Pediatric tele-echocardiography: evaluation of transmission modalities.

Pediatric cardiology consultation has been effectively delivered outside the tertiary care setting through the use of tele-echocardiography. This study examined the effectiveness of several tele-echocardiography connections and the satisfaction of the referring physicians using these services. Studies were transmitted via either a shared fiber-optic (DS3) connection (two sites), a dedicated fast-copper (ISDN-PRI) link, or by courier from a nearby (25-mile) or distant (170-mile) site. Time intervals between when echocardiograms were performed locally until they were received, interpreted, and reported were prospectively recorded. Referring physician satisfaction was assessed through a survey. The critical time between when a remote echocardiogram was performed and when its result was reported to the referring physician was primarily determined by the mode of transmission. The time interval between performing an echocardiogram and receiving the study was significantly longer for echocardiograms sent from the 170-mile courier site (2474 +/- 295 min) than either the 25-mile courier site (474 +/- 151 min), DS3 (374 +/- 121 min), or ISDN-PRI (129 +/- 16 min). Regardless of the method of transmission, all referring physicians felt that the service improved their ability to manage children, and they would recommend the service to their colleagues. Those using the courier service from the 25-mile away site were more concerned about the availability of a pediatric cardiologist and image quality, presumably due to the delay in response times. The time interval data provided in this study and the assessment of physician satisfaction provide important data as echocardiography laboratories implement tele-echocardiography services.

Tele-education in a telemedicine environment: implications for rural health care and academic medical centers.

Over 50 million people in the United States (about 20% of the population) live in rural areas, but only 9% of the nation's physicians practice in rural communities. It is difficult to recruit and retain rural health care practitioners, partly because of issues relating to professional isolation. New and enhanced telecommunications links between community and academic hospitals show promise for reducing this isolation and enhancing lifelong learning opportunities for rural health care providers. This paper will explore some of the issues involved in using interactive video (telemedicine) networks to transmit continuing medical education programming from an academic center to multiple rural hospitals. Data from a recent University of Iowa survey of the state's health educators will be presented as one approach to assessing the health care marketplace for the deployment of tele-education services.

Optimizing utilization of pediatric echocardiography and implications for telemedicine.

Telemedicine can deliver tertiary level services to remote communities where subspecialty care is limited. Locally performed echocardiography has been initiated at several locations around Iowa. The goal of this study was to examine utilization and diagnostic yield of community-based echocardiographic services. Community physicians selected patients for remote echocardiograms (echoes), and studies were performed locally by sonographers trained in recording pediatric echoes. Echoes were sent to the pediatric echocardiography laboratory by mail or via telemedicine systems. Echoes were also ordered locally by pediatric cardiologists during outreach clinics in the same communities. Numbers of normal and abnormal echoes ordered by community physicions and pediatric cardiologists were compared by chi-square analysis. Since January 1996, community physicians ordered 378 echoes, whereas 154 echoes were ordered by pediatric cardiologists at outreach clinics. Stratifying echoes by patient age found that the percentage of normal studies in patients < 1 year of age was no different between groups (27% normal by community physicians vs 15%; chi-square 0.92; p = 0.34). The percentage of normal studies ordered by community physicians was significantly greater in patients > 1 year of age (83% normal by community physicians vs 25%; chi-square 80.2; p <0.0001). Thus, (1) community physicians effectively identified patients < 1 year of age with abnormal echoes, (2) significantly fewer echoes may be required in patients > 1 year of age if patients are first evaluated by a pediatric cardiologist, and (3) patient selection will impact cost effectiveness of remotely obtained echoes.

Health care providers' perceptions of telemedicine services.

OBJECTIVE: To assess the level of consensus among the administrative and health care leaders at rural Iowa hospitals regarding service gaps and priorities for developing telemedicine services. METHODS: In the summer of 1994, a survey was conducted of all rural hospital chief executive officers, chiefs of medical staffs, and directors of nursing in Iowa concerning their perceptions of telemedicine services. RESULTS: With the exception of teleradiology, few clinical specialties received high ratings as areas of need or priorities for the development of telemedicine. There was a general lack of agreement among respondents from the same hospital on such priorities. In contrast, respondents expressed higher priorities for the development of telemedicine-based educational services. CONCLUSIONS: The interest in teleradiology is consistent with the fact that teleradiology has been more thoroughly tested for medical efficacy than other telemedicine applications. Continuing medical education may represent another potential for widespread successful telemedicine application. Financial issues were reported as the greatest barriers to the development of telemedicine systems.

Current-based transthoracic defibrillation.

This study examines in a prospective, multicenter trial the feasibility and advantage of current-based, transthoracic defibrillation. Current-based, damped, sinusoidal waveform shocks of 18, 25, 30, 35, or 40 amperes (A) were administered beginning with 25 A for polymorphic ventricular tachycardia (VT) and ventricular fibrillation (VF) or 18 A for monomorphic VT; success rates were compared with those of energy-based shocks beginning at 200 J for VF/polymorphic VT and 100 J for VT. The current-based shocks were delivered from custom-modified defibrillators that determined impedance in advance of any shock using a "test-pulse" technique; the capacitor then charged to the exact energy necessary to deliver the operator-selected current against the impedance determined by the defibrillator. Three hundred sixty-two patients received > 1 shock for VF, polymorphic VT, or monomorphic VT: 569 current- based shocks and 420 energy-based shocks. Current-based shocks of 35/40 A achieved success rates of up to 74% for VF/polymorphic VT; 30 A shocks terminated 88% of monomorphic VT episodes. Energy-based shocks of 300 J terminated 72% of VF/polymorphic VT; 200-J shocks terminated 89% of monomorphic VT. We could not demonstrate a significant increase in the success rate of current-based shocks over energy-based shocks for patients with high transthoracic impedance; this may be due to inadequate sample size. Thus, current-based defibrillation is clinically feasible and effective. A larger study will be needed to test whether current-based defibrillation is superior to energy-based defibrillation.

Circadian control of heart rate in young insulin-dependent diabetes mellitus patients.

Our objective was to determine whether young insulin-dependent diabetes mellitus (IDDM) patients without complications have abnormal circadian patterns of sympathetic or parasympathetic control of heart rate. Twenty-four-hour electrocardiographic recordings in 26 IDDM patients without complications and 27 control subjects were obtained. Patients were in good health and participated in their usual daily activities. Power spectral analysis was performed to determine the amount of heart rate variability due to all (0.01-1.0 Hz), low (0.04-0.15 Hz), and high (0.15-0.40 Hz) frequency input signals hourly and throughout the entire 24-h period. Overall 24-h heart rate power did not differ between control and IDDM subjects. In IDDM subjects high-frequency power decreased with increasing diabetes duration (r = -0.49, p = 0.013). A significant difference in circadian variation of heart rate power was only found for low frequency power (p = 0.014). These results demonstrate that young IDDM subjects without diabetic complications have normal or near normal parasympathetic and sympathetic circadian patterns of heart rate control.

Safety and utility of flecainide acetate in the routine care of patients with supraventricular tachyarrhythmias: results of a multicenter trial. The Flecainide Supraventricular Tachycardia Study Group.

Patients with supraventricular arrhythmias have been safely and effectively treated with flecainide. We conducted an open-label, 20-center trial to define further the safety and efficacy profile of oral flecainide in patients with supraventricular arrhythmias, including atrial tachycardias (ectopic or multifocal), atrial-ventricular tachycardias (reentrant), paroxysmal atrial fibrillation/flutter (PAF), and chronic atrial fibrillation (CAF). Our study population of 151 patients with documented supraventricular arrhythmias requiring treatment included 67 with paroxysmal supraventricular tachycardia (PSVT), 67 with PAF (symptoms < 15 days), and 17 with CAF (symptoms > of = 15 days)> The initial flecainide dose of 100 mg twice daily could be increased by 50 mg bid every 4 days to a maximum of 200 mg twice daily. Patients who were effectively treated could receive flecainide for 1 year. The study was terminated April 26, 1989, in response to interim results reported by the Cardiac Arrhythmia Suppression Trial (CAST). All patients were removed from the study by August 1989. At study termination 87% of PSVT, 73% of PAF, and 56% of CAF patients had improved symptomatically while on flecainide therapy. Eleven patients experienced cardiac adverse experiences: proarrhythmic events (3 patients), new or worsened congestive heart failure (7 patients), sinus pauses (1 patient). Cardiac side effects appeared to be more frequent in patients in the CAF group (5/17 patients), all of whom had structural heart disease. Overall, 45 (67%) PSVT, 43 (64%) PAF, and 9 (56%) CAF patients reported at least 1 noncardiac adverse experience; the most common were abnormal vision, dizziness, and headaches. One patient from the CAF group died; the death was considered to be unrelated to flecainide. Flecainide appears to be safe and effective treatment for patients with supraventricular arrhythmias of a variety of mechanisms and appears particularly effective for patients with PSVT. The efficacy is lowest and side effects most frequent in patients with CAF, as seen with other trials of antiarrhythmic medication in these patients. In the context of the CAST experience and other trials of antiarrhythmic drugs in patients with CAF, the balance of risk and benefit of therapy should be considered carefully before initiating treatment.

Signal-averaged ECG parameters in cardiac normals using Frank lead system and Fourier transform filter and gender specific differences: a multicenter study.

There is only limited data on normal reference values for signal-averaged electrocardiograms (SAECGs) using Frank leads and fast Fourier transform filter (FFT). Furthermore, the influence of gender on reference values and their relation to body characteristics was only the subject of a few studies on small series of normals. One hundred eighty-five cardiac normals (85 women and 100 men) were examined in this multicenter study. The obtained SAECG values (mean +/- standard deviation) are as follows: filtered QRS duration (FQRSD) = 108.6 +/- 7.5 msec; low amplitude signal duration < 40 microV (LASD) = 30.4 +/- 8.4 msec; and root mean square voltage in the terminal 40 msec (RMSV) = 43.5 +/- 20.6 microV. Between men and women, significant differences were found in FQRSD (111.7 +/- 6.5 vs 105.0 +/- 7.0 msec, P < 0.001) and in RMSV (38.6 +/- 17.4 vs 49.4 +/- 22.7 microV, P < 0.001). No difference was observed for LASD. After normalizing the three SAECG parameters for body characteristics, FQRSD normalized for height was the only variable where gender differences were eliminated. For FQRSD and LASD the 90th percentile and for RMSV the 10th percentile are proposed as cut-off values. Only for the 90th percentile of FQRSD a clear difference between men and women was observed. The following gender specific normal values for SAECG, at 40-Hz high pass filtering, using Frank leads and an FFT filter are proposed: for males, FQRSD < 122 msec; for females, FQRSD < 115 msec; for both genders, LASD < 41 msec and RMSV > 20 microV.

Atrial fibrillation and atrial flutter.

The epidemiology, pathophysiology, diagnosis, evaluation, and treatment of atrial fibrillation (AF) and atrial flutter (AFl) are reviewed, and recent developments and controversies in the approach to these arrhythmias are addressed. AF and AFl are the arrhythmias most frequently encountered in clinical practice. Although occasionally unaware of their arrhythmia, patients usually complain of palpitations, weakness, dyspnea, and decreased exercise tolerance. The initial goal of therapy is control of the ventricular rate. Rate control is accomplished with atrioventricular node-blocking agents such as digoxin, calcium-channel blockers, or beta-adrenergic blockers. Along with a rapid, irregular ventricular response, other detrimental outcomes of AF and AFl include compromised hemodynamics and increased vulnerability to thromboembolism. After the cause of the patient's arrhythmia has been evaluated, pharmacologic treatment is directed at converting the rhythm to normal sinus rhythm and maintaining it. Antiarrhythmic drugs have proved effective in about 50% of cases but may be associated with increased mortality. More effective and safer forms of drug therapy for AF and AFl are needed. Nonpharmacologic alternatives to antiarrhythmic medications for refractory AF and AFl include radio-frequency catheter ablation of the bundle of His with pacemaker placement and surgery. Patients who remain in AF despite therapy should receive long-term warfarin treatment. Drugs may be used to control the ventricular response in patients with AF and AFl, terminate and prevent the arrhythmias, and prevent thromboembolism. Nonpharmacologic treatments are reserved for patients whose arrhythmias are poorly controlled by drugs.

Noninvasive prediction of efficacy of type IA antiarrhythmic drugs by the signal-averaged electrocardiogram in patients with coronary artery disease and sustained ventricular tachycardia.

This study attempted to determine if specific changes on the signal-averaged electrocardiogram (ECG) after type IA antiarrhythmic therapy are predictive of efficacy in the treatment of ventricular tachycardia (VT). Scalar and signal-averaged ECGs were obtained at baseline and after type IA drug therapy in 15 patients with coronary artery disease and inducible VT at baseline electrophysiologic testing. Signal-averaged QRS duration, root-mean-square amplitude in the last 40 ms of signal-averaged QRS, and the duration under 40 mu v of the signal-averaged QRS (low-amplitude signal), as well as ventricular effective refractory period at electrophysiologic study, and QTc on the scalar ECG were compared. At drug study, 6 patients (group A) had persistent but slower VT, whereas 9 (group B) had VT rendered noninducible. The baseline signal-averaged QRS duration was longer in group A than in B (136 +/- 10 vs 115 +/- 13 ms; p < 0.05), as was the scalar QRS (115 +/- 19 vs 98 +/- 11 ms; p < 0.05). After antiarrhythmic therapy, group A had a greater prolongation of both signal-averaged QRS (24 +/- 10 vs 8 +/- 3 ms; p < 0.05) and low-amplitude signal (31 +/- 13 vs 3 +/- 7 ms; p < 0.05), whereas group B had a greater increase in ventricular effective refractory period (49 +/- 20 vs 20 +/- 13 ms; p < 0.05) and corrected QT interval (100 +/- 39 vs 43 +/- 23 ms; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in reflex function contributing to syncope: orthostatic hypotension, carotid sinus hypersensitivity and drug-induced dysfunction.

Orthostatic hypotension and related neurologic symptoms are frequently encountered in clinical practice. The maintenance of appropriate blood pressure and heart rate responses upon assuming the upright posture are dependent upon: 1. intact mechanical (venous valves) mechanisms, 2. functioning arterial and cardiopulmonary baroreceptors, 3. normal peripheral neural pathways, 4. normal central neural integration, and 5. appropriate neurohormonal secretion. Dysfunction at one or more of these loci may facilitate the occurrence of orthostatic hypotension and syncope. In general, the mechanisms of orthostatic hypotension may be divided into three categories. In the first category, processes interfere with normal compensatory responses to upright posture. Examples of this mechanism include age related autonomic changes, diabetic neuropathy and central nervous system disease such as Shy-Drager syndrome. The second principal mechanism involves overwhelming otherwise normal reflexes by an intense orthostatic stimulus. An obvious example of this mechanism is syncope related to hemorrhage. A final category of orthostatic hypotension relates to interference with reflex responses by drugs that may limit vasoconstriction, heart rate or cardiac output adjustments or exaggerate venous pooling. These are commonly used medications such as vasodilators, beta-adrenergic blockers and nitrates. The treatment of orthostatic hypotension revolves around the recognition of underlying causes or contributing factors amenable to correction or avoidance. Other helpful treatment options include nocturnal head-up tilting and mineralocorticoids, both of which help to expand blood volume. Many other therapeutic agents have been tried in small and selected patient populations, often with disappointing results. While many of the drugs available (phenylephrine, ephedrine, tyramine, dihydroergotamine) can improve upright blood pressure, side effects are common, and supine hypertension is problematic in many patients. Interventions of this type should be carefully initiated in a monitored setting. The carotid sinus is an important component of a neural control system responsible for heart rate and blood pressure homeostasis. Excessive heart rate and blood pressure responses to distortion of the carotid sinus are the basis for the carotid sinus syndrome (CSS). Patients with CSS tend to be elderly males and local pathology in the neck is frequently involved. Atherosclerotic coronary artery disease and hypertension are important clinical correlates. Two major categories of carotid sinus hypersensitivity (CSH) are recognized: cardioinhibitory and vasodepressor. Cardioinhibitory CSH is the most common, and in its purest form consists of sinus bradycardia or arrest, asystole or AV block during carotid sinus massage. This vagally-mediated response is eliminated by atropine. Cardiac pacing is nearly universally successful in preventing severe symptoms.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscle sympathetic nerve activity is reduced in IDDM before overt autonomic neuropathy.

Studies of heart-rate variability have demonstrated that abnormal cardiac parasympathetic activity in individuals with IDDM precedes the development of other signs or symptoms of diabetic autonomic neuropathy. To determine whether IDDM patients have impaired sympathetic activity compared with normal control subjects before the onset of overt neuropathy, we directly recorded MSNA. We also examined the effects of changes in plasma glucose and insulin on sympathetic function in each group. MSNA was recorded by using microneurographic techniques in 10 IDDM patients without clinically evident diabetic complications and 10 control subjects. MSNA was compared during a 15-min fasting baseline period and during insulin infusion (120 mU.m-2.min-1) with 30 min of euglycemia. A cold pressor test was performed at the end of euglycemia. Power spectral analysis of 24-h RR variability was used to assess cardiac autonomic function. IDDM patients had lower MSNA than control subjects at baseline (8 +/- 1 vs. 18 +/- 3 burst/min, P < 0.02). MSNA increased in both groups with insulin infusion (P < 0.01) but remained lower in IDDM patients (20 +/- 3 vs. 28 +/- 3 burst/min, P < 0.01). In the IDDM group, we found no relationships between MSNA and plasma glucose, insulin, or HbA1c concentrations. BP levels did not differ at rest or during insulin. Heart-rate variability and the MSNA response to cold pressor testing in IDDM patients did not differ from those in healthy control subjects. IDDM patients had reduced MSNA at rest and in response to insulin. The lower MSNA is not attributable to differences in plasma glucose or insulin, but, rather, is most likely an early manifestation of diabetic autonomic neuropathy that precedes impaired cardiac parasympathetic control.

Establishment of signal-averaged electrocardiographic criteria with Frank XYZ leads and spectral filter used alone and in combination with ejection fraction to predict inducible ventricular tachycardia in coronary artery disease.

Signal-averaged electrocardiographic criteria are reported for corrected Frank XYZ leads and a spectral filter. The new criteria were used alone and in combination with ejection fraction to predict inducibility of ventricular tachycardia (VT) at electrophysiologic testing. Signal-averaged electrocardiographic criteria were developed in 87 control subjects and validated in 182 patients (aged 63 +/- 10 years) with coronary artery disease and QRS duration less than 118 ms. Patients underwent electrophysiologic testing in which up to 3 extra-stimuli were used during 2 paced drives from 2 right ventricular sites. A positive finding was monomorphic VT lasting 30 seconds or needing intervention. An ejection fraction less than 40% was considered abnormal. Signal-averaged electrocardiographic variables that best characterized control subjects and separated patients with and without inducible VT were filtered QRS duration less than 120 ms, low-amplitude signal duration less than 38 ms and root-mean-square voltage greater than 20 muv. With these criteria, signal-averaged electrocardiographic and ejection fraction sensitivities were 87 and 45%, respectively, and specificities were 65 and 77%, respectively. Combining signal-averaged electrocardiography with ejection fraction improved the predictive accuracy. In conclusion, diagnostic criteria for signal-averaged electrocardiography with use of Frank XYZ leads and a spectral filter produced results similar to those reported for use of bipolar XYZ leads and a Butterworth filter. Signal-averaged electrocardiography was a better predictor of VT than was ejection fraction.

Effect of direct, reflex and exercise-provoked increases in sympathetic tone on idiopathic ventricular tachycardia.

Exercise treadmill testing and direct enhancement of sympathetic influence with agents such as isoproterenol are often used to reproduce ventricular tachycardia (VT). The cardiac effects of, and arrhythmia responses to, graded exercise, isoproterenol infusion and lower body negative pressure (the latter 2 with and without atrial and ventricular stimulation) were studied in 11 patients with idiopathic VT. During maximal exercise, substantial increases in heart rate and blood pressure occurred, but only 2 of 9 exercised patients had VT (during recovery in both). During programmed stimulation alone, VT was initiated in 6 patients. During maximum levels of lower body negative pressure (-60 cm of water in most), mean systolic blood pressure decreased by 10 mm Hg, heart rate increased by 15 beats/min, and ventricular refractory period decreased by 10 ms. In 4 patients VT occurred spontaneously during lower body negative pressure; in 2, lower body negative pressure was the only intervention producing VT. During isoproterenol infusion VT occurred spontaneously in 2 patients; both had VT initiated during other interventions. Lower body negative pressure and isoproterenol increased VT rate, but did not prolong it. It is concluded that there is significant variability in arrhythmia responses to sympathetic augmentation, suggesting that additional covariables such as parasympathetic input and ventricular volume may also have a role in arrhythmia occurrence.

Evaluation of patients with syncope. Separating the 'wheat' from the 'chaff'.

Although fairly common, syncope is a frightening symptom that can signal a serious underlying disease. The varying causes present a diagnostic challenge to the examining physician, who needs to assess whether referral for more specialized or invasive study is appropriate. The authors review common cases of syncope and outline a practical approach to rapidly identifying high-risk patients--in other words, to separating the "wheat" from the "chaff."

Clinical, hemodynamic and sympathetic neural correlates of heart rate variability in congestive heart failure.

Heart rate (HR) variability has long been recognized as a sign of cardiac health. In the presence of heart disease, HR variability decreases, an observation that has been associated with poor prognosis in a number of recent studies. HR variability is particularly altered in congestive heart failure (CHF), a condition associated with a number of typical functional hemodynamic and neurohumoral alterations. The relation of measurements of HR variability to these abnormalities in patients with heart failure has not been carefully examined. Twenty-three patients (19 men, 4 women, mean age 49 years) with New York Heart Association class II to IV CHF were studied prospectively without cardiac medications; radionuclide ventriculography, right-sided heart catheterization, peroneal microneurography, plasma norepinephrine and 24- to 48-hour ambulatory electrocardiography were performed. Average RR interval and its standard deviation, and HR power spectrum (0 to 0.5, 0.05 to 0.15 and 0.2 to 0.5 Hz) were derived from the ambulatory electrocardiographic recordings and compared with left ventricular ejection fraction, thermodilution cardiac output, pulmonary arterial wedge pressure, New York Heart Association class, age, muscle sympathetic nerve activity (peroneal nerve) and norepinephrine level by linear regression. None of the measures of HR variability were significantly related to age, left ventricular ejection fraction, cardiac output or functional classification, whereas the 0.05 to 0.15 and 0.20 to 0.50 Hz components were weakly but significantly related to cardiac output (r = 0.49 and 0.42, p = 0.02 and 0.045, respectively). In contrast, a generally stronger and negative relation was demonstrated between spectral and nonspectral measurements of HR variability, and indicators of sympathoexcitation, muscle sympathetic nerve activity and plasma norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular tachycardia rate and morphology determine energy and current requirements for transthoracic cardioversion.

BACKGROUND: The electrical current and energy required to terminate ventricular tachyarrhythmias are known to vary by arrhythmia: Ventricular tachycardia (VT) is generally considered to require less energy than ventricular fibrillation (VF). The hypothesis of our study was that current requirements for transthoracic termination of VT are further determined by VT rate and QRS complex morphology. METHODS AND RESULTS: We prospectively studied 203 patients who received a total of 569 shocks for VT or VF by following a current-based protocol. This protocol recommended shocks for VT beginning at 18 A (70 +/- 22 J) and shocks for VF beginning at 25 or 30 A (137 +/- 52 J or 221 +/- 70 J). The ventricular tachyarrhythmias were subclassified as monomorphic VT (MVT): uniform QRS complex morphology on surface electrocardiogram and heart rate greater than 100 beats per minute; polymorphic VT (PVT): nonuniform QRS complex morphology and heart rate less than or equal to 300 beats per minute; or VF: nonuniform QRS complex morphology and heart rate greater than 300 beats per minute. We found that shocks of 18 A and 25 A for terminating MVT had success rates of 69% and 82%, respectively, whereas such low-current shocks were less successful for PVT (33% at 18 A) and for VF (19% at 18 A, 53% at 25 A). High-current shocks of 35 A and 40 A were equally successful for the three ventricular tachyarrhythmias. Subdividing MVT revealed that slower MVT (heart rate less than 200 beats per minute) had a significantly better success rate with low-current shocks of 18 A and 25 A than did faster MVT (greater than 200 beats per minute) (89% versus 72% success, p less than 0.01). Bundle branch block morphology, QRS axis, and duration of ventricular tachyarrhythmia did not alter current requirements. CONCLUSIONS: Heart rate and electrocardiographic degree of organization of ventricular tachycardia are important determinants of transthoracic energy and current requirements for cardioversion and defibrillation. Transthoracic termination of MVT requires relatively low current or energy, but PVT behaves more like VF and requires higher electrical current or energy.