Methohexital impairs osmoregulation. Studies in conscious and anesthetized volume-expanded dogs

BACKGROUND

Anesthetic agents influence central regulations. This study investigated the effects of methohexital anesthesia on renal and hormonal responses to acute sodium and water loading in dogs in the absence of surgical stress.

METHODS

Fourteen experiments (two in each dog) were performed in seven well-trained, chronically tracheotomized beagle dogs kept in highly standardized environmental and dietary conditions (2.5 mmol sodium and 91 ml water/kg body weight daily). Experiments lasted 3 h, while the dogs were conscious (7 experiments) or, after 1 h control, while they were anesthetized (7 experiments) with methohexital (initial dose 6.6 mg/kg body weight and maintenance infusion 0.34 mg.min-1.kg-1 body weight) over a period of 2 h. In both experiments, extracellular volume expansion was performed by intravenous infusion of a balanced isoosmolar electrolyte solution (0.5 ml.min-1.kg-1 body weight). Normal arterial blood gases were maintained by controlled mechanical ventilation. In another five dogs the same protocol was used, and vasopressin (0.05 mU.min-1.kg-1 body weight) was infused intravenously during methohexital anesthesia.

RESULTS

Values are given as means. During methohexital anesthesia, mean arterial pressure decreased from 108 to 101 mmHg, and heart rate increased from 95 to 146 beats/min. Renal sodium excretion decreased; urine volume increased; and urine osmolarity decreased from 233 to 155 mosm/l, whereas plasma osmolarity increased from 301 to 312 mosm/l because of an increase in plasma sodium concentration from 148 to 154 mmol/l. Plasma renin activity, plasma aldosterone concentration, plasma atrial natriuretic peptide, and plasma antidiuretic hormone concentrations (range 1.8-2.8 pg/ml) did not change in either protocol. In the presence of exogenous vasopressin (antidiuretic hormone 3.3 pg/ml), water diuresis did not occur, and neither plasma osmolarity nor the plasma concentration of sodium changed.

CONCLUSIONS

Methohexital may impair osmoregulation by inhibiting adequate pituitary antidiuretic hormone release in response to an osmotic challenge.

A new method for three-dimensional reconstruction and animation of the temporomandibular joint

The aim of this project was to develop a new method for the three-dimensional reconstruction and animation of the temporomandibular joint (TMJ). Magnetic resonance (MR) tomograms of the TMJ were combined, using an extraorally placed reference system, with jaw motion data, recorded with six degrees of freedom, by means of the opto-electronic tracking system Jaws-3D. The three-dimensional reconstruction of the TMJ was calculated and animated on a graphics workstation by means of kinematic transformations. A subject without any past or present history of myoarthropathies of the masticatory system, performed jaw opening and closing and chewed chewing gum. The animation provided a three-dimensional visualisation of the movements of the entire condyle within the fossa. The condyle-fossa distance was computed for every condylar point and represented by shading the surface of the condyle with pseudo-colours. The position of the minimum distance between condyle and fossa was calculated and displayed in a plane graph representing the condylar surface.

Time-resolved detection of structural changes during the photocycle of spin-labeled bacteriorhodopsin

Bacteriorhodopsin was selectively spin labeled at residues 72, 101, or 105 after replacement of the native amino acids by cysteine. Only the electron paramagnetic resonance spectrum of the label at 101 was time-dependent during the photocycle. The spectral change rose with the decay of the M intermediate and fell with recovery of the ground state. The transient signal is interpreted as the result of movement in the C-D or E-F interhelical loop, or in both, coincident with protonation changes at the key aspartate 96 residue. These results link the optically characterized intermediates with localized conformational changes in bacteriorhodopsin during the photocycle.

Inferior vena caval pressure increase contributes to sodium and water retention during PEEP in awake dogs

This study compared the hemodynamic, renal, and hormonal effects of an experimentally induced increase in inferior vena caval pressure (IVCP) [to the same extent as during controlled mechanical ventilation (CMV) with positive end-expiratory pressure (PEEP)] with those of CMV with PEEP. Six volume-expanded conscious chronically tracheotomized dogs were studied under three conditions:

CONTROL

4 h of spontaneous breathing at 4 cmH2O continuous positive airway pressure (CPAP); CMV: CPAP for the 1st and 4th h and CMV with PEEP for the 2nd and 3rd h, resulting in a mean airway pressure of 20 cmH2O; and Increased IVCP: 4 h of CPAP, with IVCP increased during the 2nd and 3rd h by inflation of a chronically implanted cuff. Urine volume, sodium excretion, and fractional sodium excretion decreased during the 2nd and 3rd h of CMV and during increased IVCP compared with CONTROL. Glomerular filtration rate, mean arterial pressure, and antidiuretic hormone, atrial natriuretic peptide, and aldosterone plasma concentrations were not affected by CMV or Increased IVCP. Plasma renin activity decreased during CONTROL and Increased IVCP conditions but remained elevated during the 2nd and 3rd h of CMV. We conclude that, in conscious extracellular volume-expanded dogs, IVCP elevation contributes considerably to the water- and sodium-retaining effect of short-term CMV with PEEP.

Extracellular volume expansion inhibits antidiuretic hormone increase during positive end-expiratory pressure in conscious dogs

1. This study in conscious dogs examined the effects of extracellular volume expansion on plasma antidiuretic hormone, atrial natriuretic peptide and aldosterone concentrations, plasma renin activity, and haemodynamic and renal responses during controlled mechanical ventilation with 20 cmH2O positive end-expiratory pressure. 2. Twenty experiments (10 controls, 10 expansion experiments with 0.5 ml min-1 kg-1 body weight of a balanced electrolyte solution given intravenously throughout) were performed in five trained, conscious, tracheotomized dogs over 4 h: first and fourth hour, spontaneous breathing; second and third hour, 20 cmH2O positive end-expiratory pressure. 3. In the control experiments positive end-expiratory pressure increased plasma antidiuretic hormone concentration from 1.4 +/- 0.2 to 10.0 +/- 3.3 pg/ml, plasma aldosterone concentration from 113 +/- 19 to 258 +/- 58 pg/ml and heart rate from 77 +/- 5 to 94 +/- 5 beats/min. Positive end-expiratory pressure did not change plasma atrial natriuretic peptide concentration (55 +/- 5 pg/ml), plasma renin activity (2.6 +/- 0.4 pmol of angiotensin I h-1 ml-1) and mean arterial pressure 103 +/- 3 mmHg). 4. In the expansion experiments, positive end-expiratory pressure did not change plasma antidiuretic hormone concentration (1.1 +/- 0.1 pg/ml), plasma aldosterone concentration (25 +/- 2 pg/ml), plasma atrial natriuretic peptide concentration (82 +/- 8 pg/ml), plasma renin activity (0.8 +/- 0.15 pmol of angiotensin I h-1 ml-1), heart rate (92 +/- 6 beats/min) and mean arterial pressure (111 +/- 4 mmHg). 5. In the control experiments, urine volume, sodium excretion and fractional sodium excretion remained in a low range during positive end-expiratory pressure, whereas potassium excretion increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Combined transrectal ultrasonography and urodynamics in the evaluation of detrusor-sphincter dyssynergia

Thirty-one combined transrectal ultrasonographic and urodynamic studies were performed in 24 patients with spinal cord lesions at different levels between C4 and T12. Ultrasonography provided accurate real-time imaging of the bladder neck, prostatic urethra and external sphincter during the bladder filling phase as well as during the voiding phase. Bladder and rectal pressures, sphincter EMG and uroflow were recorded simultaneously. Transrectal ultrasonography contributed significantly to the accuracy of diagnosing detrusor-sphincter dyssynergia. Sphincter contractions were clearly visualised with ultrasonographic video monitoring. This imaging method was especially helpful in sorting out the problems of 3 patients with poor emptying of bladder after endoscopic sphincterotomy. One had a urethral stricture and the other 2 had sphincters that opened adequately but bladders that emptied poorly because of detrusor hypocontractility. Other problems, such as benign prostatic hyperplasia and false passage, were also easily recognised. Transrectal ultrasonography not only provides accurate information but also involves no exposure to radiation and thus precludes the need for costly lead-shielded examination rooms.

The use of transrectal longitudinal real-time ultrasonography in urodynamics

A total of 63 combined transrectal ultrasonographic and urodynamic studies was performed to evaluate the voiding dysfunction in 49 spinal cord injury patients and 7 other patients. Ultrasonography provided excellent real-time imaging of the bladder neck, prostatic urethra, prostate and external sphincter, and allowed for accurate diagnosis of detrusor-sphincter dyssynergia, detrusor-bladder neck dyssynergia, prostatic hypertrophy and bladder neck strictures. In addition to offering these capabilities, ultrasonography is less expensive than x-ray and it does not involve exposure of either the patient or examiner to radiation. However, it does not provide a means to detect vesicoureteral reflux.

Effectiveness of mattress overlays in reducing interface pressures during recumbency

This study evaluates the pressure-reduction characteristics of seven mattress overlays. Thirty subjects were evaluated on each support surface to determine the interface pressures that are generated under the most common pressure sore sites. The results of this study indicate that there is great variability in the effectiveness of traditional mattress overlays. The most effective overlays are the Roho and Akros DFD mattresses; whereas 2-inch thick convoluted foam provides no significant protection for the trochanter when the subjects were lying on their sides (lateral position).

External fixation in quadriplegia

Extremity fractures frequently occur at the time of spinal cord injury. Fractures immobilized by external fixation devices interfere with patient positioning and predispose to trophic ulcers. The devices also interfere with joint motion. Incorrect application may delay rather than promote fracture union. Patients with spinal cord injuries require appropriate fracture management.

Prevention of urinary tract infection during intermittent catheterization

We studied prospectively 40 patients with recent spinal cord injuries to determine the effectiveness of the combination of oral methenamine and an intravesicular acidifying agent for prevention of urinary tract infection during intermittent catheterization. The incidences of bacteriuria and symptomatic urinary tract infections were significantly less in the treated group. This combination appears to be a safe, effective means to reduce pyelonephritis in spinal cord injury patients on intermittent catheterization.

The spinal cord injured female in the veterans hospital: the role of the environment in rehabilitation

The outcome of rehabilitation is influenced by the interface of the patient and his/her environment. A primary element is the physical setting in which rehabilitation occurs. This paper discusses the course of rehabilitation of this center's seven, singly admitted, female spinal cord injured (SCI) patients. Four of the patients were admitted to a private room and three were admitted to an open ward with other male SCI patients. Data were collected from clinical records, letters of complaints on discharge, and personal communication. Those patients accommodated in the private room generally reported less satisfaction with rehabilitation received, remained in rehabilitation longer, and appeared to have experienced more difficulty in achieving independence commensurate with their level of injury. Psychosocial issues prevalent with the private room patients were alienation, isolation, anger, and interpersonal conflicts with the nursing staff. Surprisingly, issues of sex differences, privacy, or modesty were not expressed as primary concerns for the open ward patients. These results suggest that in the interest of maximizing the rehabilitative efforts for the females at this center, attention to the setting in which they spend most of their unscheduled time is of major importance.

Orthostatic vasomotor response in spinal man

The cardiovascular adaptation of tetraplegics to the upright position has been previously demonstrated to be deficient. Presumably this is due to the interruption of the spinal pathways linking supraspinal control centres with the peripheral sympathetic motor neurons. Review of previous studies of this phenomenon reveals that vasomotor responses have been determined primarily from blood flow measurements in the extremities. Contradictory conclusions have been drawn. Study of the visceral circulation, in particular renal blood flow, could shed more light on this poorly understood area. Renal clearance tests were carried out on seven healthy controls and eight chronic, clinically complete tetraplegic patients. Renal blood flow, mean arterial pressure, and total renal vascular resistance in both supine and passive head-up tilt positions were calculated from collected data. Renal blood flow and total renal vascular resistance showed significant decrease and increase respectively during tilting in controls and tetraplegic subjects. Although the renal circulation is autoregulated, postural change causes profound alteration of the renal blood flow mediated through the haemodynamic effects of the renal nerves. Sympathetic renal vasoconstriction is mediated by the carotid sinus reflex through the vasomotor centre in the brainstem. In the absence of supraspinal influence the renal vasculature is shown to respond to an orthostatic stimulus with a vigorous vasoconstriction. The adaptation of spinal man to the upright position may involve the recovery of a spinal vasomotor reflex involving the splanchnic circulation.

Myelopathy following cervical spine manipulation

Manipulation of the spine, a controversial mechanotherapeutic procedure, has been taught and practiced widely. Occlusion of vertebral, basillar, and cerebellar arteries with head and neck manipulation is well documented in the literature. However, there is a paucity of references about myelopathy associated with cervical spine manipulation. Three cases of cervical myelopathy following spinal manipulation are presented. All patients noted increase in cervical pain following manipulation, developed significant sensary/motor deficits within 24 hours and became tetraparetic. Two patients were found to have fracture of C5 and C6 vertebral bodies. On myelography, partial block was noted in all and widening of the spinal cord was noted in two. One patient underwent excision of C5 vertebral body and anterior interbody fusion C4/6. Two patients underwent cervical laminectomy. Hematomyelia was present in one, and in the other patient the spinal cord was reported to be hyperemic and oedematous. Only one patient showed neurological return and became ambulatory, while the other two remained tetraparetic.

Head-up tilt effect on glomerular filtration rate, renal plasma flow, and mean arterial pressure in spinal man

Changes in glomerular filtration rate (GRF), renal plasma flow (RPF), and mean arterial pressure (MAP) were measured in subjects tested in supine and head-up tilt positions with various levels of spinal cord lesion and thus with different degrees of supraspinal sympathetic vasomotor control. Responses of paraplegic subjects to head-up tilt were not significantly different from those of normal controls but GFR and RPF were significantly lower in quadriplegics in the supine position. With tilt, MAP and RPF decreased significantly, but the fall in GFR was not significant. In all 3 groups, the GFR during head-up tilt was similar, indicating that in spite of the great loss of supraspinal sympathetic control, quadriplegic subjects apparently equally-constrict their afferent and efferent renal arterioles during orthostatic stress and thus prevent excessive fall of GFR.