromised coronary circulation, where dramatic shifts in the hemodynamic status should be avoided. This is an area in which further research efforts would be useful.

F1-6
Figure 1.:
Response of the pulse oximeter waveform (Pleth) to surgical stimulation. In this case, a patient undergoing general anesthesia experiences the first surgical incision of an operative procedure. The pulse oximeter waveform is noteworthy for the sudden reduction in amplitude. This is felt to be indicative of a sudden increase in sympathetic tone causing peripheral vasoconstriction. A concomitant increase in the arterial blood pressure (BP) supports this explanation.

RHYTHM ANALYSIS

As can be seen in Figure 2, the pulse oximeter waveform can be a useful tool for detecting and diagnosing cardiac arrhythmias (14). To be used to maximum benefit, the pulse oximeter waveform is used in conjunction with the electrocardiogram. This can greatly help in correctly interpreting artifacts due to patient movement or electrical cautery. As demonstrated in these figures, the pulse oximeter waveform morphology is related to the arterial blood pressure waveform (15). As expected after each premature ventricular beat, there is a compensatory pause, which gives more time for the ventricle to fill. The next normal heartbeat is, therefore, associated with an increase in stoke volume. This is reflected in an increase of arterial blood pressure. It is thought that the same mechanism accounts for an increase in the size of the pulse oximeter amplitude after a compensatory pause. A beat-to-beat change of the pulse oximeter amplitude is often the first clue that the patient has developed an irregular heart rhythm. Comparing the pulse oximeter waveform to the electrocardiogram is an excellent way to confirm these changes.

F2-6
Figure 2.:
The impact of ventricular tachycardia on the pulse oximeter waveform (Pleth), arterial pressure waveform (BP), and electrocardiogram (ECG). The sudden reduction in the amplitude of the pulse oximeter waveform, combined with the typical ECG pattern, should give important warning regarding the presence of a dangerous situation.

PULSE ANALYSIS

A number of unanticipated uses of the pulse oximeter have been developed by clinicians. Most of these uses depend on the ability of the pulse oximeter to detect arterial pulsation. These applications take advantage of the fact that the PPG is remarkably sensitive to pulsatile blood flow.

One clever use of the pulse oximeter has been the determination of systolic blood pressure. This is done by taking advantage of the pulse oximeter’s ability to detect a peripheral pulse. The pressure at which the pulse is detected corresponds closely to the systolic blood pressure (16–18). This technique is helpful in noisy environments, or with neonates in which the use of stethoscope would be difficult. The complex relationship between arterial blood pressure and the volumetric nature of the PPG has complicated the search for the noninvasive beat-to-beat measurements desired by clinicians (15,19). It is hoped that standardization of the equipment, and better understanding of the underlying physiology of the PPG, may allow for obtaining this elusive goal in the future.

A number of studies have been published using the pulse oximeter’s plethysmographic capability to detect tissue perfusion. The advantage the pulse oximeter offers is the ability to do noninvasive, continuous monitoring of peripheral blood flow with readily available technology. Using either transmission or reflective plethysmographic techniques, a number of tissues have been studied. The traditional pulse oximeter depends on transmission plethysmography, with the light taking a direct path through the tissue being studied (i.e., the fingertip or earlobe). Reflective plethysmography takes advantage of the back-scattering of light to the surface (i.e., forehead). Published studies using these techniques to determine tissue perfusion have been performed on small bowel (20), reimplanted fingers (21), and free flaps (22).

RESPIRATORY VARIABILITY ANALYSIS

With ventilation (spontaneous and positive pressure) there is fluctuation of both the baseline (D/C) and pulsatile (A/C) components of the plethysmographic waveform. The ability to detect the influence of the respiratory system on the cardiovascular system opens intriguing possibilities. At the minimum, it is believed that the respiratory rate can be reliably determined using the plethysmographic waveform (23–27).

The effect of positive pressure ventilation on the arterial pressure waveform has been well described (28). It is theorized that with each positive pressure breath venous return to the heart is impeded resulting in a temporary reduction in cardiac output. As a patient becomes volume depleted, with a resulting decrease in venous pressure, positive pressure ventilation has an exaggerated impact on the arterial blood pressure. A similar effect on the plethysmograph has been described (29,30). Figure 3 demonstrates this phenomenon. Monitoring the respiratory variability seen in the pulse oximeter waveform may be a useful method of detecting occult hemorrhage, with its resulting hypovolemia (31,32). There are ongoing research efforts designed to find the best site and method of analysis for quantifying the effects of ventilation on the plethysmographic waveform (33,34).

F3-6
Figure 3.:
The effect of blood loss on the pulse oximeter waveform (Pleth) and arterial pressure waveform (BP). The upper diagram shows the baseline waveforms of the patient under general anesthesia with positive pressure ventilation. The lower diagram is after a 1000 mL blood loss. The effect of positive pressure ventilation is apparent.

THE PATH FORWARD

The availability of increasingly powerful methods of digital signal processing are allowing for a renaissance in the field of PPG research. Calculations that once required mainframe computers are now performed almost instantaneously with digital signal processing chips. This has allowed for the detailed re-examination of the plethysmograph. Combined with improved understanding of the underlying physiology of the waveform it is easy to predict the emergence of multifunction pulse oximeters.

To uncover the true potential of this waveform, we need standardization and quantification of the plethysmograph as it is presented to the clinician. I believe that the clinician has a vital role to play in the discovery and verification of new uses of the waveform. As was pointed out (7) the clinician attempting to solve clinical questions by innovative means is often faced with highly processed information from their monitoring devices. In their zeal to simplify the clinician’s life, medical device manufacturers strive to present as “clean” a signal as possible, not wanting to distract the care provider with the “messy details.” The downside of this approach is the potential over simplification of complex physiology. It must be remembered that what is viewed as an artifact from one prospective (i.e., respiratory variation of the PPG while determining heart rate) becomes signal in another (i.e., using the same respiratory variation of the PPG to predict fluid responsiveness).

In conclusion, I believe we need: 1) better equipment generating waveforms that can be quantified in a standardized manner, 2) well-designed prospective studies demonstrating that we are measuring clinically relevant information, and 3) outcome studies showing that this information will help the clinician provide better care for their patients.

REFERENCES

1. Hertzman AB, Spielman C. Observations on the finger volume pulse recorded photoelectrically. Am J Physiol 1937;119:334–5
2. Foster AJ, Neuman C, Rovenstine E. Peripheral circulation during anesthesia, shock and hemorrage; the digital plethysmograph as a clinical guide. Anesthesiology 1945;6:246–57
3. Shelley KH, Shelley S. Pulse oximeter waveform: photoelectric plethysmography. In: Lake CL, Hines RL, Blitt CD, eds. Clinical monitoring: practical applications for anesthesia and critical care. Philadelphia, PA: WB Saunders, 2001:420–8
4. Trafford J, Lafferty K. What does photoplethysmography measure ? Med Biol Engl Comput 1984;22:479–80
5. Spigulis J. Optical noninvasive monitoring of skin blood pulsations. Appl Opt 2005;44:1850–7
6. Kim JM, Arakawa K, Benson KT, Fox DK. Pulse oximetry and circulatory kinetics associated with pulse volume amplitude measured by photoelectric plethysmography. Anesth Analg 1986;65:1333–9
7. Feldman JM. Can clinical monitors be used as scientific instruments? Anesth Analg 2006;103:1071–2
8. Awad A, Ghobashy MA, Ouda W, Stout RG, Silverman DG, Shelley KH. Different responses of ear and finger pulse oximeter wave form to cold pressor test. Anesth Analg 2001;92:1483–6
9. Dorlas JC, Nijboer JA. Photo-electric plethysmography as a monitoring device in anaesthesia. Application and interpretation. Br J Anaesth 1985;57:524–30
10. Ezri T, Steinmetz A, Geva D, Szmuk P. Skin vasomotor reflex as a measure of depth of anesthesia. Anesthesiology 1998;89:1281–2
11. Luginbuhl M, Reichlin F, Sigurdsson GH, Zbinden AM, Petersen-Felix S. Prediction of the haemodynamic response to tracheal intubation: comparison of laser-doppler skin vasomotor reflex and pulse wave reflex. Br J Anaesth 2002;89:389–97
12. Tanaka G, Sawada Y, Yamakoshi K. Beat-by-beat double-normalized pulse volume derived photoplethysmographically as a new quantitative index of finger vascular tone in humans. Eur J Appl Physiol 2000;81:148–54
13. Roizen MF, Horrigan RW, Frazer BM. Anesthetic doses blocking adrenergic (stress) and cardiovascular responses to incision—mac bar. Anesthesiology 1981;54:390–8
14. Blanc VF, Haig M, Troli M, Sauve B. Computerized photo-plethysmography of the finger. Can J Anaesth 1993;40:271–8
15. Awad A, Ghobashy MA, Stout RG, Silverman DG, Shelley KH. How does the plethysmogram derived from the pulse oximeter relate to arterial blood pressure in coronary artery bypass graft patients? Anesth Analg 2001;93:1466–71
16. Talke P, Nichols RJ, Traber D. Does measurement of systolic blood pressure with a pulse oximeter correlate with conventional methods? J Clin Monit 1990;6:5–9
17. Wallace C, Baker J, Alpert C. Comparison of blood pressure measurement by doppler and by pulse oximetry techniques. Anesth Analg 1987;66:1018–19
18. Jonsson B, Laurent C, Skau T, Lindberg LG. A new probe for ankle systolic pressure measurement using photoplethysmography (ppg). Ann Biomed Engl 2005;33:232–9
19. Shelley KH, Murray WB, Chang D. Arterial-pulse oximetry loops: a new method of monitoring vascular tone. J Clin Monit 1997;13:223–8
20. Ferrara J, Dyess D, Lasecki M. Surface oximetry: a new method to evaluate intestinal perfusion. Am Surg 1988;54:10–14
21. Graham B, Paulus D, Caffee H. Pulse oximetry for vascular monitoring in upper extremity replantation surgery. J Hand Surg [Am] 1986;11:687–92
22. Stack BC Jr, Futran ND, Shohet MJ, Scharf JE. Spectral analysis of photoplethysmograms from radial forearm free flaps. Laryngoscope 1998;108:1329–33
23. Leonard PA, Douglas JG, Grubb NR, Clifton D, Addison PS, Watson JN. A fully automated algorithm for the determination of respiratory rate from the photoplethysmogram. J Clin Monit Comput 2006;20:33–6
24. Nilsson L, Johansson A, Kalman S. Respiration can be monitored by photoplethysmography with high sensitivity and specificity regardless of anaesthesia and ventilatory mode. Acta Anaesthesiol Scand 2005;49:1157–62
25. Johansson A. Neural network for photoplethysmographic respiratory rate monitoring. Med Biol Engl Comput 2003;41:242–8
26. De Meersman RE, Zion AS, Teitelbaum S, Weir JP, Lieberman J, Downey J. Deriving respiration from pulse wave: a new signal-processing technique. Am J Physiol 1996;270:H1672–5
27. Foo JY, Wilson SJ. Estimation of breathing interval from the photoplethysmographic signals in children. Physiol Meas 2005;26:1049–58
28. Michard F. Changes in arterial pressure during mechanical ventilation. Anesthesiology 2005;103:419–28
29. Partridge BL. Use of pulse oximetry as a noninvasive indicator of intravascular volume status. J Clin Monit 1987;3:263–8
30. Natalini G, Rosano A, Franceschetti ME, Facchetti P, Bernardini A. Variations in arterial blood pressure and photoplethysmography during mechanical ventilation. Anesth Analg 2006;103:1182–8
31. Monnet X, Lamia B, Teboul J. Pulse oximeter as a sensor of fluid responsiveness: do we have our finger on the best solution? Crit Care 2005;9:429–30
32. Cannesson M, Besnard C, Durand PG, Bohé J, Jacques D. Relation between respiratory variations in pulse oximetry plethysmographic waveform amplitude and arterial pulse pressure in ventilated patients. Crit Care 2005;9:562–8
33. Shelley KH, Jablonka DH, Awad AA, Stout RG, Rezkanna H, Silverman DG. What is the best site for measuring the effect of ventilation on the pulse oximeter waveform? Anesth Analg 2006;103:372–7
34. Shelley KH, Awad AA, Stout RG, Silverman DG. The use of joint time frequency analysis to quantify the effect of ventilation on the pulse oximeter waveform. J Clin Monit Comput 2006;20:81–7
International Anesthesia Research Society