Progressive muscle relaxation

Matsumoto, M. and Smith, J. C. (2001). Progressive muscle relaxation, breathing exercises and ABC relaxation theory. Journal of Clinical Psychology 12 1551-7. 

Systemic arterial blood pressure decreases progressively with increasing depth of anaesthesia with isoflurane. It also increases heart rate but arrhythmias are not precipitated. Isoflurane depresses respiration as concentration is increased. Uterine and skeletal muscle relaxation is similar to enflurane. 

The effects on muscle relaxation are dose-dependent. When administered as an epidural or to produce peripheral nerve blockade, use of low-dose bupivacaine will produce partial relaxation. Progressively higher doses produce more effect, and high doses (e.g.. use of a 75 percent solution) will produce complete muscle relaxation. 

A mechanism for the pathophysiology of high-flow priapism is described by Bastuba et al. (1994). Unlike a traditional arteriovenous fistula, the condition is described as an arterial-lacunar fistula where the helicine arteries are bypassed, and the blood passes directly into the lacunar spaces. The high-flow in the lacunar space creates shear stress in adjacent areas, leading to increased nitric oxide release, activation of the cGMP pathway, smooth muscle relaxation and trabecular dilatation. These authors also postulate that the delay in onset of high-flow priapism may be secondary to a delay in the complete necrosis of the arterial wall after the initial trauma or secondary to clot formation at the site of injury followed by the normal lytic pathway, which progresses in a few days. 

Deaths by deliberate or accidental overdose have occurred with muscle relaxants including metaxalone, particularly in combination with any antidepressants and/or alcohol. When determining the LD, in rats and mice, progressive sedation, hypnosis, and finally respiratory fafliu-e were noted as the dosage increased. In dogs, no LD could be determined as the higher doses produced an emetic action in 15 to 30 minutes. 

Figure 2. Muscle stimulation, a) a single nerve impulse (stimulus) causes a single contraction (a twitch). There is a small delay following the stimulus before force rises called the latent period, b) A train of stimuli at a low frequency causes an unfused tetanus. Force increases after each progressive stimulus towards a maximum, as calcium levels in the myofibrillar space increase. But there is enough time between each stimulus for calcium to be partially taken back up into the sarcoplasmic reticulum allowing partial relaxation before the next stimulus occurs, c) A train of stimuli at a higher frequency causes a fused tetanus, and force is maximum. There is not enough time for force to relax between stimuli. In the contractions shown here, the ends of the muscle are held fixed the contractions are isometric.

A. Horska and R. G. Spencer, Measurement of spin-lattice relaxation times and kinetic rate constants in rat muscle using progressive partial saturation and steady-state saturation transfer. Magn. Reson. Med., 1996, 36, 233-240. 

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