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Biomedical subjects

R Fletcher

Publications and source records attributed to R Fletcher.

At least 37 records · Page 2Linked to original sources
PubMed1990

Increasing fibrillation duration enhances relative asymmetrical biphasic versus monophasic defibrillator waveform efficacy.

Biphasic waveforms reduce defibrillation threshold compared with corresponding monophasic waveforms. However, effects of fibrillation duration on relative efficacy of monophasic and biphasic waveforms are unknown. This study used a newly developed defibrillation model, the isolated right- and left-sided working rabbit heart, with epicardial defibrillation electrodes, to compare threshold for a monophasic waveform (5 msec rectangular) and an asymmetrical biphasic waveform (5 msec each pulse, V2 = 50% V1). Mean voltage defibrillation threshold (V50) was determined from sigmoidal probability of successful defibrillation versus shock intensity curves after 5, 15, and 30 seconds of fibrillation in a paired study with 10 hearts. Results showed that biphasic waveforms had significantly lower voltage and energy thresholds at all fibrillation durations and that their relative efficacy improved with increasing fibrillation duration. Biphasic voltage threshold was 38.2 +/- 2.2, 44.7 +/- 4.8, and 46.6 +/- 3.2 V after 5, 15, and 30 seconds of fibrillation compared with monophasic thresholds of 51.7 +/- 4.4 (p less than 0.002), 63.0 +/- 7.6 (p less than 0.05), and 72.1 +/- 3.9 V (p less than 0.005). Biphasic waveform energy threshold was 0.67 that for the monophasic waveform after 5 seconds of fibrillation (0.12 +/- 0.01 versus 0.18 +/- 0.03 J, p less than 0.05). The ratio between biphasic waveform threshold and monophasic waveform threshold (B/M) decreased to 0.62 at 15 seconds. At 30 seconds, B/M was 0.52 (0.17 +/- 0.02 versus 0.33 +/- 0.04 J, p less than 0.02). This study also showed that biphasic waveform threshold was a nonlinear function of monophasic waveform threshold so that improved biphasic defibrillator waveform efficacy was greatest for hearts having higher monophasic thresholds.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Limitations of cough in maintaining blood flow during asystole: assessment by two-dimensional and Doppler echocardiography.

It has been previously demonstrated that patients can maintain consciousness by vigorously coughing during episodes of prolonged ventricular asystole or ventricular fibrillation. Using two-dimensional Doppler echocardiography and arterial blood pressure recordings, we evaluated the changes that occur during coughing in nine patients who were dependent on pacemakers and in whom periods of asystole could be induced. During asystole in each patient the mitral valve stayed partially open and the left ventricular area comprised 87% and 90% (apical four-chamber view, short-axis view, respectively) of the area during diastole of paced rhythm. Peak arterial pressure during coughing and asystole was 80% of peak systolic pressure of paced rhythm (P = 0.001). Left ventricular area during coughing was 110% (both apical four-chamber view and short-axis views) of the area during asystole when patients were not coughing. Both the mitral and aortic valves showed no appreciable motion during coughing. During coughing the mean flow velocity across the mitral valve was 0.06 +/- 0.03 m/sec and the flow velocity integral was 0.03 +/- 0.02 cm. Thus coughing during asystole produced minimal flow despite a rise in brachial arterial pressure.

Aged↗
PubMed1989

Exercise blood pressure response and left ventricular hypertrophy.

Although hypertension is the major cause of left ventricular hypertrophy (LVH), numerous studies failed to demonstrate a close correlation between resting blood pressure (BP) and degree of LVH. Some authors have shown better correlation between BP at work and left ventricular mass (LV mass), whereas other studies supported an association between catecholamines or angiotensin II and LV mass. In this study we investigated the relationship of resting and exercise BP and catecholamines to the degree of LVH. Nineteen patients with established mild to moderate hypertension were studied. Blood pressure was measured following a ten-minute rest and every three minutes during exercise using a Bruce protocol. Electrolytes, epinephrine (EP), and norepinephrine (NE) were measured at rest, at peak exercise, and at ten-minutes postexercise. Resting BP averaged 154 +/- 24/99 +/- 9 mm Hg and at three minutes of exercise 195 +/- 30/101 +/- 6 (P less than .001). Resting EP was 51 +/- 20 pg/mL, NE 314 +/- 187, and at peak exercise EP was 107 +/- 61 (P less than .001) and NE 1016 +/- 566 (P less than .001). The average LV mass was 277 +/- 85 g. A significant correlation was found only between systolic BP at three minutes of exercise and LV mass (r = .479, P less than .04). No other variable correlated significantly with LV mass. These data suggest that systolic BP achieved at low level of exercise (5 mets), corresponding to usual daily activities, may be the most important determinant of LVH in patients with hypertension.

Blood Pressure↗
PubMed1989

Relationship between alveolar deadspace and arterial oxygenation in children with congenital cardiac disease.

Fifty-eight children were studied during nitrous oxide in oxygen and fentanyl anaesthesia before undergoing closed or open cardiac surgery. FlO2 was 0.5. Alveolar deadspace was measured using the carbon dioxide single breath test (SBT-CO2) obtained from a computerized online system for monitoring expired CO2 and airway flow, based on the Servo ventilator, Arterial blood was sampled simultaneously for measurement of PaCO2 and PaO2. There was a marked reciprocal relationship between PaO2 and the alveolar deadspace fraction. In children with a normal pulmonary circulation and good oxygenation, alveolar deadspace fraction was approx. 0.05. Shunts which reduced PaO2 to 10 kPa produced a deadspace fraction of 0.15. When PaO2 was 3-4 kPa, alveolar deadspace fraction was approx. 0.4. In well-oxygenated children, alveolar deadspace fraction was only slightly greater than predicted by a model of the effects of pure right-to-left shunting. In severely cyanotic children, the discrepancy between predicted and observed VDalv/VTalv was greater. The mean arterial-end-tidal CO2 difference was zero in children in whom PaO2 was greater than 10 kPa, despite a measurable alveolar deadspace. In severely hypoxic children, the difference was 1-2 kPa. In a retrospective analysis of published data from anaesthetized adults without intracardiac shunting, no relationship was found between alveolar deadspace and PaO2.

Anesthesia, General↗
PubMed1989

Effects of lung surgery and one-lung ventilation on pulmonary arterial pressure, venous admixture and immediate postoperative lung function.

We studied 17 patients, with unilateral lesions in lung or thoracic wall, during thoracotomy performed in the full lateral position. Balanced anaesthesia was used with pethidine and 50% nitrous oxide, in oxygen. The procedure included a period of one-lung ventilation (OLV). Haemodynamic and gas exchange measurements were performed before and after pleurotomy, during OLV, after re-expansion of the lung and after closure of the thoracic wall. The function of each lung was assessed separately during the first and last stages. Mean venous admixture was 9-12% before and after OLV and 31% during OLV. There was a positive correlation between venous admixture and pulmonary arterial pressure during OLV. End-tidal PCO2, carbon dioxide elimination and compliance of the operated side were reduced significantly at the end of the procedure; this is consistent with reduced blood flow and increased water content in that lung.

Anesthesia, General↗
PubMed1989

Changes in the arterial to end-tidal PCO2 differences during coronary artery bypass grafting.

The arterial to end-tidal PCO2 difference, PaCO2-PE,CO2, was measured at four different stages during coronary artery bypass grafting in 43 patients: 1) before sternotomy; 2) after sternotomy, sternum retracted; 3) after bypass, sternum still retracted; 4) sternum closed. Mean PaCO2-PE,CO2 initially was 0.6 kPa and changed scarcely at all during the procedure. There were, however, some moderate individual changes in PaCO2-PE,CO2 during surgery, the range of changes compared to the initial value being from -1.4 to +1.3 kPa. The standard deviations for the changes from stage to stage were 0.3-0.4 kPa. PaCO2-PE,CO2 changed by more than 0.5 kPa on only 12 occasions in ten patients. The limitations of PaCO2-PE,CO2 as an index of the alveolar deadspace fraction and the efficiency of ventilation are discussed.

Adult↗
PubMed1989

Monitoring of physiological parameters during high frequency ventilation (HFV).

Non-biological descriptors such as jet feeding pressure and oscillator stroke volume are often used to describe HFV. This results in confusion and hinders acceptance of HFV. The goal of this paper is to show how physiological parameters which are valid during HFV can be monitored. Airway pressure measured in narrow tubes with high linear flow rates is underestimated. A relevant airway pressure must be measured well below the tracheal tube. Pressure measured higher up should be validated against peripheral pressure measurements. Minute ventilation and expired CO2 concentration can be determined with a ServoVentilator and a CO2 analyzer arranged at its exit port. Minute ventilation and CO2 elimination can thereby be continuously monitored during high frequency jet ventilation or so-called "combined high frequency jet ventilation" to prevent undetected disturbance of ventilation and perfusion. Physiological dead space can be studied for optimization of ventilatory pattern. The principle of gas analysis at the exit port of the ventilator may be used for FRC determinations with sulfur-hexafluoride.

Animals↗
PubMed1989

Advances in catheter ablation use of unipolar electrograms.

Ventricular tachycardia in man can be eliminated by relatively small lesions in the reentrant circuit. This report includes a review of available energy sources, and methods for localizing arrhythmias. Methods to assure contact and prevent perforation using low frequency electrograms are presented including the new finding of reverse ST deflection with contact. Experience with laser energy in dogs showed discrete homogenous lesions. When compared with DC shock the animals showed far less arrhythmia and the lack of far field effect greatly reduced echo abnormalities in the post shock period. Studies with radiofrequency show ability to produce localized lesions similar to the laser but with a more flexible catheter. Localization requires a correlation of techniques including pacemapping, activation maps and pacing during tachycardia. Early activation (less than -60 ms) at times 180-320 ms, with comparable pace to QRS during tachycardia with no change in morphology best localized the slow zone of the reentrant circuit. The low frequency unipolar electrogram from the tip and immediately proximal electrode revealed contact with ST deviation. The distal deviation was always greater than the proximal RV free wall and posterior basal produced depression rather than elevation of the electrogram. While the mechanism of ST reversal with contact is not understood and may relate to the type of indifferent reference used (Wilson central terminal), the ST depression reveals the same information about contact that elevation does in most areas of the heart studied in our patients.

Animals↗
PubMed1988

Carbon dioxide production in cyanotic children during anaesthesia with controlled ventilation.

Carbon dioxide production and ventilatory efficiency were measured during undisturbed anaesthesia with intermittent positive pressure ventilation in 34 children about to undergo closed or open cardiac surgery. Anaesthesia was provided with fentanyl or halothane and nitrous oxide. There were 15 cyanotic and 19 acyanotic children. Children with cyanotic heart disease produced approximately 20% less carbon dioxide per unit body weight than acyanotic children, but ventilation was approximately 20% less efficient. Adequate ventilation should therefore be obtained when "normal" ventilation in relation to body weight is used in cyanotic children.

Anesthesia, Inhalation↗
PubMed1988

Invasive and noninvasive measurement of the respiratory deadspace in anesthetized children with cardiac disease.

To compare the magnitude of the different "invasive" and "noninvasive" dead space variables and the effect on them of ventilator setting, CO2 single breath tests (SBT-CO2) were obtained using an on-line computerized system based on the Servo ventilator and CO2 Analyzer 930, in 50 children anesthetized for cardiac surgery. The variables were the airway deadspace (VDaw), Bohr's deadspace (VDBohr) obtained noninvasively using end-tidal PCO2 (PETCO2) for alveolar PCO2 in the deadspace equation, and the physiologic deadspace, VDphys. In 42 children with normal single breath tests, VDaw was two-thirds of VDBohr; in 9 children in whom phase III of SBT-CO2 (the "alveolar plateau") was steeper than normal, it was only half of VDBohr. Steeper slopes of phase III were seen particularly in the present of left-right (LR) shunting. VDphys was very similar in magnitude to VDBohr in all children, except those with right-left (RL) shunts. VDaw was the major component of VDphys only in children with normal arterial-end-tidal PCO2 differences, i.e., those without RL shunts. When two ventilator frequencies giving the same alveolar ventilation were compared in children with normal gas exchange, VDBohr as a fraction of tidal volume was least at the lower frequency, as it also is in adults. The data confirm that noninvasive CO2 monitoring and measurement of deadspace gives useful indexes of the adequacy of ventilation in all children except those with RL shunts.

Anesthesia↗