The maintenance need for water in parenteral fluid therapy

PURPOSE OF REVIEW

Maintenance fluid therapy represents the volume of fluids and amount of electrolytes and glucose needed to replace anticipated physiological losses from breath, sweat and urine and to prevent hypoglycaemia. For 50 years, this therapy was based on Holliday and Segar's formula, which proposed to match children's water and electrolyte requirements on a weight-based calculation using hypotonic solutions. Recent publications highlight the risk of hyponatraemia in the postoperative period and the facilitating role of a hypotonic infusion, leading some people to recommend replacing hypotonic with isotonic solutions.

RECENT FINDINGS

The postoperative period is at risk for nonosmotic secretion of antidiuretic hormone, which reduces the ability of the kidneys to excrete free water. In the context of antidiuretic hormone release, the associated low urine output makes maintenance volume requirement decrease to 50% of the calculated hourly rate. While isotonic fluids are recommended during anaesthesia, controversies still exist on the nature of fluid for maintenance therapy in the postoperative period. The proof for a benefit of isotonic fluids in this context is weak; further investigations are needed to make a decision. Whatever the choice, an individualized maintenance infusion protocol for each patient is necessary.

SUMMARY

As free water excretion is altered for all children in the postoperative period, it is necessary to reduce the volume of maintenance fluid therapy to half the previously recommended volume. The choice of an isotonic solution should be more pertinent to that of a hypotonic solution, but evidence is lacking for a definitive answer.

HYDRODYNAMICS OF AORTIC BLOOD FLOW

A thermistor was used to record the temperature at regular intervals across the aortic diameter of dogs during the continuous infusion of cold indicator solution at various upstream locations. The results showed that the indicator mixed across the greater part of the diameter of the aorta in the ascending portion and proximal half of the arch but maintained a more streamlined pattern in the distal portion of the arch and descending thoracic aorta. These observations were confirmed by cineangiography. When two thermistors were located in different positions in the aortic root, and when cold saline was injected suddenly in their vicinity, the temperatures recorded by the two thermistors equalized immediately following the onset of systole. Sound waves also were detected during the systolic interval in the ascending but not in the descending aorta.

These findings are consistent with the presence of disturbed, but not necessarily turbulent flow in the ascending aorta; and with predominantly streamlined, but not necessarily completely laminar flow in the descending portion. The factors producing disturbed flow in the aortic root and the transition of flow to a more streamlined pattern in the descending portion are discussed in hydrodynamic terms.

It is postulated that during systole the blood ejected into the aorta separates from a slower moving boundary layer near the walls and at the edges of the valve cusps. This separation produces disturbed flow and mixing in the entering jet. While the thin layer adjacent to the walls as well as stagnant areas behind the valves do not fully mix with the central stream, they represent a relatively small proportion of the total aortic blood flow. Additional mixing is produced by the prominent backflow in early diastole and by the branchings of the coronary and brachiocephalic arteries.

PRESSOR EFFECT OF RENIN

The pressor activity of renin administered intravenously at various rates and at various states of purity was measured and the effects of anesthesia, heparinization, and atropinization were investigated. Under all of these conditions, the blood pressure of normotensive and hypertensive dogs was elevated considerably throughout the period of infusion and for some time thereafter. The pressor activity of single doses of angiotensin and of renin was abolished and that of norepinephrine was inhibited, up to 85%, during the infusion of renin. The refractory state (socalled tachyphylaxis) to renin is not specific, since the continuous infusion of methoxamine likewise abolished the pressor activity of superimposed single doses of renin or angiotensin. At a very high rate of infusion, renin caused two exceptional and highly abnormal effects, the return of the blood pressure to the base line during the infusion, and the failure of the infused renin to inhibit the pressor action of single doses of norepinephrine.

ASSOCIATION OF INTRAVENOUS ALBUMIN WITH ALKALINE PHOSPHATASE ACTIVITY

After albumin transfusion levels of alkaline phosphatase are raised. These levels were studied in selected groups of patients and in albumin prepared for transfusion using a heat sensitivity test, and by this means the significance of raised levels of alkaline phosphatase may be interpreted correctly.

BRADYKININ: VASCULAR RELAXANT, CARDIAC STIMULANT

Bradykinin infusion causes an increase in cardiac output in rats whether the autonomic nervous system is blocked or not. After autonomic blockade, bradykinin causes a lesser decrease in the total peripheral resistance but a greater increase in cardiac output, resulting in an elevation of arterial pressure. The increase in cardiac output is caused by a small increase in heart rate and a substantial increase in stroke volume. The fact that these increases are observed after autonomic blockade suggests that bradykinin increases cardiac output by direct stimulation of the heart.

RENAL EFFECTS OF ANGIOTENSIN II INFUSIONS IN NORMOTENSIVE PREGNANT AND NONPREGNANT WOMEN

Normotensive pregnant women are relatively resistant to angiotensin II in that they give smaller pressor responses and show smaller reductions in urine flow and sodium and chloride excretions than do normotensive nonpregnant women.

Women early in the third trimester of pregnancy are more resistant to angiotensin II than are women at term, with respect to the effect upon sodium and chloride excretions.

The less pronounced renal effects of angiotensin II in pregnant women cannot be attributed merely to error caused by increase in the dead space of the urinary tract.

HEMODYNAMIC DETERMINANTS OF THE AMPLITUDE OF THE FIRST HEART SOUND

The relationships between the amplitude of the first heart sound and various hemodynamic parameters and their analog functions were systematically investigated in 51 dogs under morphine and pentobarbital anesthesia. Simultaneous hemodynamic and phonocardiographic tracings were obtained by cardiac catheterization, high speed recording, and analog computers. Changes in cardiodynamics were induced by: l -epinephrine, levarterenol, phenylephrine, isoproterenol, methoxamine, veratrine, vasopressin, histamine, atropine, saline infusion, mechanical obstruction of the aorta, pulmonary artery, or both venae cavae, embolization of the coronary arteries with plastic microspheres, mechanical rupture of aortic valve leaflets, or hemorrhage. In this way, the relationships were examined over a wide range of pressures, volumes, and cardiodynamics. The results showed that the amplitude of the first sound is not directly related to stroke output, P-R interval, heart rate, end diastolic, or systolic ventricular or aortic pressures. Only the peak rate of rise of left ventricular systolic pressure (first derivative maximum) had a consistent relationship to first sound amplitude. The right ventricle seems to contribute very little to the external first sound. These observations suggest that the amplitude of the first sound is related to left ventricular contractility.