Signal quality assessment of peripheral venous pressure

Develop a signal quality index (SQI) for the widely available peripheral venous pressure waveform (PVP). We focus on the quality of the cardiac component in PVP. We model PVP by the adaptive non-harmonic model. When the cardiac component in PVP is stronger, the PVP is defined to have a higher quality. This signal quality is quantified by applying the synchrosqueezing transform to decompose the cardiac component out of PVP, and the SQI is defined as a value between 0 and 1. A database collected during the lower body negative pressure experiment is utilized to validate the developed SQI. All signals are labeled into categories of low and high qualities by experts. A support vector machine (SVM) learning model is trained for practical purpose. The developed signal quality index coincide with human experts' labels with the area under the curve 0.95. In a leave-one-subject-out cross validation (LOSOCV), the SQI achieves accuracy 0.89 and F1 0.88, which is consistently higher than other commonly used signal qualities, including entropy, power and mean venous pressure. The trained SVM model trained with SQI, entropy, power and mean venous pressure could achieve an accuracy 0.92 and F1 0.91 under LOSOCV. An exterior validation of SQI achieves accuracy 0.87 and F1 0.92; an exterior validation of the SVM model achieves accuracy 0.95 and F1 0.96. The developed SQI has a convincing potential to help identify high quality PVP segments for further hemodynamic study. This is the first work aiming to quantify the signal quality of the widely applied PVP waveform.

Improving pulse oximetry accuracy in dark-skinned patients: technical aspects and current regulations

Recent concerns regarding the clinical accuracy of pulse oximetry in dark-skinned patients, specifically in detecting occult hypoxaemia, have motivated research on this topic and recently reported in this journal. We provide an overview of the technical aspects of the issue, the sources of inaccuracy, and the current regulations and limitations. These insights offer perspectives on how pulse oximetry can be improved to address these potential limitations.

Using the ear photoplethysmographic waveform as an early indicator of central hypovolemia in healthy volunteers utilizing LBNP induced hypovolemia model

OBJECTIVE

To study the photoplethysmographic (PPG) waveforms of different locations (ear and finger) during lower body negative pressure (LBNP) induced hypovolemia. Then, to determine whether the PPG waveform can be used to detect hypovolemia during the early stage of LBNP.
Approach: 36 healthy volunteers were recruited for progressive LBNP induced hypovolemia, with an endpoint of -60 mmHg or development of hypoperfusion symptoms, whichever comes first. Subjects tolerating the entire protocol without symptoms were designated as high tolerance (HT), while symptomatic subjects were designated as low tolerance (LT). Subjects were monitored with an electrocardiogram, continuous noninvasive blood pressure monitor (CNAP), and two pulse oximetry probes, one on the ear (Xhale) and one the finger (Nellcor). Stroke volume (SV) was measured non-invasively utilizing NICOM (Cheetah Medical). The waveform morphology was analyzed using novel PPG waveforms indices, including phase hemodynamic index (PHI) and amplitude hemodyamaic index (AHI) and were evaluated from the ear PPG and finger PPG at different LBNP stages.
Main Results: The PHI, particularly the phase relationship between the second harmonic and the fundamental component of the ear PPG denoted as 〖∇φ〗_2, during the early stage of LBNP (-15 mmHg) in the HT and LT groups is statistically significantly different (p value=0.0033) with the area under curve 0.81 (CI: 0.616-0.926). The other indices are not significantly different. The 5 fold cross validation shows that 〖∇φ〗_2 during the early stage of LBNP (-15 mmHg) as the single index could predict the tolerance of the subject with the sensitivity, specificity, accuracy and F1 as 0.771±0.192, 0.71±0.107, 0.7±0.1 and 0.771±0.192 respectively.
Significance: The ear's PPG PHI which compares the phases of the fundamental and second harmonic has the potential to be used as an early predictor of central hypovolemia. &#xD.

Inaccuracy of pulse oximetry with dark skin pigmentation: clinical implications and need for improvement

Recent reports highlight potential inaccuracies of pulse oximetry in patients with various degrees of skin pigmentation. We summarise the literature, provide an overview of potential clinical implications, and provide insights into how pulse oximetry could be improved to mitigate against such potential shortcomings.

Photoplethysmography

The photoplethysmographic (PPG) waveform, also known as the pulse oximeter waveform, is one of the most commonly displayed clinical waveforms. First described in the 1930s, the technology behind the waveform is simple. The waveform, as displayed on the modern pulse oximeter, is an amplified and highly filtered measurement of light absorption by the local tissue over time. It is optimized by medical device manufacturers to accentuate its pulsatile components. Physiologically, it is the result of a complex, and not well understood, interaction between the cardiovascular, respiratory, and autonomic systems. All modern pulse oximeters extract and display the heart rate and oxygen saturation derived from the PPG measurements at multiple wavelengths. "As is," the PPG is an excellent monitor for cardiac arrhythmia, particularly when used in conjunction with the electrocardiogram (ECG). With slight modifications in the display of the PPG (either to a strip chart recorder or slowed down on the monitor screen), the PPG can be used to measure the ventilator-induced modulations which have been associated with hypovolemia. Research efforts are under way to analyze the PPG using improved digital signal processing methods to develop new physiologic parameters. It is hoped that when these new physiologic parameters are combined with a more modern understanding of cardiovascular physiology (functional hemodynamics) the potential utility of the PPG will be expanded. The clinical researcher's objective is the use of the PPG to guide early goal-directed therapeutic interventions (fluid, vasopressors, and inotropes), in effect to extract from the simple PPG the information and therapeutic guidance that was previously only obtainable from an arterial pressure line and the pulmonary artery catheter.

Impact of lower body negative pressure induced hypovolemia on peripheral venous pressure waveform parameters in healthy volunteers

Lower body negative pressure (LBNP) creates a reversible hypovolemia by sequestrating blood volume in the lower extremities. This study sought to examine the impact of central hypovolemia on peripheral venous pressure (PVP) waveforms in spontaneously breathing subjects. With IRB approval, 11 healthy subjects underwent progressive LBNP (baseline, -30, -75, and -90 mmHg or until the subject became symptomatic). Each was monitored for heart rate (HR), finger arterial blood pressure (BP), a chest respiratory band and PVP waveforms which are generated from a transduced upper extremity intravenous site. The first subject was excluded from PVP analysis because of technical errors in collecting the venous pressure waveform. PVP waveforms were analyzed to determine venous pulse pressure, mean venous pressure, pulse width, maximum and minimum slope (time domain analysis) together with cardiac and respiratory modulations (frequency domain analysis). No changes of significance were found in the arterial BP values at -30 mmHg LBNP, while there were significant reductions in the PVP waveforms time domain parameters (except for 50% width of the respiration induced modulations) together with modulation of the PVP waveform at the cardiac frequency but not at the respiratory frequency. As the LBNP progressed, arterial systolic BP, mean BP and pulse pressure, PVP parameters and PVP cardiac modulation decreased significantly, while diastolic BP and HR increased significantly. Changes in hemodynamic and PVP waveform parameters reached a maximum during the symptomatic phase. During the recovery phase, there was a significant reduction in HR together with a significant increase in HR variability, mean PVP and PVP cardiac modulation. Thus, in response to mild hypovolemia induced by LBNP, changes in cardiac modulation and other PVP waveform parameters identified hypovolemia before detectable hemodynamic changes.

Modulation of finger photoplethysmographic traces during forced respiration: venous blood in motion?

Photoplethysmographic (PPG) signals were recorded from the fingers of 10 healthy volunteers during forced respiratory inspiration. The aim of this pilot study was to assess the effect of negative airway pressure on the blood volumes within the tissue bed of the finger, and the resultant modulation of PPG signals. The acquired signals were analysed and oxygen saturations estimated from the frequency spectra in the cardiac and respiratory frequency ranges. Assuming that respiratory modulation affects blood volumes in veins to a greater extent than in arteries, the local venous oxygen saturation was estimated. Estimated venous oxygen saturation was found to be 3.1% (±4.2%) lower than the estimated arterial saturation.

Statistical approach for the detection of motion/noise artifacts in Photoplethysmogram

Motion and noise artifacts (MNA) have been a serious obstacle in realizing the potential of Photoplethysmogram (PPG) signals for real-time monitoring of vital signs. We present a statistical approach based on the computation of kurtosis and Shannon Entropy (SE) for the accurate detection of MNA in PPG data. The MNA detection algorithm was verified on multi-site PPG data collected from both laboratory and clinical settings. The accuracy of the fusion of kurtosis and SE metrics for the artifact detection was 99.0%, 94.8% and 93.3% in simultaneously recorded ear, finger and forehead PPGs obtained in a clinical setting, respectively. For laboratory PPG data recorded from a finger with contrived artifacts, the accuracy was 88.8%. It was identified that the measurements from the forehead PPG sensor contained the most artifacts followed by finger and ear. The proposed MNA algorithm can be implemented in real-time as the computation time was 0.14 seconds using Matlab®.

Early detection of spontaneous blood loss using amplitude modulation of Photoplethysmogram

The present study was designed to investigate can the amplitude modulation (AM) of Photoplethysmogram (PPG) be used as an indicator of blood loss and if so what is the best PPG probe site. PPG from ear, finger and forehead probe sites, standard ECG, and Finapres blood pressure waveforms were continuously recorded from 8 healthy volunteers during baseline, blood withdrawal of 900 ml followed by the blood reinfusion. The instantaneous amplitude modulations present in heart rate (AM(HR)) and breathing rate (AM(BR)) band frequencies of PPG were extracted from high-resolution time-frequency spectrum. HR and pulse pressure showed no significant changes during the protocol. The AM(HR) significantly (P<0.05) decreased at 100 ml through 900 ml blood loss from ear and finger probe sites. The mean percent decrease in AM(HR) at 900 ml blood loss compared to baseline value was 45.2%, 42.0%, and 42.3% for ear, finger and forehead PPG signals, respectively. In addition, significant increases in AM(BR) were found due to blood loss in ear and finger PPG signals. Even without baseline AM(HR) values, 900 ml blood loss detection was shown possible with specificity and sensitivity both 87.5% from ear PPG signals. The present technique has great potential to serve as a valuable tool in the intraoperative and trauma settings to detect hemorrhage.

Respiratory physiology and the impact of different modes of ventilation on the photoplethysmographic waveform

The photoplethysmographic waveform sits at the core of the most used, and arguably the most important, clinical monitor, the pulse oximeter. Interestingly, the pulse oximeter was discovered while examining an artifact during the development of a noninvasive cardiac output monitor. This article will explore the response of the pulse oximeter waveform to various modes of ventilation. Modern digital signal processing is allowing for a re-examination of this ubiquitous signal. The effect of ventilation on the photoplethysmographic waveform has long been thought of as a source of artifact. The primary goal of this article is to improve the understanding of the underlying physiology responsible for the observed phenomena, thereby encouraging the utilization of this understanding to develop new methods of patient monitoring. The reader will be presented with a review of respiratory physiology followed by numerous examples of the impact of ventilation on the photoplethysmographic waveform.

Autonomic control mechanism of maximal lower body negative pressure application

Autonomic control mechanisms during progressive hemorrhage in humans remain complex and unclear. The present study investigates the autonomic reflexes during maximal application of lower body negative pressure (LBNP) that mimics severe hemorrhage in conscious human subjects (n=10) using analyses of heart rate variability (HRV) and systolic blood pressure variability (BPV) and baroreflex sensitivity. Spectral analysis of HRV included linear power spectral density (PSD), and nonlinear principal dynamic modes (PDM) methods. The maximal LBNP application decreased (P&#60;0.01) the systolic and pulse pressures (PP), root mean square successive differences, normalized high frequency (HF) power of HRV, and transfer function gains at low frequency (LF) and HF bands. Meanwhile, increases (P&#60;0.05) in heart rate, diastolic blood pressure (DBP), LFHRV, LF/HFHRV, and sympathetic activity of HRV using PDM were observed during maximal LBNP tolerance. After the termination of LBNP, no significant changes (P>0.05) were found in all the parameters except DBP and PP between recovery and baseline conditions. Rapid application of maximal LBNP that simulated severe hemorrhage was found to be associated with unloading of baroreflex mediated increased sympathetic reflex.

A novel approach using time-frequency analysis of pulse-oximeter data to detect progressive hypovolemia in spontaneously breathing healthy subjects

Accurate and early detection of blood volume loss would greatly improve intraoperative and trauma care. This study has attempted to determine early diagnostic and quantitative markers for blood volume loss by analyzing photoplethysmogram (PPG) data from ear, finger and forehead sites with our high-resolution time-frequency spectral (TFS) technique in spontaneously breathing healthy subjects (n = 11) subjected to lower body negative pressure (LBNP). The instantaneous amplitude modulations present in heart rate (AM HR) and breathing rate (AMBR) band frequencies of PPG signals were calculated from the high-resolution TFS. Results suggested that the changes (P < 0.05) in AMBR and especially in AMHR values can be used to detect the blood volume loss at an early stage of 20% LBNP tolerance when compared to the baseline values. The mean percent decrease in AMHR values at 100% LBNP tolerance was 78.3%, 72.5%, and 33.9% for ear, finger, and forehead PPG signals, respectively. The mean percent increase in AMBR values at 100% LBNP tolerance was 99.4% and 19.6% for ear and finger sites, respectively; AMBR values were not attainable for forehead PPG signal. Even without baseline AMHR values, our results suggest that hypovolemia detection is possible with specificity and sensitivity greater than 90% for the ear and forehead locations when LBNP tolerance is 100%. Therefore, the TFS analysis of noninvasive PPG waveforms is promising for early diagnosis and quantification of hypovolemia at levels not identified by vital signs in spontaneously breathing subjects.

Measuring venous oxygenation using the photoplethysmograph waveform

OBJECTIVE

We investigate the hypothesis that the photoplethysmograph (PPG) waveform can be analyzed to infer regional venous oxygen saturation.

METHODS

Fundamental to the successful isolation of the venous saturation is the identification of PPG characteristics that are unique to the peripheral venous system. Two such characteristics have been identified. First, the peripheral venous waveform tends to reflect atrial contraction. Second, ventilation tends to move venous blood preferentially due to the low pressure and high compliance of the venous system. Red (660 nm) and IR (940 nm) PPG waveforms were collected from 10 cardiac surgery patients using an esophageal PPG probe. These waveforms were analyzed using algorithms written in Mathematica. Four time-domain saturation algorithms (ArtSat, VenSat, ArtInstSat, VenInstSat) and four frequency-domain saturation algorithms (RespDC, RespAC, Cardiac, and Harmonic) were applied to the data set.

RESULTS

Three of the algorithms for calculating venous saturation (VenSat, VenInstSat, and RespDC) demonstrate significant difference from ArtSat (the conventional time-domain algorithm for measuring arterial saturation) using the Wilcoxon signed-rank test with Bonferroni correction (p < 0.0071).

CONCLUSIONS

This work introduces new algorithms for PPG analysis. Three algorithms (VenSat, VenInstSat, and RespDC) succeed in detecting lower saturation blood. The next step is to confirm the accuracy of the measurement by comparing them to a gold standard (i.e., venous blood gas).

Patients and jargon: are we speaking the same language?

STUDY OBJECTIVE

To assess the ability of surgical patients to understand words commonly used during the anesthetic preoperative visit.

DESIGN

Questionnaire study.

SETTING

Preanesthetic holding area of a university hospital.

PATIENTS

96 perioperative ASA physical status I, II, III, and IV outpatients and patients to be admitted.

INTERVENTIONS

Patients were asked to complete a questionnaire that asked each to define 10 terms commonly used during the preoperative interview. Patients also answered three demographic questions as part of the survey.

MEASUREMENTS

Understanding of 10 commonly used terms, first language, age, and highest education level were all recorded.

MAIN RESULTS

Of the 10 terms, 4 had a greater than 80% correct response rate: EKG, i.v., general anesthesia, and local or regional anesthesia, with correct response rates of 92.7%, 91.7%, 81.3%, and 81.3%, respectively. The terms with the poorest understanding were NPO (31.3%), MI (32.3%), and pulse ox (39.6%). The rest of the terms, with their correct response rates, were as follows: GERD (67.7%), hypertension (70.8%), and intubate (60.4%). Whereas higher education was associated with correct answer score, age was not.

CONCLUSIONS

Most patients understand the words EKG and i.v.. Further clarification might be needed when discussing general and regional anesthesia, and other words should be avoided or else explained.

Impact of Withdrawal of 450 ml of Blood on Respiration-Induced Oscillations of the Ear Plethysmographic Waveform

OBJECTIVE

It has been widely appreciated that ventilation-induced variations in systolic blood pressure during mechanical ventilation correlate with changes in intravascular volume. The present study assessed whether alterations in volume status likewise can be detected with noninvasive monitoring (ear plethysmograph) in non-intubated subjects (awake volunteers).

METHODS

Eight healthy adults were monitored with EKG, noninvasive blood pressure, an unfiltered ear plethysmograph, and a respiratory force transduction belt before (PRE) and after (POST) withdrawal of 450 ml of blood from an antecubital vein. Spectral-domain analysis was used to determine the peak ventilatory frequency and the power of the associated variation in the ear plethysmographic tracing; Interphase differences in the respiration-induced plethysmographic variations were assessed by Wilcoxon signed rank test. In addition, the changes in the ear plethysmographic tracing were compared to changes in heart rate and blood pressure.

RESULTS

There was a significant increase in respiratory-associated oscillations at the respiratory frequency between the PRE and POST phases (p = 0.012). These changes were detected despite lack of changes in heart rate or blood pressure.

CONCLUSIONS

Respiration-induced changes of the ear plethysmographic waveform during spontaneous ventilation increase significantly as a consequence of withdrawal of approximately one unit of blood in healthy volunteers.

Optimizing the medical management of diabetic patients undergoing surgery

Patients with diabetes are prone to metabolic derangements because of their lack of effective insulin. Comorbid conditions, such as coronary artery disease, nephropathy, and autonomic neuropathy warrant preoperative assessment to ensure safety in the perioperative period. Preoperative evaluation must include assessment of chronic complications of diabetes. A thorough history and physical should guide preoperative testing which should be aimed at detecting correctable abnormalities and assessing the extent of end-organ disease. Surgery poses special challenges to patients with diabetes because the stress response, interruption of food intake, altered consciousness, and circulatory alterations all lead to unpredictable glucose and electrolyte levels. The management of insulin perioperatively depends on the preparation normally taken by the patient, and the glucose level on the morning of surgery. The goal is to avoid hypoglycemia and extreme hyperglycemia. Oral hypoglycemic agents should be held on the morning of surgery. Metformin should be discontinued 48 hours prior to and subsequent to surgery in order to reduce the risk of lactic acidosis. The avoidance of hypoglycemia and excessive hyperglycemia intraoperatively is best achieved with frequent monitoring of blood glucose and treating abnormalities according to patients' preoperative regimen and current condition. Maintaining blood glucose levels below 110 mg/dL reduces morbidity and mortality in critically ill patients. Measure blood glucose immediately following surgery because progression of the stress response postoperatively, in addition to possible nausea and vomiting, can complicate the patient's management. Precautions should be taken to prevent damage to peripheral nerves while diabetics are on the operating table because their nerves and limbs are already vulnerable to pressure and stretch injuries secondary to neurologic and vascular disease. With thorough and careful management, metabolic control in the perioperative period is a goal that is attainable for most patients.

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

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).

The Use of Joint Time Frequency Analysis to Quantify the Effect of Ventilation on the Pulse Oximeter Waveform

OBJECTIVE

In the process of determining oxygen saturation, the pulse oximeter functions as a photoelectric plethysmograph. By analyzing how the frequency spectrum of the pulse oximeter waveform changes over time, new clinically relevant features can be extracted.

METHODS

Thirty patients undergoing general anesthesia for abdominal surgery had their pulse oximeter, airway pressure and CO(2) waveforms collected (50 Hz). The pulse oximeter waveform was analyzed with a short-time Fourier transform using a moving 4096 point Hann window of 82 seconds duration. The frequency signal created by positive pressure ventilation was extracted using a peak detection algorithm in the frequency range of ventilation (0.08-0.4 Hz = 5-24 breaths/minute). The respiratory rate derived in this manner was compared to the respiratory rate as determined by CO(2) detection.

RESULTS

In total, 52 hours of telemetry data were analyzed. The respiratory rate measured from the pulse oximeter waveform was found to have a 0.89 linear correlation when compared to CO(2) detection and airway pressure change. the bias was 0.03 breath/min, SD was 0.557 breath/min and the upper and lower limits of agreement were 1.145 and -1.083 breath/min respectively. The presence of motion artifact proved to be the primary cause of failure of this technique.

CONCLUSION

Joint time frequency analysis of the pulse oximeter waveform can be used to determine the respiratory rate of ventilated patients and to quantify the impact of ventilation on the waveform. In addition, when applied to the pulse oximeter waveform new clinically relevant features were observed.

Analysis of the Ear Pulse Oximeter Waveform

OBJECTIVE

For years researchers have been attempting to understand the relationship between central hemodynamics and the resulting peripheral waveforms. This study is designed to further understanding of the relationship between ear pulse oximeter waveforms, finger pulse oximeter waveforms and cardiac output (CO). It is hoped that with appropriate analysis of the peripheral waveforms, clues can be gained to help to optimize cardiac performance.

METHODS

Part 1: Studying the effect of cold immersion test on plethysmographic waveforms. Part 2: Studying the correlation between ear and finger plethysmographic waveforms and (CO) during CABG surgery. The ear and finger plethysmographic waveforms were analyzed to determine amplitude, width, area, upstroke and downslope. The CO was measured using continuous PA catheter. Using multi-linear regression, ear plethysmographic waveforms, together with heart rate (HR), were used to determine the CO Agreement between the two methods of CO determination was assessed.

RESULTS

Part 1: On contralateral hand immersion, all finger plethysmographic waveforms were reduced, there was no significant change seen in ear plethysmographic waveforms, except an increase in ear plethysmographic width. Part 2: Phase 1: Significant correlation detected between the ear plethysmographic width and other ear and finger plethysmographic waveforms. Phase 2: The ear plethysmographic width had a significant correlation with the HR and CO. The correlation of the other ear plethysmographic waveforms with CO and HR are summarized (Table 5). Multi-linear regression analysis was done and the best fit equation was found to be: CO=8.084 - 14.248 x Ear width + 0.03 x HR+ 92.322 x Ear down slope+0.027 x Ear Area Using Bland & Altman, the bias was (0.05 L) but the precision (2.46) is large to be clinically accepted.

CONCLUSION

The ear is relatively immune to vasoconstrictive challenges which make ear plethysmographic waveforms a suitable monitor for central hemodynamic changes. The ear plethysmographic width has a good correlation with CO.

Topical non-iontophoretic application of acetylcholine and nitroglycerin via a translucent patch: a new means for assessing microvascular reactivity

OBJECTIVE

Assessments of endothelial cell function with acetylcholine have typically used systemic, regional intra-arterial, or iontophoretic delivery of drug. Each of these techniques induces systemic and/or local changes that compromise their safety or effectiveness. Using translucent drug preparations applied under laser Doppler flowmetry (LDF) probes, we tested whether local vasodilation can be induced with non-iontophoretic transdermal delivery of acetylcholine and how such dilation would compare to the dilation achieved with topical nitroglycerin in healthy volunteers.

METHODS

Ten subjects without known vascular disease were recruited for LDF monitoring at sites of drug application for this preliminary investigation. Topical acetylcholine chloride, nitroglycerin, and placebo were applied via translucent patches to the forehead directly below LDF probes.

RESULTS

LDF readings increased by 406 percent (245 percent to 566 percent) and 36 percent (26 percent to 46 percent), respectively, at the acetylcholine and placebo sites (p = .005 by Wilcoxon Signed Rank Test (WSRT) for acetylcholine vs. placebo); and they increased by 365 percent (179 percent to 550 percent) at the nitroglycerin site (p = .005 by WSRT for nitroglycerin vs. placebo; p = .6 vs. acetylcholine).

CONCLUSION

Transdermal delivery of acetylcholine can induce significant local vasodilatory responses comparable to those achieved with nitroglycerin without requiring iontophoresis. The means of transdermal delivery and monitoring described herein may constitute a new minimally invasive way to interrogate the microvasculature and thereby assess the microcirculatory changes induced by various disorders and therapeutic interventions.

The Effect of Venous Pulsation on the Forehead Pulse Oximeter Wave Form as a Possible Source of Error in Spo2 Calculation

Reflective forehead pulse oximeter sensors have recently been introduced into clinical practice. They reportedly have the advantage of faster response times and immunity to the effects of vasoconstriction. Of concern are reports of signal instability and erroneously low Spo(2) values with some of these new sensors. During a study of the plethysmographic wave forms from various sites (finger, ear, and forehead) it was noted that in some cases the forehead wave form became unexpectedly complex in configuration. The plethysmographic signals from 25 general anesthetic cases were obtained, which revealed the complex forehead wave form during 5 cases. We hypothesized that the complex wave form was attributable to an underlying venous signal. It was determined that the use of a pressure dressing over the sensor resulted in a return of a normal plethysmographic wave form. Further examination of the complex forehead wave form reveal a morphology consistent with a central venous trace with atrial, cuspidal, and venous waves. It is speculated that the presence of the venous signal is the source of the problems reported with the forehead sensors. It is believed that the venous wave form is a result of the method of attachment rather than the use of reflective plethysmographic sensors.

Special anesthetic concerns in mentally handicapped institutionalized patients undergoing gynecological procedures in an outpatient setting

STUDY OBJECTIVE

To evaluate the anesthesia issues involved in caring for mentally handicapped outpatients.

DESIGN

Retrospective chart review.

SETTING

University-affiliated outpatient ambulatory center.

PATIENTS

Twenty adult patients scheduled for gynecological procedures.

INTERVENTIONS

None.

MEASUREMENTS

Data collection sheet was used to record patients' age, ASA status, procedure, premedication, intravenous placement, degree of agitation, airway control, induction (method and drugs), intraoperative anesthesia care, postoperative medications, total time in hospital, postanesthesia care unit time and disposition.

MAIN RESULTS

Agitation was present in 100% of the patients. A significant number of these patients were ASA III, needing oral or intramuscular sedation (35%) or mask induction prior to placement of an intravenous line. Severely agitated patients had the longest stays in the postanesthesia care unit (PACU).

CONCLUSIONS

Agitation was the main reason why 90% of the patients required intubation for relatively minor procedures. Agitation was the main factor leading to prolonged recovery room time.

How does the plethysmogram derived from the pulse oximeter relate to arterial blood pressure in coronary artery bypass graft patients?

Twenty patients scheduled for coronary artery bypass grafting had their ear and finger oximeter and radial artery blood pressure (Bp(meas)) waveforms collected. The ear and finger pulse oximeter waveforms were analyzed to extract beat-to-beat amplitude and area and width measurements. The Bp(meas) waveforms were analyzed to measured systolic blood pressure (BP), mean BP, and pulse pressure. The correlation coefficient was determined between the derived waveforms from the pulse oximeter and Bp(meas) for the first 10 patients. The ear pulse oximeter width (Width(Ear)) had the best correlation (r = 0.8). Linear regression was done between Width(Ear) and Bp(meas) based on slope (b) and intercept (a) values, BP was calculated (Bp(calc)) in the next 10 patients as: [equation: see text] where i = systolic BP, mean BP, and pulse pressure. The initial bias was too large to be clinically useful. To improve clinical applicability a period of calibration was introduced in which the first 50 readings of Width(Ear) and Bp(meas) for each patient were used to calculate the intercept. After calibration the systolic BP, mean BP and pulse pressure bias values were -2.6, -1.88 and -1.28 mm Hg, and the precision values were 15.9 10.09, and 9.94 mm Hg, respectively. The present attempt to develop a clinically useful method of noninvasive BP measuring was partly successful with the requirement of a calibration period.

IMPLICATIONS

Statistical comparison was made between measured blood pressure (BP) from arterial line and calculated BP derived from ear pulse oximeter waveform in 10 patients undergoing coronary artery bypass graft surgery. Using 62,077 paired readings, the mean difference for systolic BP, mean BP, and pulse pressure between the 2 methods was -2.6, -1.88, and -1.28 mm Hg, respectively.

Different Responses of Ear and Finger Pulse Oximeter Wave Form to Cold Pressor Test

The cold pressor test is often used to assess vasoconstrictive responses because it simulates the vasoconstrictive challenges commonly encountered in the clinical setting. With IRB approval, 12 healthy volunteers, aged 25--50 yr, underwent baseline plethysmographic monitoring on the finger and ear. The contralateral hand was immersed in ice water for 30 s to elicit a systemic vasoconstrictive response while the recordings were continued. The changes in plethysmographic amplitude for the first 30 s of ice water immersion (period of maximum response) of the finger and ear were compared. The data indicate a significant disparity between the finger and the ear signals in response to the cold stimulus. The average finger plethysmographic amplitude measurement decreased by 48% +/- 19%. In contrast, no significant change was seen in the ear plethysmographic amplitude measurement, which decreased by 2% +/- 10%. We conclude that the ear is relatively immune to the vasoconstrictive effects. These findings suggest that the comparison of the ear and finger pulse oximeter wave forms might be used as a real-time monitor of sympathetic tone and that the ear plethysmography may be a suitable monitor of the systemic circulation.

Arterial-pulse oximetry loops: a new method of monitoring vascular tone

OBJECTIVE

We report the off-line calculation of the vascular compliance of the finger and suggest the continuous on-line use of this methodology as an aid to monitoring the peripheral vascular resistance. This method consists of the simultaneous analysis of the waveform signals from the pulse oximeter monitors and the arterial pressure as indicators of "volume" and pressure respectively to continuously calculate the vascular "compliance" (volume change per unit pressure change). This should be seen as a "relative compliance" as the pulse plethysmograph signal is not calibrated. This new methodology allows for continuous monitoring of peripheral vascular compliance as a beat-to-beat indicator of peripheral vascular resistance. The vaso-constrictors, phenylephrine and ephedrine, were shown to decrease the compliance as predicted.

METHODS

The arterial pressure and pulse oximeter waveforms were obtained during routine anesthetic care. The waveforms were collected with a computer data-acquisition system and then analyzed "off-line" as an indirect indicator of total vascular tone. Demographic and clinical information including drug administration were recorded.

RESULTS

A case report is presented using this new form of analysis. Vascular compliance changes induced by phenylephrine and ephedrine were studied. A dose response curve of peripheral vascular compliance to phenylephrine was generated from these data.

CONCLUSIONS

By plotting the pulse oximeter waveforms versus the arterial waveforms, multiple volume versus pressure (relative compliance) loops were obtained. Analysis of these loops may assist in the monitoring of vascular compliance.

The detection of peripheral venous pulsation using the pulse oximeter as a plethysmograph

The pulse oximeter can serve as a sensitive photoelectric plethysmograph in the operating room. It was noted in several cases that the plethysmographic waveform showed a high degree of variability during diastole. Three patients are described with discrete diastolic peaks on the plethysmograph. Further investigation revealed that these diastolic peaks appear to correlate with peripheral venous pulsation, which seems to have a central venous origin. Evidence is presented that the plethysmographic detection of the venous-pulse may be useful in estimating the changing volume status of the patient.