Conduction of electrical current to and through the human body: a review.

OBJECTIVE: The objective of this article is to explain ways in which electric current is conducted to and through the human body and how this influences the nature of injuries. METHODS: This multidisciplinary topic is explained by first reviewing electrical and pathophysiological principles. There are discussions of how electric current is conducted through the body via air, water, earth, and man-made conductive materials. There are also discussions of skin resistance (impedance), internal body resistance, current path through the body, the let-go phenomenon, skin breakdown, electrical stimulation of skeletal muscles and nerves, cardiac dysrhythmias and arrest, and electric shock drowning. After the review of basic principles, a number of clinically relevant examples of accident mechanisms and their medical effects are discussed. Topics related to high-voltage burns include ground faults, ground potential gradient, step and touch potentials, arcs, and lightning. RESULTS: The practicing physician will have a better understanding of electrical mechanisms of injury and their expected clinical effects. CONCLUSIONS: There are a variety of types of electrical contact, each with important characteristics. Understanding how electric current reaches and travels through the body can help the clinician understand how and why specific accidents occur and what medical and surgical problems may be expected.

Loose cuff hypertension.

In this study, an analysis of the effects of cuff looseness on mean blood pressure readings was performed. Using a standard adult blood pressure cuff, pressure readings were taken on each arm at a cuff looseness of 0, 2, 4, and 6 cm beyond patient arm circumference. The cuff was then switched to the opposite arm and the procedure repeated. Blood pressure readings taken from the left arm with the cuff at an appropriately snug fit served as the reference. Increasing cuff looseness simulates the possibly incorrect blood pressure cuff placement by health care workers in the clinical setting. Data from 24 subjects support the claims that mean blood pressure increases with respect to increasing cuff looseness. It was shown that measurements taken on left and right arms will result in significantly different blood pressure readings (p < 0.001). It is therefore crucial to properly place the cuff at a snug fit on the patient's arm for each measurement procedure, to prevent false readings. Lack of consistent cuff size and snugness procedures can lead to misdiagnosis of hypertension, acute patient discomfort, and inconvenient costs to the patient and health care provider.

Methods for calculating coronary perfusion pressure during CPR.

Coronary perfusion pressure (CPP) is a major indicator of the effectiveness of cardiopulmonary resuscitation in human and animal research studies, however, methods for calculating CPP differ among research groups. Here we compare the 6 published methods for calculating CPP using the same data set of aortic (Ao) and right atrial (RA) blood pressures. CPP was computed using each of the 6 calculation methods in an anesthetized pig model, instrumented with catheters with Cobe pressure transducers. Aortic and right atrial pressures were recorded continuously during electrically induced ventricular fibrillation and standard AHA CPR. CPP calculated from the same raw data set by the 6 calculation methods ranged from -1 (signifying retrograde blood flow) to 26 mmHg (mean +/- SD of 15 +/- 11 mmHg). The CPP achieved by standard closed chest CPR is typically reported as 10-20 mmHg. Within a single study the CPP values may be comparable; however, the CPP values for different studies may not be a reliable indicator of the efficacy of a given CPR method. Electronically derived true mean coronary perfusion pressure is arguably the gold standard method for representing coronary perfusion pressure.

An optical oscillometric method for neonatal and premature infant blood pressure monitoring.

PURPOSE: A preliminary study of a new optical oscillometric method to noninvasively measure systolic, mean, and diastolic blood pressures, in addition to heart and respiratory rates in very small extremities, is described. It employs transillumination of an extremity and measures the optical oscillation amplitude during cuff deflation from suprasystolic to zero pressure. The amplitude of the optical pulsatile oscillations is similar to that produced with the conventional pneumatic oscillometric method; however, the pulsatile optical signal is much larger and is present at all times when the cuff is deflated. METHODS AND DESIGN: Two types of blood pressure verification studies were performed: (1) a weanling piglet study using a weanling piglet tail and (2) a human study using the little fingers of adult participants. For the weanling piglet study, direct femoral artery pressure, tail-cuff pressure, and optical oscillations were recorded in 5 anesthetized weanling piglets ranging in weight from 2 to 4 kg. Ten measurements were made in the pressure range of 30 to 175 mm Hg. For human study, data were obtained from 23 adult participants of both sexes with a little finger circumference of 4 cm or less. Radial artery pressure, measured with the conventional pneumatic oscillometric method, was used as the standard and was compared with the simultaneous optical oscillometric pressure in the little finger of the opposite arm. MAIN OUTCOME MEASURES: This is an initial study demonstrating the optical oscillometric technique as a viable alternative for noninvasive blood pressure measurement in low birth-weight infants. PRINCIPAL RESULTS: The weanling piglet data show a high correlation between direct arterial pressure and this new optical oscillometric method over a pressure range of approximately 30 to 175 mm Hg. The correlation coefficients of linear regression were 0.93, 0.93, and 0.91, respectively. The human little finger data show a high correlation between the pneumatic oscillometric mean arterial pressure and this new optical oscillometric method over a pressure range of approximately 40 to 140 mm Hg. The correlation coefficient of linear regression was 0.87. CONCLUSIONS: This new optical oscillometric technique simplifies noninvasive blood pressure measurement because it was designed specifically for small-diameter extremities such as those found in low birth-weight infants. This new optical oscillometric device has the added benefit of continually monitoring pulse and respiration rates.

Electrophysiology of connection current spikes.

Connection to a 60-Hz or other voltage source can result in cardiac dysrhythmias, a startle reaction, muscle contractions, and a variety of other physiological responses. Such responses can lead to injury, especially if significant ventricular cardiac dysrhythmias occur, or if a person is working at some height above ground and falls as a result of a musculoskeletal response. Physiological reactions are known to relate to intensity and duration of current exposure. The connection current that flows is a function of the applied voltage at the instant of connection, and the electrical impedance encountered by the voltage source in contact with the skin or other body tissues. In this article we describe a rarely investigated phenomenon, namely a contact, or connection, current spike that is many times higher than the steady-state current. This current spike occurs when an electrical connection is made at a non-zero voltage time in a sine wave or other waveform. Such current spikes may occur when electronic or manual switching or connecting of conductors occurs in electronic instrumentation connected to a patient. These findings are relevant to medical devices and instrumentation and to electrical safety in general.

Rhythmic abdominal compression CPR ventilates without supplemental breaths and provides effective blood circulation.

OBJECTIVES: Standard chest-compression CPR has an out-of-hospital resuscitation rate of less than 10% and can result in rib fractures or mouth-to-mouth transfer of infection. Recently, we introduced a new CPR method that utilizes only rhythmic abdominal compressions (OAC-CPR). The present study compares ventilation and hemodynamics produced by chest and abdominal compression CPR. METHODS: Twelve swine (29-34kg) were anesthetized, intubated and allowed to breathe spontaneously. Physiologic dead space, resting tidal volume, compression-induced lung air flow, and blood pressures were recorded. Ventricular fibrillation (VF) was electrically induced and subjects were treated with either standard CPR or OAC-CPR at various force and rate settings. Minute alveolar ventilation (MAV) and mean coronary perfusion pressure (CPP) were compared. RESULTS: For OAC-CPR, ventilation per compression tended to increase with increasing force and decreasing rate. Chest only compressions produced no MAV, while OAC-CPR at 80cycles/min or less, matched the MAV for spontaneous respiration. For all rates, abdominal compressions met, or exceeded, the CPP of chest compressions performed at 100lbs. CONCLUSIONS: OAC-CPR generated ventilatory volumes significantly greater than the dead space and produced equivalent, or larger, CPP than with chest compressions. Thus, OAC-CPR ventilates a subject, eliminating the need for mouth-to-mouth breathing, and effectively circulates blood during VF without breaking ribs. Furthermore, this technique is simple to perform, can be administered by a single rescuer, and should reduce bystander reluctance to administer CPR.

A new cardiopulmonary resuscitation method using only rhythmic abdominal compression: a preliminary report.

This article introduces 2 new cardiopulmonary resuscitation (CPR) concepts: (1) the use of only rhythmic abdominal compression (OAC) to produce blood flow during CPR with ventricular fibrillation and (2) a new way of describing coronary perfusion effectiveness, namely, the area between the aortic and right atrial pressure curves, summed over 1 minute, the units being millimeters of mercury per second. We call this unit the coronary perfusion index (CPI). True mean coronary perfusion pressure is CPI/60. We also relate CPI during CPR with ventricular fibrillation to the CPI for the normally beating heart in the same animal, obtained before each experiment. This 11-pig (25-35 kg) study compares the CPI for standard chest-compression CPR and that obtained with OAC-CPR. The coronary perfusion ratio for OAC-CPR compared with standard chest-compression CPR was 1.6 +/- 0.73 (P = .024). In other words, OAC-CPR produced 60% more coronary perfusion than standard chest-compression CPR, with no damage to visceral organs.

Sustained abdominal compression during CPR raises coronary perfusion pressures as much as vasopressor drugs.

OBJECTIVES: This study investigated sustained abdominal compression as a means to improve coronary perfusion pressure (CPP) during cardiopulmonary resuscitation (CPR) and compared the resulting CPP augmentation with that achieved using vasopressor drugs. METHOD: During electrically induced ventricular fibrillation in anesthetized, 30kg juvenile pigs, Thumper CPR was supplemented at intervals either by constant abdominal compression at 100-500mmHg using an inflated contoured cuff or by the administration of vasopressor drugs (epinephrine, vasopressin, or glibenclamide). CPP before and after cuff inflation or drug administration was the end point. RESULTS: Sustained abdominal compression at >200mmHg increases CPP during VF and otherwise standard CPR by 8-18mmHg. The effect persists over practical ranges of chest compression force and duty cycle and is similar to that achieved with vasopressor drugs. Constant abdominal compression also augments CPP after prior administration of epinephrine or vasopressin. CONCLUSIONS: During CPR noninvasive abdominal compression with the inflatable contoured cuff rapidly elevates the CPP, sustains the elevated CPP as long as the device is inflated, and is immediately and controllably reversible upon device deflation. Physical control of peripheral vascular resistance during CPR by abdominal compression has some advantages over pharmacological manipulation and deserves serious reconsideration, now that the limitations of pressor drugs during CPR have become better understood, including post-resuscitation myocardial depression and the need for intravenous access.

Chest compression force of trained and untrained CPR rescuers.

The objective of this study was to measure the force exerted by 83 trained CPR rescuers and 104 untrained adult laypersons (college students and staff). A bathroom scale was used to measure the force exerted by these subjects with their hands on the bathroom scale in the CPR position. The weight range for both groups was the same. Of the trained rescuers, 60% pressed with more than 125 lbs, whereas only 37% of the laypersons pressed with more than 125 lbs. In view of the American Heart Association (AHA) guidelines (2000) to depress the chest 1.5 to 2 inches, which requires 100-125 lbs, it would appear that most laypersons do not exert enough force for effective CPR.

The natural biochemical changes during ventricular fibrillation with cardiopulmonary resuscitation and the onset of postdefibrillation pulseless electrical activity.

OBJECTIVE: The objective of this study was to document the biochemical changes during ventricular fibrillation (VF) with cardiopulmonary resuscitation (CPR), and to identify factors associated with postdefibrillation pulseless electrical activity (PD-PEA). BACKGROUND: It has been reliably estimated that as much as 60% of out-of-hospital sudden cardiac death can be attributed to the onset of PD-PEA (Niemann JT, Cruz B, Garner D et al. Immediate countershock versus CPR before countershock in a 5-minute swine model of ventricular fibrillation arrest. Ann Emerg Med 2000;36:543-6). Previous attempts to treat reversible causes of pulseless electrical activity have not been successful clinically (Niemann JT, Stratton SJ, Cruz B, Lewis RJ. Outcome of out-of-hospital postcountershock asystole and pulseless electrical activity versus primary asystole and pulseless electrical activity. Crit Care Med 2001;29:2366-70). METHODS: This investigation used 22 studies on 14 anesthetized pigs breathing 100% oxygen. Ventricular fibrillation was induced with a right ventricular catheter electrode, and the chest was compressed with a pneumatically driven Chest Thumper (Michigan Instruments) (80-100 lb at 60/min). The electrocardiogram and aortic pressure were recorded continuously. Arterial pH, P(O2), P(CO2), Na+, K+, Ca2+, Cl-, SaO2, glucose, hematocrit, and hemoglobin level were measured at selected times. Ventricular defibrillation was achieved with transchest electrodes. RESULTS: Typically, during VF with CPR, mean aortic pressure was 20 to 25 mm Hg. In all cases aortic P(O2) decreased to about 20% of the initial value in 10 minutes, and aortic blood K+ increased by 50% in 6 minutes. By 5 to 8 minutes, the incidence of PD-PEA was 50%. CONCLUSION: Ventricular fibrillation duration, arterial K+, and arterial P(CO2) were statistically correlated with the onset of PD-PEA in this study. In addition, trends suggest an association of mean arterial blood pressure and arterial P(O2) with the onset of PD-PEA.

Heel calf capillary-support pressure in lithotomy positions.

A three-part analysis was undertaken to increase understanding of the occurrence of pressure ulcers in lithotomy positions. An innovative measuring device was used to determine capillary pressure. Ankle blood pressure was measured compared to ankle height in 11 participants. Ankle systolic and diastolic pressure decreased approximately 20 mmHg per foot of elevation. Calf and heel capillary-support pressures were measured in 15 participants in the standard lithotomy position. Capillary-support pressure for the calf was substantially less than for the heel. Heel capillary-support pressures were measured in 16 participants in the high lithotomy position. As heel height increased, capillary-support pressure also increased.

The duration of ventricular fibrillation required to produce pulseless electrical activity.

The duration of untreated (no cardiopulmonary resuscitation) ventricular fibrillation (VF) needed to produce postdefibrillation pulseless electrical activity (PEA) was determined in 9 anesthetized swine ranging in weight from 20 to 30 kg. VF was induced electrically by a right ventricular catheter electrode, while arterial pressure and the electrocardiogram were recorded. VF was confirmed by the presence of VF waves in the electrocardiogram and a loss of pulsatile arterial pressure. VF was allowed to persist for 15-second increments (eg, 15, 30, 45, etc), after which defibrillation was achieved with transchest electrodes and the presence or absence of PEA was noted. If PEA was present, rhythmic chest compressions were applied to rescue the animal. Just after initiation of VF and just before defibrillation, VF wave frequency was measured. PEA was encountered in 100% of the trials after 180 seconds of VF. The threshold duration for PEA was 60 seconds. VF wave frequency decreased with the passage of time. At VF initiation, VF wave frequency (f0) ranged from 6 to 15 per second, with a mean of 10.1+/-2.1 per second. At 180 seconds (f180), the mean frequency was 4.0+/-0 per second. It was only possible to eliminate PEA and restore pumping in 1 animal when untreated VF lasted more than 180 seconds. There was no clear transition in the frequency of the VF waves with the passage of time that could predict the possibility of postdefibrillation PEA. Moreover, because of the different initial VF wave frequencies and the different rates of decrease with time, a measurement of VF wave frequency is unlikely to be informative on how long VF had been present. A consistent finding in this swine study of prolonged untreated VF was a rise in blood K+ which increased from a normal prefibrillation value of about 4 mEq/L to 8 to 12 mEq/L at 180 seconds. The longer the duration of VF, the higher the K+.

Evolution of our knowledge of sudden death due to commotio cordis.

Commotio cordis refers to circulatory arrest due to a nonpenetrating blow to the chest. First discovered in 1932 in a study using large rabbits, it came to the attention of clinicians who encountered children dying suddenly from a chest blow while engaging in sports activities. This review traces the history of commotio cordis, establishes the conditions necessary for sudden death from a nonpenetrating chest blow, and presents the first ECG record showing that a chest blow landing in the ventricular vulnerable period can produce ventricular fibrillation. The conditions necessary for sustaining ventricular fibrillation and numerous examples of sudden death by commotio cordis are presented.

Accuracy limitations of chronaxie values.

The strength-duration curve is a plot of the threshold current (I) versus pulse duration (d) required to stimulate excitable tissue. On this curve are two points: 1) rheobase (b) and 2) chronaxie (c). Rheobase is the threshold current for an infinitely long-duration stimulus. Chronaxie, the excitability constant, is the duration of a pulse of current of twice rheobasic strength. The mathematical expression for the strength-duration curve is I = b(1 + c/d). Although there are many published values for chronaxie for various excitable tissues, the range of variability for a given tissue type is quite large. This paper identifies five factors that can affect the accuracy of chronaxie measurement and shows that the most reliable values can be obtained with a rectangular pulse delivered from a constant-current source.

Biaxial strength of multilaminated extracellular matrix scaffolds.

Xenogeneic extracellular matrix (ECM) can be harvested and configured to function as a bioscaffold for tissue and organ reconstruction. The mechanical properties of the ECM vary depending upon the tissue from which it is harvested. Likewise, the manufacturing steps required to develop ECMs into medical grade devices will affect the surface morphology and the mechanical properties of the bioscaffold; important properties for constructive tissue remodeling. The present study compared the ball-burst strength of five different ECM scaffolds before and after treatment with peracetic acid (PAA): porcine small intestinal submucosa (SIS), porcine urinary bladder submucosa (UBS), porcine urinary bladder matrix (UBM), a composite of UBS + UBM, and canine stomach submucosa (SS). This study also compared the mechanical properties of 2- and 4-layer ECM scaffolds. Results showed 2-layer SS devices had the highest ball-burst value of all 2-layer ECM devices. Moreover, all 4-layer ECM devices had similar ball-burst strength except for 4-layer UBM devices which was the weakest. PAA-treatment decreased the ball-burst strength of SS and increased the ball-burst strength of UBS 2-layer devices. This study showed the material properties of the ECM scaffolds could be engineered to mimic those of native soft tissues (i.e. vascular, musculotendinous, etc) by varying the number of layers and modifying the disinfection/sterilization treatments used for manufacturing.