Outcomes of primary phakic trabeculectomies without versus with 0.5- to 1-minute versus 3- to 5-minute mitomycin C

PURPOSE: To compare the intraocular pressure and hypotony outcomes of primary phakic trabeculectomies with no mitomycin C (MMC), shorter MMC, and longer MMC exposure.METHODS: We evaluated primary phakic trabeculectomies with no MMC (36 eyes of 36 patients), 0.5- to 1-minute MMC (50 eyes of 50 patients), and 3- to 5-minute MMC (38 eyes of 38 patients) at the concentration of 0.5 mg/ml. Successful trabeculectomy was defined as an intraocular pressure of 21 mm Hg or less without development of a marked visual acuity loss associated with prolonged hypotony (intraocular pressure < 6 mm Hg over 3 months) and without the need for additional surgery to control intraocular pressure or treat postoperative complications.RESULTS: The three groups were similar in demographics, preoperative intraocular pressure, and medical dependency. However, the incidence of hypotony during the postoperative periods of 3 to 12 months was significantly higher in the 3- to 5-minute MMC group (P < .05, chi-square test). Severe visual acuity loss associated with hypotony was also more frequently found in the 3- to 5-minute MMC group than in the 0.5- to 1-minute (P = .009, chi-square test) group or the control group (P = .014, chi-square test). In addition, the success probabilities were significantly different among the three groups (P = .001, Kaplan-Meier survival analysis with log-rank test) and were the highest in the 0.5- to 1-minute MMC group and the lowest in the 3- to 5-minute MMC group.CONCLUSION: Shorter application (0.5 to 1 minute) of MMC appears to be optimal for the successful outcome of primary phakic trabeculectomy compared with no MMC or longer application of MMC at a concentration of 0.5 mg/ml.     

Pulse oximetry: Analysis of theory,technology, and practice

Interest in two-wavelength classic, that is, nonpulse, oximetry began early in the 20th century. Noninvasive in vivo measurements of oxygen saturation showed promise, but the methods were beset by several problems. The pulse oximetry technique, by focusing on the pulsatile arterial component, neatly circumvented many of the problems of the classic nonpulse arterial approach. Today's pulse oximeter owes a good measure of its success to the technologic advances in light emission and detection and the ready availability of microcomputers and their software. Many clinicians have recognized how valuable the assessment of the patient's oxygenation in real time can be. This appreciation has propelled the use of pulse oximeters into many clinical fields, as well as nonclinical fields such as sports training and aviation. Understanding how and what pulse oximetry measures, how pulse oximetry data compare with data derived from laboratory analysis, and how the pulse oximeter responds to dyshemoglobins, dyes, and other interfering conditions must be understood for the correct application and interpretation of this revolutionary monitor.     

Two computer-based anesthetic monitors: The duke automatic monitoring equipment (Dame) system and the microdame

From 1972 to 1983 the Duke University Department of Anesthesiology designed, built, and maintained most of its own operating room patient monitoring equipment. Construction of a new hospital facility in 1980 provided the opportunity to design and test a new computer-based system, the Duke Automatic Monitoring Equipment (DAME) System. The system consists of microcomputer-based instrumentation on monitoring carts, which communicate with a central minicomputer that allows selection ot different software monitoring packages based on the needs of the patient. Multiple problems, including frequent total monitoring failures during surgery, plagued the DAME System in its first year of operation. Despite resolution of many of these problems, user acceptance was poor because of the large size and weight of the monitoring carts, the inadequate quality of displayed physiological waveforms, and inability to overcome the difficulties of the man-machine interface. Because the remaining problems could not be rectified with the existing monitoring carts, a new generation of monitors was designed. The smaller, multiprocessor microDAME was designed to be as automatic and user tolerant as possible. It would omit much of the flexibility that had proved undesirable in the DAME system. When the microDAME was nearly completed, however, departmental research in that area ceased. It remains for others to apply our experiences to further improve operating room patient monitors.     

The effect of cuff pressure deflation rate on accuracy in indirect measurement of blood pressure with the auscultatory method

The importance of cuff deflation rate in the auscultatory method of measuring blood pressure was investigated using a computer-based model. To determine the relationship between the cuff deflation rate and the measurement error, two cuff deflation protocols were used, one based on heart rate (mm Hg per heartbeat), the other on a constant rate (mm Hg per second). The different deflation protocols and rates were tested using a constant blood pressure of 120/80 mm Hg and heart rates ranging from 40 to 120 beats/min. It was confirmed that a cuff deflation rate that is time based will introduce larger errors at low heart rates. Using heart rate as a basis for cuff deflation rate yields a constant error that is independent of heart rate. The currently used standard of 3 mm Hg/s could result in a maximum error of 2.5 mm Hg in both systolic and diastolic pressures at a heart rate of 72 beats/min. The maximum systolic and diastolic errors increase to more than 4 mm Hg at 40 beats/min. A deflation rate of 2 mm Hg/beat, however, yields a maximum error of 2 mm Hg for both systolic and diastolic pressures, independent of heart rate. A cuff deflation rate based on heart rate is recommended to help minimize changes in measurement error when measuring blood pressure if a wide range of heart rates will be encountered.Supported by grants from IVAC, San Diego, CA, and Physio Control, Redmond, WA.     

Experimental myocardial ischemia and infarction Production of diffuse myocardial lesions in unanesthetized calves

To produce a chronic, ischemic heart failure preparation of controllable severity, plastic microspheres (6 to 14 μ in diameter) were injected into the left main coronary artery of unanesthetized calves. Two dose levels were used (4 and 5 mg/kg, Groups I and II, respectively). Hemodynamic and electrocardiographic changes were not observed during the microsphere infusion. Arterial-coronary sinus lactate differences shifted from positive control values (mean +5.3 mg/ 100 ml) to negative during and throughout the injection (mean −6.4 mg/100 ml). Significant hemodynamic abnormalities at rest (Group II only) were first noted on postinfusion day 1, and unaccompanied by subsequent significant physiologic recovery. These included an increase in left ventricular end-diastolic pressure (+18 to 21 mm Hg), reduction in stroke volume (−32 percent), cardiac output (−25 percent), dP/dt (−22 percent), mean ejection rate (−18 percent) and left ventricular ejection time, and a prolongation of preejection period. Cardiac response to angiotensin-induced stress was impaired in both test groups. Mean heart weight and left ventricular wall thickness increased by 65 and 45 percent, respectively, in both groups. Histopathologic examination demonstrated microinfarcts widely distributed throughout the myocardium supplied by the left coronary artery at various stages of healing, sometimes in the same heart, with typical time-dependent morphologic characteristics. The myocardial fibers were hypertrophied. In addition, atypical areas were found on glycogen selective staining.

The results suggest that the extent of microcirculatory ischemia and infarction determines the degree of myocardial impairment at rest and during stress irrespective of compensatory hypertrophy.     

Vagal tone is reduced during paced breathing in patients with the chronic fatigue syndrome

Patients with chronic fatigue syndrome (CFS) often complain of an inability to maintain activity levels and a variety of autonomic-like symptoms that make everyday activiry intolerable at times. The purpose of the study was to determine if there were differences in vagal activiry at fixed breathing rates in women with CFS. Twelve women with the diagnosis of CFS between the ages of 32 and 59 years volunteered for the study. Healthy women, who were between the ages of 30 and 49, served as controls. Full signal electrocardiograph and respiratory signals were collected during a paced breathing protocol of three fixed breathing rates (8, 12 and 18 breaths/min) performed in the sitting and standing postures. Vagal activity was analyzed by means of heart rate spectral analysis to determine the subject's response to specific breathing rates and postures. Heart rate variability was used as a non-invasive method of measuring the parasympathetic component of the autonomic nervous system. Using this method, although there was significantly less vagal power in the sitting versus the standing postures for both groups, the overall vagal power was significantly lower (p < 0.034) in the CFS group versus healthy controls. Vagal power was also significantly lower (p < 0.01 to p < 0.05) at all breathing rates in both postures except while standing and breathing at 18 breaths/min. Knowledge of the differences in vagal activity for CFS patients may allow stratification for the analysis of other research variables.     

Challenges and Regulatory Considerations in the Acoustic Measurement of High‐Frequency (>20 MHz) Ultrasound

This article examines the challenges associated with making acoustic output measurements at high ultrasound frequencies (>20 MHz) in the context of regulatory considerations contained in the US Food and Drug Administration industry guidance document for diagnostic ultrasound devices. Error sources in the acoustic measurement, including hydrophone calibration and spatial averaging, nonlinear distortion, and mechanical alignment, are evaluated, and the limitations of currently available acoustic measurement instruments are discussed. An uncertainty analysis of acoustic intensity and power measurements is presented, and an example uncertainty calculation is done on a hypothetical 30‐MHz high‐frequency ultrasound system. This analysis concludes that the estimated measurement uncertainty of the acoustic intensity is +73%/?86%, and the uncertainty in the mechanical index is +37%/?43%. These values exceed the respective levels in the Food and Drug Administration guidance document of 30% and 15%, respectively, which are more representative of the measurement uncertainty associated with characterizing lower‐frequency ultrasound systems. Recommendations made for minimizing the measurement uncertainty include implementing a mechanical positioning system that has sufficient repeatability and precision, reconstructing the time‐pressure waveform via deconvolution using the hydrophone frequency response, and correcting for hydrophone spatial averaging.     

Encircling overlapping multipulse shock waveforms for transthoracic defibrillation

Objectives. This study was performed to determine the efficacy of new encircling overlapping multipulse, multipathway waveforms for transthoracic defibrillation.

Background. Alternative waveforms for transthoracic defibrillation may improve shock success.

Methods. First, we determined the shock success achieved by three different waveforms at varying energies (18–150 J) in 21 mongrel dogs after short-duration ventricular fibrillation. The waveforms tested included the traditional damped sinusoidal waveform, a single pathway biphasic waveform, and a new encircling overlapping multipulse waveform delivered from six electrode pads oriented circumferentially. Second, in 11 swine we compared the efficacy of encircling overlapping multipulse shocks given from six electrode pads and three capacitors versus encircling overlapping shocks given from a device utilizing three electrodes and one capacitor.

Results. In the first experiment, the encircling overlapping waveform performed significantly better than biphasic and damped sinusoidal waveforms at lower energies. The shock success rate of the overlapping waveform (six pads) ranged from 67 ± 4% (at 18–49 J energy) to 99 ± 3% at ≥150 J; at comparable energies biphasic waveform shock success ranged from 26 ± 5% (p < 0.01 vs. encircling overlapping waveforms) to 99 ± 5% (p = NS). Damped sinusoidal waveform shock success ranged from 4 ± 1% (p < 0.01 vs. encircling overlapping waveform) to 73 ± 9% (p = NS). In the second experiment the three electrode pads, one capacitor encircling waveform achieved shock success rates comparable with the six-pad, three-capacitor waveform; at 18–49 J, success rates were 45 ± 15% versus 57 ± 12%, respectively (p = NS). At 100 J, success rates for both were 100%.

Conclusions. We conclude that encircling overlapping multipulse multipathway waveforms facilitate transthoracic defibrillation at low energies. These waveforms can be generated from a device that requires only three electrodes and one capacitor.     

Defibrillation energy requirements using a left anterior chest cutaneous to subcutaneous shocking vector: Implications for a total subcutaneous implantable defibrillator

BACKGROUND: Subcutaneous implantable defibrillators (ICDs) are being developed to facilitate ICD implantation. OBJECTIVE: The purpose of this study was to estimate the human defibrillation energy requirement (DER) using a left chest cutaneous (Q) to subcutaneous (SQ) shocking vector. METHODS: Twenty patients undergoing implantation of an indicated ICD were enrolled (15 males, age = 63 +/- 12 years; ejection fraction = 0.27 +/- 0.14). Defibrillation testing was performed using an investigational system consisting of an external defibrillator and a constructed connector to deliver a shock between a pectoral SQ can and a cardiac apical Q electrode. Two attempts at defibrillation using this configuration were allowed. Stage 1 testing started at 70 J with a step-down/step-up to 50 or 100 J, respectively. Stage 2 testing began at 50 J with a step-down/step-up to 30 or 70 J. RESULTS: During stage 1, a 70-J shock was successful in 7/9 (78%) patients. A second attempt was successful in 7/7 patients using a 50-J shock. In the two remaining patients, a second attempt using a 100-J shock was successful. During stage 2, a 50-J shock was successful in 10/11 (91%) patients. The protocol could not be completed in 2/11 patients. Of the remaining nine patients, a second defibrillation was successful in seven (78%) using a 30-J shock. CONCLUSIONS: The defibrillation energy requirement (DER) of this study vector was 50 J or less in most patients. This low DER supports further investigation of a totally SQ-ICD. However, the DER of 100 J in two patients indicates that further investigation is needed regarding DER variability and safety margins.