Synchronizing Gait with Cardiac Cycle Phase Alters Heart Rate Response during Running.

Timing foot strike to occur in synchrony with cardiac diastole may reduce left ventricular afterload and promote coronary and skeletal muscle perfusion. PURPOSE: This study aimed to assess heart rate (HR) and metabolic responses to running when foot strikes are timed to occur exclusively during 1) the systolic phase of the cardiac cycle or 2) the diastolic phase. METHODS: Ten elite male distance runners performed a testing session on a treadmill at 4.72 m·s while matching their steps to an auditory tone and wearing a chest strap that transmitted accelerometer and ECG signals. Testing comprised eight prompted 3-min stages, where a real-time adaptive auditory tone guided subjects to step with each ECG R-wave (systolic stepping) or alternatively, at 45% of each R-R interval (diastolic stepping), followed by a 3-min unprompted control stage. Metabolic variables were measured continuously. RESULTS: HR (P < 0.001) and minute ventilation (P < 0.001) were significantly lower during diastolic stepping compared with systolic stepping, whereas O2 pulse (P < 0.001) was correspondingly significantly higher during diastolic stepping. CONCLUSION: Synchronizing foot strikes when running to the diastolic portion of the cardiac cycle results in a significantly reduced HR and minute ventilation compared with stepping during systole. This cardiac and ventilatory response to diastolic stepping may be beneficial to distance running performance.

Pulse oximetry saturation patterns detect repetitive reductions in airflow.

OBJECTIVE: Postoperative patients exhibiting signs or symptoms of obstructive sleep apnea (OSA) have been identified to be at increased risk for respiratory compromise. One of the key markers associated with OSA is repetitive reductions in airflow (RRiA). A real-time pulse oximeter saturation pattern recognition algorithm (OxiMax SPD™ intended for adult in-hospital use only) designed to detect specific signatures in the SpO(2) trend associated with RRiA may provide caregivers early indication of its presence so they can treat the patient appropriately. The purpose of our study was to test the performance of saturation pattern detection (SPD) in a clinical study targeting subjects with a high prevalence of RRiA. METHODS: Overnight polysomnograph (PSG) recordings were collected on 104 sleep lab patients. RRiA was defined in terms of specific criteria from four PSG signals, evaluated in consecutive 10 min epochs. PSG scoring was conducted blind to calculation of SPD. Statistical measures of sensitivity, specificity and area under the receiver operating characteristic (ROC) curve were calculated for the detection of RRiA by SPD. RESULTS: Data were analyzed for 92 valid sets of patient recordings, encompassing 3,917 epochs. At the highest available SPD alert setting, the sensitivity was 80.2% (95% C.I. = 76.8-83.3%), the specificity was 88.3% (87.2-89.3). Area under the ROC curve was 0.87 (0.84-0.89). CONCLUSIONS: The real-time SPD algorithm was able to detect episodes of RRiA in sleep lab patients with a high degree of sensitivity and specificity.

Accuracy of methemoglobin detection by pulse CO-oximetry during hypoxia.

BACKGROUND: Methemoglobin in the blood cannot be detected by conventional pulse oximetry, although it can bias the oximeter's estimate (Spo2) of the true arterial functional oxygen saturation (Sao2). A recently introduced "Pulse CO-Oximeter" (Masimo Rainbow SET(R) Radical-7 Pulse CO-Oximeter, Masimo Corp., Irvine, CA) is intended to additionally monitor noninvasively the fractional carboxyhemoglobin and methemoglobin content in blood. The purpose of our study was to determine whether hypoxia affects the new device's estimated methemoglobin reading accuracy, and whether the presence of methemoglobin impairs the ability of the Radical-7 and a conventional pulse oximeter (Nonin 9700, Nonin Medical Inc., Plymouth, MN) to detect decreases in Sao2. METHODS: Eight and 6 healthy adults were included in 2 study groups, respectively, each fitted with multiple sensors and a radial arterial catheter for blood sampling. In the first group, IV administration of approximately 300 mg sodium nitrite increased subjects' methemoglobin level to a 7% to 8% target and hypoxia was induced to different levels of Sao2 (70%-100%) by varying fractional inspired oxygen. In the second group, 15% methemoglobin at room air and 80% Sao2 were targeted. Pulse CO-oximeter readings were compared with arterial blood values measured using a Radiometer multiwavelength hemoximeter. Pulse CO-oximeter methemoglobin reading performance was analyzed by observing the incidence of meaningful reading errors at the various hypoxia levels. This was used to determine the impact on predictive values for detecting methemoglobinemia. Spo2 reading bias, precision, and root mean square error were evaluated during conditions of elevated methemoglobin. RESULTS: Observations spanned 66.2% to 99% Sao2 and 0.6% to 14.4% methemoglobin over the 2 groups (170 blood draws). Masimo methemoglobin reading bias and precision over the full Sao2 span was 7.7% +/- 13.0%. Best accuracy was found in the 95% to 100% Sao2 range (1.9% +/- 2.5%), progressing to its worst in the 70% to 80% range (24.8% +/- 15.6%). Occurrence of methemoglobin readings in error >5% increased over each 5-point decrease in Sao2 (P < 0.05). Masimo Spo2 readings were biased -6.3% +/- 3.0% in the 95% to 100% Sao2 range with 4% to 8.3% methemoglobin. Both the Radical-7 and Nonin 9700 pulse oximeters accurately detected decreases in Sao(2) <90% with 4% to 15% methemoglobin, despite displaying low Spo2 readings when Sao2 was >95%. CONCLUSIONS: The Radical-7's methemoglobin readings become progressively more inaccurate as Sao2 decreases <95%, at times overestimating true values by 10% to 40%. Elevated methemoglobin causes the Spo2 readings to underestimate Sao2 similar to conventional 2-wavelength pulse oximeters at high saturation. Spo2 readings from both types of instruments continue to trend downward during the development of hypoxemia (Sao2 <90%) with methemoglobin levels up to 15%.

The light-tissue interaction of pulse oximetry.

The underlying science of pulse oximetry is based on a simple manipulation of the Lambert-Beer law, which describes the attenuation of light traveling through a mixture of absorbers. Signals from detected red and infrared light that has traveled through blood-perfused tissues are used to estimate the underlying arterial hemoglobin oxygen saturation. However, light scatters in tissue and influences some of the simplifications made in determining this relationship. Under most clinical circumstances, the empirical process that manufacturers use to calibrate the system during its design readily accommodates this and results in accurate readings. The same tissue light scattering properties allow sensors to be configured for use on opposing or adjacent surfaces, provided that the placement sites offer sufficient signal strength and are absent factors known to influence accuracy. In this paper I review the light-tissue interaction in pulse oximetry and describe some of the assumptions made and their implications. Certain deviations from the nominal conditions, whether clinical in nature or misuse of the product, can affect system performance. Consequently, users should be cautious in modifying sensors and/or using them on tissue sites not intended by the manufacturer (off-label use). While perhaps helpful for obtaining pulsatile signals or extending the lifetime of a sensor, some practices can disrupt the optical integrity of the measurement and negatively impact the oxygen saturation reading accuracy.

Forehead pulse oximetry: Headband use helps alleviate false low readings likely related to venous pulsation artifact.

BACKGROUND: This study investigated whether a tensioning headband that applies up to 20 mmHg pressure over a forehead pulse oximetry sensor could improve arterial hemoglobin oxygen saturation reading accuracy in presence of venous pooling and pulsations at the forehead site. METHODS: Healthy volunteers were studied breathing room air in supine and various levels of negative incline (Trendelenburg position) using the forehead sensor with the headband adjusted to its maximum and minimum recommended pressure limits. Saturation readings obtained from the forehead sensor with the subjects supine and the headband in place were used as a baseline to compare the effects of negative incline on reading accuracy when using and not using the headband. Occurrences of false low-saturation readings detected by forehead sensors were compared with those from digit sensors. RESULTS: No difference was observed between saturation readings obtained from the forehead sensor in supine and negative incline positions when the headband was applied. Forehead sensor readings obtained while subjects were inclined and the headband was not used were significantly lower (P < 0.05) than the supine readings. There was no statistically significant difference between the digit and forehead sensor in reporting false low-saturation readings when the headband was applied, regardless of body incline. CONCLUSIONS: Application of up to 20 mmHg pressure on the forehead pulse oximetry sensor using an elastic tensioning headband significantly reduced reading errors and provided consistent performance when subjects were placed between supine and up to 15 degrees head-down incline (Trendelenburg position).

The influence of larger subcutaneous blood vessels on pulse oximetry.

OBJECTIVE: Recent studies have renewed interest in reflectance pulse oximetry, specifically for monitoring the patient's forehead. Blood circulation on the forehead immediately above the eyebrow is fed by arteries that branch from the internal carotid artery and lack the vasoconstrictor response present in more peripheral regions. Some investigators question, however, the reliability of monitoring SpO2 on the forehead due to prior reported inaccurate readings with reflectance sensors. The present study evaluates pulse oximetry accuracy when reflectance sensors are placed over potentially pulsing or moving larger arterial vessels, or over more homogeneous microvasculature devoid of larger subcutaneous vessels. METHODS. Ten healthy adult volunteers were fitted with reflectance pulse oximetry sensors and exposed to a controlled desaturation to 70%. Sensors were placed immediately above the left and right eyebrows as well as over the temple. Additionally, numerical modeling was used to simulate light signals and photon migration through a homogeneous tissue bed with an added static or dynamic artery. RESULTS: Sensors placed above the eyebrows tracked one another with significantly better accuracy than when comparing temple with the brow placement (RMS of the Differences = 1.12% vs. 4.24%, respectively). Photon migration simulations indicate that the detected light bypasses the interior of larger vessels, while vessel presence affects the red and IR light pulse amplitudes independent of SaO2. CONCLUSIONS: Placement of reflectance pulse oximetry sensors directly over larger cardio-synchronously pulsing or moving vasculature can significantly degrade SpO2 reading accuracy. Reflectance sensors placed low on the forehead directly over the eyebrow and slightly lateral to the iris appear to avoid such vasculature and provide consistent and accurate estimates of SaO2.

Forehead oximetry in critically ill patients: the case for a new monitoring site.

Pulse oximetry is a ubiquitous monitor in anesthesia and critical care and is often considered the fifth vital sign. Under conditions of normal perfusion and temperature, the finger probe is the most common and effective sensor. In the presence of hypotension, hypoperfusion,and hypothermia, however, the finger sensor is often unable to detect a pulsatile signal. Another site and sensor are necessary to monitor these patients effectively. This article describes the search for this site, the choice of the forehead, and preliminary data regarding the use of this sensor site.

Design and validation of pulse oximetry for low saturation.

As pulse oximetry migrates into new areas of patient care, it becomes increasingly important to rigorously test performance in the environment of use before claiming success. If evaluated only under tightly or ideally controlled laboratory or bench-top conditions, results may have little relationship to real-world behavior. As an example, fetal pulse oximetry sensors are subject to challenges not typically found with air-breathing patients. The "presenting part" is an attractive site for fetal oximetry, however the presence of resolvable pulses by itself does not ensure accurate readings. This study explores the effects of physiological and mechanical perturbations at low saturation expected during use on fetuses with such sensors. Numerical modeling and animal studies demonstrate that at 40% true arterial saturation (SaO2), calculated saturation values (SpO2) may err by more than 30%, even in the presence of otherwise normal pulse oximetry signals.