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TECHNOLOGY, COMPUTING, AND SIMULATION

What Is the Best Site for Measuring the Effect of Ventilation on the Pulse Oximeter Waveform?

From the *Department of Anesthesiology, Yale University, New Haven, Connecticut; and Department of Anesthesia, Benha Faculty of Medicine, Zagazig University, Egypt.

Address correspondence and reprint requests to Kirk Shelley MD, PhD, Department of Anesthesiology, 333 Cedar Street, P.O. Box 208051, New Haven, CT 06520-8051. Address e-mail to kirk.shelley{at}yale.edu.

	Abstract

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The cardiac pulse is the predominant feature of the pulse oximeter (plethysmographic) waveform. Less obvious is the effect of ventilation on the waveform. There have been efforts to measure the effect of ventilation on the waveform to determine respiratory rate, tidal volume, and blood volume. We measured the relative strength of the effect of ventilation on the reflective plethysmographic waveform at three different sites: the finger, ear, and forehead. The plethysmographic waveforms from 18 patients undergoing positive pressure ventilation during surgery and 10 patients spontaneously breathing during renal dialysis were collected. The respiratory signal was isolated from the waveform using spectral analysis. It was found that the respiratory signal in the pulse oximeter waveform was more than 10 times stronger in the region of the head when compared with the finger. This was true with both controlled positive pressure ventilation and spontaneous breathing. A significant correlation was demonstrated between the estimated blood loss from surgical procedures and the impact of ventilation on ear plethysmographic data (r_s = 0.624, P = 0.006).

	Introduction

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The pulse oximeter is the most commonly used patient monitor both in and out of the operating room. Its popularity is undoubtedly attributable to its ability to monitor arterial oxygen saturation as well as heart rate noninvasively. In addition, it is remarkably easy to use and comfortable for the patient. It would only seem logical that we should strive to maximize the benefit we derive from using this technology.

In the process of determining oxygen saturation, the pulse oximeter functions as a photoelectric plethysmograph. In this role, it noninvasively measures changes in the light absorption of the vascular bed (e.g., finger, ear, or forehead). The resulting waveform contains a complex mixture of influences, from the arterial, venous, autonomic, and respiratory systems, on the peripheral circulation. There is interest in quantifying the effects of ventilation on the pulse oximeter waveform to determine the respiratory rate (1–3). It has also been suggested that the degree of ventilation-induced fluctuation is related to the respiratory tidal volume (4) and blood volume (5,6). Figure 1 illustrates an example of the effect of ventilation on the plethysmographic waveform.

View larger version (31K): [in this window] [in a new window]   Figure 1. Tracings of the plethysmograph of the ear (A) and peak airway pressure (B) from a patient undergoing an abdominal procedure. It demonstrates the effect of positive pressure ventilation after a 25-s period of apnea.

This study was designed to determine which of the commonly used pulse oximeter sites (finger, ear, and forehead) allow for the best extraction of the respiratory signal during both positive pressure and spontaneous ventilation. To study positive pressure ventilation, patients undergoing open lower abdominal surgical procedures under general anesthesia were monitored. To study spontaneous ventilation, patients with chronic renal failure undergoing outpatient hemodialysis were monitored.

	METHODS

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Each portion of the study was reviewed and approved by our institutional Human Investigation Committee. Verbal consent was obtained for the placement of the additional monitors during the given clinical procedure. Written consent was obtained to review any relevant medical records.

Patients undergoing elective lower abdominal procedures (gynecological and urological) using general anesthesia were studied. As a part of the anesthetic, the trachea was intubated after induction with propofol (2.0–3.0 mg/kg IV) and vecuronium (0.1 mg/kg IV). Sevoflurane 1%–3% in combination with nitrous oxide 60% in oxygen was administered for maintenance of anesthesia. The ventilator was set to a tidal volume of 10 mL/kg at an inspiratory:expiratory ratio of 1:2, with the ventilation rate adjusted at the discretion of the anesthesia care team. An upper body Bair Hugger warming unit (Model 505; Augustine Medical, Inc., Eden Prairie, MN) was applied at the start of the procedure and used throughout the case. Additional medications (fentanyl, morphine, and ondansetron) were given as needed. At the end of surgery, inhaled anesthetics were discontinued and residual neuromuscular block was reversed with neostigmine (0.05 mg/kg IV) and glycopyrrolate (0.01 mg/kg IV). During the surgery, estimated blood loss (EBL) was closely followed by recording suction canister measurements and weighing sponges (7), as well as recording replacement fluid given to the patient.

In addition to the standard monitors for anesthesia, three identical reflective infrared plethysmographic probes (MLT1020; ADInstruments, CO Springs, CO) were placed on each patient at the finger (with a clip), forehead (covered by clear dressing), and ear (clip). The plethysmographic, CO₂ and airway pressure (PAW) waveforms were recorded at 100 Hz with Powerlab/16SP with a Quad Bridge Amp (ML795 & ML112; ADInstruments) with a 0.01 Hz high-pass filter. The continuous waveforms from the entire case (from intubation to extubation) for each of these waveforms underwent spectral analysis using Chart software (v 5.02; ADInstruments with program setting of Spectrum, 16 k, Hamming, Amplitude Density, 160-s window, 50% overlap). The respiratory frequency was determined by identifying the peak frequency of the PAW power spectrum. The influence of respiration on each of the plethysmographic signals was determined by measuring the amplitude of the power spectrum at the respiratory frequency. Once quantified the relative strength of the respiratory influence on the plethysmographic from each site was compared with the two another sites and reported as a ratio.^*

The effects of spontaneous ventilation were assessed in patients undergoing hemodialysis. In addition to the routine noninvasive arterial blood pressure monitor, a respiratory belt transducer (MLT1132; ADInstruments) was placed around the chest and reflective infrared plethysmographic probes were placed on the finger, forehead, and ear and were recorded and analyzed as above. The power spectrum for the respiratory belt waveform provided the respiratory frequency.

The results are reported as mean ± sd and median (percentiles). Data were assessed for normality by the Kolmogorov-Smirnov test. Data not normally distributed were analyzed by nonparametric tests. Differences between groups were assessed by analysis of variance and paired Student’s t-test, or by Friedman test and Wilcoxon signed ranks test as appropriate. Linear correlations were tested using Pearson’s product moment or Spearman rank method. For all comparisons, a P value <0.05 was considered statistically significant; all tests were two-tailed. All statistical analyses were performed using SPSS (version 13.0; SPSS, Chicago, IL).

	RESULTS

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A total of 53.3 h (18 cases) of waveforms were collected and analyzed (Table 1). Figure 2 illustrates the plethysmographic tracings and peak PAW for a sample period of time from a representative subject. Figure 3 provides the power spectra generated from the entire case. It demonstrates the presence of a frequency peak in the plethysmographs at the respiratory frequency (at approximately 0.16 Hz). The amplitude of that peak is related to the relative strength of the respiratory influence on the plethysmographic waveform with the impact on ear > forehead > finger. Overall, the amplitude of the peak at the respiratory frequency for both the ear and forehead plethysmograph was significantly greater than on the finger waveform using a Friedman test (Fig. 4) (P < 0.001). The average ear/finger strength ratio was 17.88 ± 9.78 (P < 0.001 for ear versus finger); the average forehead/finger ratio, 11.09 ± 5.61 (P < 0.001 for forehead versus finger), and the average ear/forehead strength ratio, 1.71 ± 0.89 (P = 0.029 for ear versus forehead).

View larger version (65K): [in this window] [in a new window]   Figure 2. Tracings of the plethysmographs of the finger (A), ear (B), forehead (C), and peak airway pressure (D) from a patient undergoing radical prostatectomy (60-s segment).

View larger version (10K): [in this window] [in a new window]   Figure 3. The frequency spectrum of the plethysmographic waveforms (finger: A, ear: B, forehead: C) and airway pressure: D) for the entire case depicted in Figure 2. (x-axis: Hz; y-axis: V/Hz2) * 10–3) The arrows indicate the ventilation frequency. The higher harmonics are the result of the fact the ventilation curve is not a sine wave but closer to a saw tooth pattern (Fig. 2 - D).

View larger version (9K): [in this window] [in a new window]   Figure 4. A comparison of the amplitude density (volt/Hz2)) from the plethysmographic waveform at the ventilation frequency from 18 patients undergoing surgical procedures. The data were collected at three different sites (finger, ear, and forehead). The circles depict the mean and the error bars indicate the 95% confidence intervals.

The EBL and amplitude of frequency peak of the ear plethysmographic at the respiratory frequency were not normally distributed among subjects. A significant correlation was shown between the EBL and power at the respiratory frequency of the ear plethysmographic signal (r_s = 0.624, P = 0.006). In contrast, there was no correlation between the EBL and forehead respiratory power (r = 0.245; P = 0.328) or the EBL and finger respiratory power (r = –0.008; P = 0.975).

A total of 36.2 h (10 cases) of waveforms was collected and analyzed from spontaneously ventilating dialysis patients (Table 2). The respiration-induced variation of the ear and forehead plethysmographic signals was found to be significantly greater than the finger signal by one-way between-groups analysis of variance using Tukey’s HSD comparison F = 13.7, P < 0.001 (Fig. 5). The average ear/finger strength ratio was 12.53 ± 7.99 (P < 0.001); average forehead/finger, 10.75 ± 5.3 (P < 0.001); and average ear/forehead strength ratio, 1.10 ± 0.60 (P = 0.90).

View larger version (10K): [in this window] [in a new window]   Figure 5. A comparison of the amplitude density (volt/Hz2) from the plethysmographic waveform at the respiratory frequency from 10 patients undergoing renal dialysis. The data were collected at three different sites (finger, ear, and forehead). The circles depict the mean and the error bars indicate the 95% confidence intervals.

	DISCUSSION

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On average, the effect of ventilation was expressed 18 times more strongly in the ear plethysmograph when compared with the finger plethysmograph with positive pressure ventilation and 12 times more strongly with spontaneous ventilation. Two factors likely contributed to this finding. First, the shorter distance between the head and chest (compared with the distance between the finger and chest) means that there is less distance for attenuation of the ventilatory signal within the vasculature. This attenuation of the signal could come from the dampening that occurs because of the natural elasticity of the vessels or from the influence of valves within the venous system. Second, the vasculature of the head is relatively insensitive to local sympathetically mediated vasoconstriction that may mask respiration-generated oscillations (8).

The difference between the ear and forehead signals, as well as the higher correlation of oscillatory power of the ear signal with EBL, may be attributable to the method of probe application as opposed to differences in the underlying vasculature. The forehead signal appeared to have a greater degree of background noise and the presence of an intermittent venous pulse (9). It is possible that the method of attachment (adhesive dressing without a headband) of the forehead plethysmographic probe may have contributed to both of these problems.

Admittedly, the present findings cannot be seamlessly translated into routine clinical care with current clinical monitors and data processing programs. First, the respiration-induced changes reported herein may not be as readily identifiable with currently available commercial pulse oximeters. Those devices typically provide a highly processed signal with auto-centering algorithms (with a high-pass filter) to eliminate oscillatory "noise" and thereby emphasize the pulsatile component. This filtering process may obscure or eliminate the effects of ventilation on the waveform. Second, this investigation used the raw data waveform from reflective infrared plethysmographs, consistent with other investigations of the impact of ventilation on plethysmographic signal (1,10,11). Alternatively, a commercial pulse oximeter uses a mixture of reflective and transmission probes. Although the waveforms from reflective and transmission probes are similar, they are not identical (12–14).

The use of spectral analysis to analyze the waveform from the entire case in the present study has the advantage being relatively immune to isolated artifacts such as motion. It, therefore, was preferable for the overall assessment of the impact of ventilation on the plethysmographic signals. However, the integrated assessment of such long study intervals is not preferred for analysis of short-term events such as acute blood loss or ventilator changes. Despite this potential weakness, a significant correlation was found between the ear signal and total EBL. The ability of the plethysmograph, most notably at the ear, to identify and perhaps quantify blood loss may have significant impact on the potential utility of such monitoring for the noninvasive assessment of intravascular volume status. This can be more readily accomplished by monitoring selected segments of data.

The increasing sophistication of digital signal processing techniques is allowing for a re-examination of the waveforms (i.e., CO₂, airway pressure, pulse waveform) routinely used by clinicians. The choice of probe site can have a significant impact on the strength of the respiratory signal detected within the plethysmographic waveform. The region of head and face has a respiratory signal that is more than 10 times stronger than that found in the finger.

There is an increasing interest in the possible use of pulse pressure variation from the pulse oximeter waveform (15,16) to guide fluid therapy. So far there has been limited success. To quote Michard (17):

"...The pulse oximeter plethysmographic waveforms have been compared to the arterial pressure variation, but despite significant relations between the two phenomena, discrepancies have been reported, supporting the notion that pulse oximetry cannot be recommended to accurately assess the respiratory variation in arterial pressure in mechanically ventilated patients..."

Up to this point, plethysmographic studies have all involved the finger plethysmograph. We hope that this study will encourage researchers to examine new sites.

	ACKNOWLEDGMENTS

 
We thank Martin D. Slade of Yale University School of Medicine, Departments of Internal Medicine and Epidemiology & Public Health for his input regarding the statistical analysis.

	Footnotes

 
* It should be noted that the use of this set-up in a subsequent study resulted in a burn on the forehead of a research subject and that at this time the use of the ADInstruments IR plethysmographic transducer cannot be recommended. This occurred with the probe attached with a Tegaderm without external pressure applied. The mechanism of injury appears to have been attributable to a low voltage current leak from the probe as described in Leeming MN, Jacobs RG, Howland WS. Low voltage, direct current plethysmograph burns. Med Res Eng 1971;10:19–21.

Accepted for publication March 28, 2006.

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