Nitrogen versus muscle calcium in the genesis of abnormal muscle function in malnutrition

Assessment of involuntary muscle function in patients after critical injury or severe sepsis

BACKGROUND

Study of involuntary skeletal muscle function (MFA) has been well accepted in the area of nutrition assessment and potentially offers a means for following progress of the critically ill patient. We report on the application of this technique to intensive care patients.

METHODS

MFA was performed by study of the contraction/relaxation characteristics of the adductor pollicis muscle of the thumb after ulnar nerve stimulation. Serial measurements were made in 16 critically injured patients and 28 patients with severe sepsis and were compared with those obtained from 26 control subjects. Extent of loss of total body protein (TBP) was quantified with in vivo neutron activation.

RESULTS

Significant difficulties exist in applying this technique to intensive care patients. In the critically injured, only five acceptable traces could be obtained from a possible 58 measurements. For patients with severe sepsis it was possible to obtain an acceptable trace on 12 of 56 occasions. Neuromuscular blockade and lack of patient cooperation were significant impediments to MFA study. Although frequently perceived as unpleasant by these patients, there was no long-term morbidity associated with MFA. No significant differences were seen in maximal relaxation rate at 30 Hz (MMR30) or force frequency ratios (F10/50 and F30/ 50) between trauma patients and controls. In the sepsis patient group, a significantly higher F10/50 was measured (52% +/- 3% severe sepsis vs 40% +/- 1% control subjects, p < .01). Six patients had MFA measured approximately 21 days after the illness, by which stage they had lost 11% of their initial TBP. Compared with control subjects, no significant differences were observed in MRR30 or F30/50, whereas a higher value for F10/50 was measured (48% +/- 1% critical illness vs 40% +/- 1% control subjects, p < .01).

CONCLUSIONS

The MFA technique is difficult to apply to intensive care patients. No significant disturbance to MFA is seen after critical injury. Severe sepsis results in an elevation of F10/ 50 only. When able to be obtained, MFA results do not reflect the extent of proteolysis but are indicative of the state of cellular energetics.

Effects of hypocaloric feeding and refeeding on myocardial Ca and ATP cycling in the rat

Hypocaloric feeding (HCF) depresses heart function causing cardiac atrophy, bradycardia, and decreased cardiac output. We tested the hypothesis that HCF results in decreased myocardial Ca- and ATP cycling. We reduced protein-calorie intake of adult rats by 20% for 7 days and then allowed them to recover for 3 days. Changes in ionized Ca concentration (nM/s) of 2.5% myocardial homogenates that were attributable to the Ca-ATPase pump and Ca-release channel (CRC), respectively, of the sarcoplasmic reticulum (SR) were depressed 41 and 85% by HCF from 61.6 +/- 9.4 and 24.7 +/- 3.3, to 36.1 +/- 2.8 and 3.6 +/- 2.9. Activity of the Ca-pump was restored after 3 days of refeeding, whereas the CRC remained 23% depressed (all p < 0.05). Additionally, the CRC activity was inhibited to a 3-fold greater extent than controls by HCF, but was disinhibited within one day of refeeding. The greater effect on CRC than Ca-pump activity resulted in net Ca-uptake being unaffected by HCF. In addition to depression of Ca-cycling, ATP sythetase and total ATPase activities (IU/g), respectively, were depressed 20 and 15% by HCF from 174 +/- 19 and 51.3 +/- 3.8 to 140 +/- 15 and 43.7 +/- 4.7, but were restored to control values within one day of refeeding. We conclude that HCF produces a compensatory, reversible, and asymmetric downregulation and inhibition of Ca-cycling, with the CRC being preferentially affected.

Microcomputer system for the analysis of muscle function

Though skeletal muscle function has recently been increasingly studied, a system for routine clinical examination in patients is not widely available. A microcomputer based system has been developed for the measurement and analysis of force-frequency characteristics, relaxation rate, fatiguability and endurance in the adductor pollicis muscle. The mobile system consists of a modified displacement transducer fitted with a linear-output Hall-effect sensor, a specially designed signal processing unit and a microcomputer for control of both the electrical stimulus and a suite of menu-driven, user-interactive programs. One hundred normal volunteers have been studied to produce a normal database for comparison with malnourished surgical patients. The system compares favourably with other systems in use for objective measurement of skeletal muscle function and introduces a refined technique for evaluating endurance.

Muscle power after glucose-potassium loading in undernourished patients

The force-frequency characteristics and maximal relaxation rate of the adductor pollicis muscle were measured before and after 48 hours of intravenous loading with glucose (104.5 kJ (25 kcal)/kg/24 h) and potassium (20 mmol(mEq)/500 ml glucose) in eight undernourished patients about to undergo surgery. Both variables of skeletal muscle performance, which were depressed when compared with data from 100 healthy volunteers, improved significantly after glucose-potassium loading. The improvement was accompanied by restoration of muscle glycogen values and return of respiratory exchange ratios towards unity. These results imply that if muscle power is a yardstick for preoperative nutritional rehabilitation then a simple regimen of energy-electrolyte repletion may be cost effective in preparing undernourished patients for major surgery.