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Biomedical subjects

L A Geddes

Publications and source records attributed to L A Geddes.

At least 109 records · Page 6Linked to original sources
PubMed1985

Electroventilation.

Electroventilation is a term used to describe the production of inspiration by applying rhythmic bursts of short duration stimuli to extrathoracic electrodes to stimulate motor nerves to the inspiratory muscles. In the dog, the optimum site for the electrodes was found to be on the upper chest wall, bilaterally. The inspired volume increased with increasing current intensity. The maximum tidal volume attainable was about four times resting tidal volume. The ability of electroventilation to maintain arterial blood oxygen saturation without the production of cardiac arrhythmias was demonstrated in pentobarbital-anesthetized dogs. The technique has several potential applications and offers promise in emergency and critical-care medicine.

Animals↗
PubMed1985

Choice of the optimum pulse duration for precordial cardiac pacing: a theoretical study.

In precordial pacing with skin-surface electrodes, the goal is to excite the ventricles with minimal stimulation of overlying tissues. A theoretical analysis is presented to identify the relevant factors; the two most important are electrode location and pulse duration. Using the basic law of stimulation, we developed a model which indicates that the optimum pulse duration for the closed-chest pacing pulse is one that is long with respect to the membrane time constant of cardiac muscle. Current-versus-duration curves are presented for pacing and pain, based on experimentally obtained data. For pacing with minimum pain, the optimum stimulus duration was found to be about 10 ms.

Adult↗
PubMed1985

Correlation of the cardiogenic air flow in the respiratory airway (i.e. the pneumocardiogram) with left ventricular stroke volume in dogs.

The pneumocardiogram (PNCG) is a record of the pulsatile flow of air in the trachea coincident with each heart beat. In 5 anesthetized mongrel dogs, the cardiogenic air flow through an endotracheal tube was measured with a research pneumotachograph. The correlation between the pneumocardiographic volume of air, obtained by integrating the PNCG over the period of ventricular ejection, and stroke volume (SV), measured by the saline-dilution method, was determined for a physiologic range of SV. Although the pneumocardiographic volume tracked measured SV, the former was always less. The range of correlation coefficients was 0.60 to 0.91. The results suggest that the PNCG may be suitable for continuous tracking of changes in SV on a noninvasive, beat-by-beat basis. The technique is ideally applicable to intubated human subjects, particularly those undergoing closed-chest surgical procedures.

Animals↗
PubMed1985

Indirect auscultatory systolic and diastolic pressures in the anesthetized dog.

The auscultatory method was used to obtain indirect systolic and diastolic pressures in 13 dogs anesthetized with either halothane or sodium pentobarbital (30 mg/kg of body weight). Korotkoff sounds were obtained, using a 1-cm (diameter) piezoelectric element cemented to the inner surface of a pediatric cuff (width 5.5 cm) which was placed on a shaved thoracic limb (membrum thoracicum). The signal from the piezoelement was amplified by a differential amplifier (30 to 200 Hz) and a commercially available audio amplifier. Indirect pressure (I) was compared with direct pressure (D) in the brachial, femoral, or carotid artery. The linear regression lines and correlation coefficients (r) for the data were as follows: systolic, I = 0.94 (D) + 1.1, r = 0.98; diastolic, I = 0.99 (D) + 3.2, r = 0.99. The quality of the Korotkoff sounds and the accuracy of the determinations were best in the halothane-anesthetized dogs. These results indicate that indirect auscultatory systolic and diastolic pressures are in excellent agreement with the directly measured pressures.

Anesthesia↗
PubMed1984

Cardiac, thoracic, and abdominal pump mechanisms in cardiopulmonary resuscitation: studies in an electrical model of the circulation.

To investigate alternative mechanisms generating artificial circulation during cardiopulmonary resuscitation (CPR), an electrical model of the circulation was developed. Heart and blood vessels were modeled as resistive-capacitive networks; pressures in the chest, abdomen, and vascular compartments as voltages; blood flow as electric current; blood inertia as inductance; and the cardiac and venous valves as diodes. External pressurization of thoracic and abdominal vessels, as would occur in CPR, was simulated by application of half-sinusoidal voltage pulses. Three modes of creating artificial circulation were studied: cardiac pump (CP), in which the atria and ventricles of the model were pressurized simultaneously; thoracic pump (TP), in which all intrathoracic elements of the model were pressurized simultaneously; and abdominal pump (AP), in which the abdominal aorta and inferior vena cava of the model were pressurized simultaneously. Flow was greatest with the CP, less with the TP, and least with the AP mechanism. However, the AP could be practically combined with either the CP or TP by interposition of abdominal compressions between chest compressions (IAC-CPR). Our model predicts that this combined method can substantially improve artificial circulation, especially when cardiac compression does not occur and chest compression invokes only the thoracic pump mechanism.

Abdomen↗
PubMed1984

Theoretical advantages of abdominal counterpulsation in CPR as demonstrated in a simple electrical model of the circulation.

Recent animal studies and preliminary clinical observations suggest that the addition of interposed abdominal compressions (IAC) to ventilation and chest compression of standard cardiopulmonary resuscitation (CPR) augments blood flow, blood pressures, and immediate survival. To investigate the physical basis for enhanced circulation during IAC-CPR, we developed an electrical model of the circulation. Heart and blood vessels were modeled as resistive-capacitive networks, pressures as voltages, blood flow as electric current, blood inertia as inductance, and the cardiac and venous valves as diodes. External pressurization of the heart and great vessels, as would occur in CPR, was simulated by application by half-sinusoidal voltage pulses between vascular capacitances and ground. Closed-chest CPR was simulated by pressurization of all intrathoracic capacitances. IAC was simulated by similar pressurization of the inferior vena cava and abdominal aorta, 180 degrees out of phase with chest compression. During simulation of CPR, IAC improved cranial and myocardial perfusion at all levels of chest compression pressure by amounts linearly related to peak abdominal pressure, suggesting that the abdomen can function as a second, independent blood pump during CPR. Brain and heart flow were improved further during simulated vasoconstriction in kidneys, abdominal viscera, and extremities. Based on the fundamental properties of the cardiovascular system represented in the model, abdominal counterpulsation provides a rational basis for flow augmentation during CPR.

Blood Circulation↗
PubMed1984

Safety factor for precordial pacing: minimum current thresholds for pacing and for ventricular fibrillation by vulnerable-period stimulation.

Temporary cardiac pacing by means of rapidly applied, precordial electrodes can be of great value in selected patients suffering cardiac arrest or profound bradycardia, post-defibrillation atrioventricular block, or digitalis intoxication. This study provides data pertaining to the safety of precordial pacing measured as the ratio of the minimum current required to induce ventricular fibrillation (VF) to the minimum current required to pace the ventricular myocardium. Single rectangular pulse stimuli having durations of 1-50 ms were evaluated. In 6 anesthetized dogs, a cutaneous electrode was centered over the shaved apex beat area of the left chest and paired with a larger electrode sutured to the right chest wall. Using a specially fabricated, synchronized, high-energy pulse generator, the "most vulnerable" time in the cardiac cycle at which the least current was required to induce VF with a single shock was identified. This current was compared to the much lower current required to pace the heart in the diastolic interval. While the current required to produce either fibrillation or pacing decreased as pulse duration increased, the safety factor (i.e., the ratio of fibrillation current to pacing current) remained nearly constant, averaging 12.6 for all pulse durations examined. That is, on the average, a stimulus of any given duration between 1 and 50 ms required at least 12 times the current required for pacing to produce ventricular fibrillation. We conclude that in normal canine hearts, the risk of inducing VF during precordial pacing is small.

Animals↗
PubMed1984

Precordial pacing windows.

The threshold peak current for cardiac pacing with 10 ms rectangular pulses applied to precordial electrodes was determined in 18 anesthetized dogs (average weight 20.8 kg). The entire left and right precordium was mapped with a 1 cm diameter (cathode) electrode, paired with a 10 cm diameter reference electrode on the opposite chest. It was found that the lowest current required for pacing corresponded to the region of the apex beat on the left chest. At this site the average pacing threshold with the 1 cm electrode was 34.3 mA (peak). No sharply defined low pacing current area was found on the right chest, indicating that the location for the right chest electrode is not critical. The term "pacing window" is introduced to identify the region on the precordium where the pacing threshold is lowest. Studies were also performed to determine the manner in which the pacing current increased as electrode diameter was increased beyond the diameter of the pacing window. It was determined that little is to be gained with a left chest electrode larger than 5 cm in diameter in this animal study.

Animals↗
PubMed1984

Increasing cardiac rate by measurement of right ventricular temperature.

A pacemaker that increases pacing rate in response to exercise enables the pacemaker patient to improve his state of well-being. Possible sensors include atrial rate, Q-T interval, pH, venous oxygen saturation, respiratory rate, cardiac output, body motion, and blood temperature. To determine the efficacy of right ventricular blood temperature as an indicator of exercise, an experimental temperature-controlled pacemaker was developed and was evaluated in dogs. A resulting algorithm was implemented in a microcomputer-based VVI pacemaker which reliably detected exercise and rest from blood temperature when totally implanted. The pacemaker generated two pacing rates to provide increased cardiac output. Data is being obtained to establish feasibility in humans.

Animals↗