The effect of negative pressure breathing on urine flow

When the Renal (Function) Begins to Fall: A Mini-Review of Acute Kidney Injury Related to Acute Respiratory Distress Syndrome in Critically Ill Patients

Acute kidney injury (AKI) is one of the most frequent causes of organ failure encountered in patients in the intensive care unit (ICU). Because of its predisposition to occur in the most critically ill patients, it is not surprising to observe a high frequency of AKI in patients with acute respiratory distress syndrome (ARDS). However, few studies have been carried out to assess the epidemiology of AKI in subgroups of ARDS patients using recommended KDIGO criteria. Moreover, the mechanisms involved in the physio-pathogenesis of AKI are still poorly understood, in particular the impact of mechanical ventilation on the kidneys. We carried out a review of the literature, focusing on the epidemiology and physiopathology of AKI in patients with ARDS admitted to the ICU. We addressed the importance of clinical management, focusing on mechanical ventilation for improving outcomes, on AKI. Finally, we also propose candidate treatment strategies and management perspectives. Our literature search showed that AKI is particularly common in ICU patients with ARDS. In association with the classic risk factors for AKI, such as comorbidities and iatrogeny, changes in mechanical ventilation parameters, which have been exclusively evaluated for their outcomes on respiratory function and death, must be considered carefully in terms of their impact on the short-term renal prognosis.

[Interactions between lung and kidney in the critically ill]

Interactions between lung and kidney can significantly affect the course of acute diseases, a phenomenon that was first observed in the 1950s by describing pulmonary dysfunction in uremic patients. From animal experiments there is ample evidence for remote lung injury following acute kidney injury (AKI), with an increased risk for the development of pulmonary edema and acute respiratory distress syndrome (ARDS). Coincident ARDS and AKI are associated with higher rates of intubation and mechanical ventilation, significantly prolonged weaning from mechanical ventilation and increased mortality. On the other hand, acute lung diseases and mechanical ventilation can promote the development of AKI and are associated with increased mortality when AKI is also present. These bidirectional interactions may include hemodynamic adverse effects during mechanical ventilation or volume overload as well as the release or decreased clearance and metabolism of proinflammatory mediators (e.g., interleukin-6 and tumor necrosis factor-α), which may induce and aggravate distant organ injury. The aim of this work is to examine the interactions between lung and the kidney in critically ill patients, as well as discuss potential preventive approaches.

The regulation of antidiuretic hormone release in man: I. Effects of change in position and ambient temperature on blood ADH levels

It has been postulated that alterations in the intravascular distribution of blood affect antidiuretic hormone (ADH) secretion in man. The studies reported here were designed to alter blood distribution by thermal and by positional change to test this thesis.HUMAN BLOOD ADH LEVELS HAVE BEEN SHOWN TO VARY WITH POSITION: a mean value of 0.4 +/- 0.6 (SD) muU/ml was obtained while the subject was supine, a value of 1.4 +/- 0.7 muU/ml while sitting, and 3.1 +/- 1.5 muU/ml while standing. In 79 control subjects, sitting comfortably for 30 min in a normal environment, a blood ADH level of 1.65 +/- 0.63 muU/ml was found. It is suggested that subjects assume this position during experiments in which blood is drawn for measurement of ADH levels.In eight seated subjects the ADH level rose from 1.6 +/- 0.4 to 5.2 +/- 0.8 muU/ml after a 2 hr exposure at 50 degrees C and fell to 1.0 +/- 0.26 muU/ml within 15 min at 26 degrees C.Six subjects with a mean ADH level of 2.2 +/- 0.58 muU/ml sat quietly in the cold (13 degrees C) for 1 hr, and the ADH level fell to 1.2 +/- 0.36 muU/ml. After 15 min at 26 degrees C, the level rose to 3.1 +/- 0.78 muU/ml. The serum sodium and osmolal concentrations remained constant during all studies.Water, sodium, and total solute excretion decreased during exposure to the heat, whereas the urine to plasma (U/P) osmolal ratio increased. During cold exposure, water, sodium, and total solute excretion increased, and there was a decrease in the U/P osmolal ratio.These data are interpreted as indicating that changes in activity of intrathoracic stretch receptors, in response to redistribution of blood, alter ADH secretion independently of changes in serum osmolality. The rapidity of change of blood ADH concentration indicates a great sensitivity and a prime functional role for the "volume receptors" in the regulation of ADH secretion.

Mechanical ventilation and the kidney

Acute lung injury (ALI) and acute kidney injury (AKI) are complications often encountered in the setting of critical illness. Both forms of end-organ injury commonly occur in similar settings of systemic inflammatory response syndrome, shock, and evolving multiple organ dysfunction. Recent elucidation of the pathobiology of critical illness has led to a more basic mechanistic understanding of the complex interplay between injured organs in patients with multiple organ dysfunction syndrome; this has been aptly called 'the slippery slope of critical illness' [Kidney Int Suppl 1998;66:S25-S33]. Distant organ effects of apparently isolated injuries to the lungs, gut, and kidneys have all been discovered in recent years. In this article, we will review the harmful bidirectional interaction between ALI and AKI, which appears to be a common clinical syndrome with routine clinical implications. We will review the current understanding of lung-kidney interactions from both perspectives, including the renal effects of ALI and mechanical ventilation, and the pulmonary sequelae of AKI. In this review of the emerging evidence of deleterious bidirectional organ cross talk between lung and kidney, we will focus on the role of ventilator-induced kidney injury in the pathogenesis of AKI in patients with ALI.

Effect of mechanical ventilation on the kidney

Mechanical ventilation is a standard component of intensive care unit management of critically ill patients and is widely used for respiratory support. Recent animal and clinical studies have shown that positive pressure ventilation can worsen pre-existing lung injury and produce ventilator-induced lung injury, which has been linked with the development of systemic inflammation and multi-system organ dysfunction, including renal failure. Although the physiological consequences of mechanical ventilation on pulmonary and cardiovascular function have been extensively studied, its effects on renal function are not as well defined. Previous experimental studies and few clinical reports have shown a significant effect of mechanical ventilation on renal function. Interestingly, recent data are emerging which suggest that renal dysfunction also has a direct, adverse effect on pulmonary function. This chapter reviews the information in these areas and provides a framework for future investigation in this field.

Mechanical ventilation and renal function: an area for concern?

Mechanical ventilation is a standard component of intensive care unit management of critically ill patients and widely used for respiratory support. Patients requiring ventilation often have renal dysfunction that can occur as a consequence of the underlying disease or be related to the therapy. Although the physiological consequences of mechanical ventilation on pulmonary and cardiovascular function have been extensively studied, its effects on renal function are not as well defined. Previous experimental studies and few clinical reports have shown a significant effect of mechanical ventilation on renal function. This review compiles the information in this area and provides a framework for future investigation in this field.

Effects of nonpeptide V1a and V2 antagonists on blood pressure fast oscillations in conscious rats

This paper describes the effects of vasopressin nonpeptide selective V1a (OPC-21268) and V2 (OPC-31260) antagonists on fast blood pressure (BP) oscillations in conscious non-haemorrhaged and haemorrhaged rats. Equidistant sampling at 20 Hz allowed direct spectral analysis of BP on 30 overlapping 2048 point-time series. In non-haemorrhaged rats, V1a antagonist (5 mg/kg; i.v) reduced BP and low-frequency (LF-BP) component while subsequent administration of V2 antagonist (1 mg/kg; i.v) reversed these changes and enhanced the very low-frequency (VLF-BP) component. In haemorrhaged rats (5-15 ml/kg/min) V2 antagonist pre-treatment enhanced the VLF-BP component during normotensive bleeding, while the V1a antagonist pre-treatment modified BP variability after hypotensive haemorrhage by enhancing the HF-SBP component. The results suggest that under normotensive conditions vasopressin by the stimulation of both V1a and V2 receptors buffers BP variability in the VLF-BP frequency domain. In addition, under hypotensive conditions vasopressin, by the stimulation of V1a receptors buffers the respiration-induced HF-BP oscillation.

Hemodynamic effects of continuous positive and negative pressure breathing in normal man

Alterations in cardiac output, central blood volume, and arterial and central venous pressures were studied in 36 young men subjected to continuous negative or continuous positive pressure breathing. Negative pressure breathing increased cardiac output, stroke volume, and, to a lesser extent, central blood volume. Positive pressure, without overbreathing, decreased both cardiac output and central blood volume, but hyperventilation reversed these effects. On the basis of these observations, it is concluded that decreased intrathoracic pressure is important in maintaining or increasing the blood available to the heart in man, particularly during circulatory stress.

Polyuria in paroxysmal tachycardia and paroxysmal atrial flutter and fibrillation

When Paul Wood spoke about this to the British Cardiac Society in 1961, we were impressed by the originality of his work on a subject that so many had noticed but failed to investigate. A few months before his death in July 1962, when I asked him if his paper was ready for the journal, he said that he had not yet been able to get all the evidence he hoped for to establish the full explanation. Unfortunately, he had not the time to complete this. I have, therefore, tried to set out as clearly as possible the stage that he had reached. Maurice Campbell

Mechanisms of tubular sodium chloride transport

Extracellular fluid volume is determined by sodium and its accompanying anions. There are control mechanisms which regulate sodium balance in the body. These include high and low pressure baroreceptors, intrarenal baroreceptors, renal autoregulation, tubuloglomerular feedback, aldosterone, and numerous other physical and hormonal factors. Sodium transport by the nephron involves active and passive processes which occur in several different nephron segments. Mechanisms of cotransport, Na(+)-H+ exchange, antiporters and ion-specific channels are all utilized by the nephron to maintain sodium balance. These regulatory factors and transport mechanisms for sodium in the kidney will he discussed in detail.

Extrarenal regulation of salt and water exchange: volume receptors

This paper reviews the complicated interplay which may be involved in the retention of water and salt by the kidney. For the sake of brevity, it does not treat either the mechanisms which regulate the intake of water or the disposition of fluid throughout the body. The neurohumoral pathways by which a change in intravascular volume can effect a retention of water by the kidney receives particular attention. The difficulties in dissociating intrarenal from extrarenal determinants of salt and water excretion are considered in detail.

Physiological mechanisms in regulation of blood pressure fast frequency variations

Spectral analysis of blood pressure fast oscillations (short-term variability), both in humans and animals, reveals three major frequential domains: the very low-, low- and the high-frequency domain. In this paper, experimental data providing evidence for physiological mechanisms involved in the regulation of blood pressure oscillations such as sympathetic nervous system, renin-angiotensin system, NO, respiration, heart function, and circulating blood volume are reviewed. In addition, novel evidence is provided by the author for vasopressin modulation of the low- and high-frequency blood pressure components. This review suggests that the multiplicity of factors involved in the genesis of the blood pressure spectral components imply utmost caution in interpreting spectral results.

Interactions between hemodynamic and hormonal modifications during PEEP-induced antidiuresis and antinatriuresis

The interactions between hemodynamic and hormonal modifications during antidiuresis and antinatriuresis induced by positive end-expiratory pressure (PEEP) were studied in six patients under 15 cm H2O PEEP before PEEP and after the addition of lower body positive pressure (LBPP) to PEEP (PEEP+LBPP). We measured or calculated the following: cardiac index, systemic arterial, right atrial, pulmonary arterial, and pulmonary artery occlusive pressures; indexed renal blood flow (iodohippurate 131 sodium clearance); total blood volume (chromium 51 radiolabeled RBCs); glomerular filtration rate; urinary output; fractional excretion of sodium (FE Na+); plasma concentrations of antidiuretic hormone (ADH), plasma renin activity (PRA), norepinephrine and epinephrine; urinary concentration of PGE2 (PGE2u). Although LBPP application corrected PEEP deleterious effects on systemic and renal hemodynamics, sustained fall in Vu and in FE Na+ were observed. Antidiuresis was not due to ADH release. Sympathetic activation and high PRA appeared the main determinants of renal function alterations in PEEP ventilation.

Intermittent negative pressure ventilation may increase urine flow in normal subjects

In order to analyze the effect of intermittent negative pressure ventilation (NPV) on renal function, we studied 20 healthy male volunteers (mean age 29 +/- 4.1 years). NPV was performed with an "Emerson Chest Respirator Pump", adjusted to a breathing frequency of 10 respirations per minute, with inspiratory time/total respiratory time ratio of 0.4 and negative pressure of 25 cmH2O. The experimental protocol was carried out in two phases of two hours each--spontaneous breathing and NPV breathing. At the end of each phase, urine volume of the whole period was collected as well as venous blood sample for biochemical determinations. During NPV there was significant increase (P < 0.05) in urine flow rate (1.43 +/- 0.81 to 2.76 +/- 1.95 ml/min) as well as in natriuresis (258 +/- 201 to 389 +/- 175 mcEq/min), kaliuresis (61 +/- 45 to 98 +/- 49 mcEq/min), fractional sodium excretion (1.38 +/- 0.88 to (1.96 +/- 0.98%), osmolar clearance (3.13 +/- 1.82 to 4.32 +/- 1.24 ml/min) and pH (7.37 +/- 0.04 to 7.41 +/- 0.07) with unchanged creatinine and free water clearances. We concluded that NPV increases urine flow rate, kaliuresis and natriuresis but the data we have do not allow us to explain the mechanisms underlying such a phenomenon.

Atrial regulation of intravascular volume: observations on the tachycardia-polyuria syndrome

The atria, strategically located at the junction of the venous and arterial circulation, contain a network of neural and humoral structures by which they sense and regulate intravascular volume. Atrial receptors, most commonly consisting of complex unencapsulated nerve endings discharging into myelinated vagal fibers, are located in the intrapericardial portions of the caval and pulmonary veins and the adjacent atrial walls. Receptor activation by atrial distension results in increased afferent vagal fiber discharge, which in turn leads to tachycardia (Bainbridge's reflex) and decreased renal sympathetic nerve activity, renal vasomotor tone, and antidiuretic hormone activity. In addition, atrial distension also releases ANF, a peptide with potent diuretic, natriuretic, and vasorelaxant actions. The combined effect of these neurohumoral changes is the production of a large hypotonic diuresis. In the clinical setting the volume-regulating role of the atria is demonstrated by the tachycardia-polyuria syndrome. Laboratory and clinical evidence points to the activation of atrial neurohumoral mechanisms in response to atrial distension as the mediators of the polyuria that often accompanies paroxysmal tachycardias. The involvement of these mechanisms in other forms of cardiac congestion and the capability to easily measure in the blood an index of atrial distension, namely ANF, provide the opportunity to elucidate the pathophysiology and hence to open new therapeutic avenues in many cardiac disorders.

Atrial natriuretic factor: a historic perspective

Atrial natriuretic factor (ANF) is a relatively newly discovered hormone. Its discovery was the culmination of almost a half century of research. This article reviews some of the data leading up to its discovery. In addition, the issue of its physiologic role in volume expansion is considered.

Initial fluid therapy for children with meningitis with consideration of the syndrome of inappropriate anti-diuretic hormone

A review of 85 children with meningitis admitted to the University Children's Hospital at Hermann Hospital revealed that only six patients (7%) had sufficient clinical and laboratory evidence to be compatible with the diagnosis of inappropriate secretion of antidiuretic hormone (SIADH). This is in contrast with various communications in the pediatric literature that have reported an association between the two conditions of as high as 85%. Hyponatremia (serum sodium 135 mEq/L) was observed in 27 (32%), but neither the initial laboratory data nor the subsequent follow-up data supported the diagnosis of SIADH. Moderate to severe dehydration was documented in 16 patients (19%) and suspected in more than 50% of the cases (serial weight determinations indicating an increase in weight from admission to discharge). Although the incidence of neurologic sequelae in this series was not influenced significantly by the presence or absence of hyponatremia, SIADH, or fluid restriction, the numbers are still small. Based on these data, routine fluid restriction cannot be recommended unless there is confirmatory evidence of SIADH.

Athletes, astronauts and orthostatic tolerance

Specific alterations in autonomic functions induced by endurance training may lead to a reduced ability to withstand orthostatic stress. This possibility has caused some authorities to suggest that, because of potentially greater pooling of blood in the lower extremities during gravitational loading, endurance-trained athletes may make poor astronauts. Although results from spaceflight studies have provided little evidence to support this suggestion, data from water-immersion studies indicate that endurance-trained athletes do become more orthostatically intolerant following a few hours of simulated weightlessness. Unfortunately, other evidence supporting the hypothesis that endurance training reduces orthostatic tolerance has not received adequate publication in the open scientific literature. On the other hand, a number of studies which have been openly reported clearly refute this hypothesis. Nevertheless, the established physiological differences between endurance athletes and non-athletes are themselves sufficient to suggest that the hypothesis could be tenable. Consequently, it has to be concluded that the presently available information is both qualitatively and quantitatively inadequate to permit any definite statement regarding a possible relationship between aerobic power (VO2max) and orthostatic tolerance.

Central transmural venous pressure and plasma arginine vasopressin during negative pressure breathing in man

After overnight food and fluid restriction, 8 normal healthy males were examined in the upright sitting position before (prestudy), during and after (recovery) negative pressure breathing (NPB) with a pressure (P = difference between airway pressure and barometric pressure) of -9.6 +/- 0.5 to -10.4 +/- 0.4 mm Hg for 30 min. Plasma arginine vasopressin (pAVP) did not change significantly comparing prestudy with 10 and 30 min of NPB or comparing recovery with NPB at 10, 20 or 30 min. However, at 20 min of NBP, pAVP was slightly lower than at prestudy (p less than 0.05). Central venous pressure (CVP) decreased significantly during NPB, and central transmural venous pressure (CVP-P) increased significantly from -0.9 +/- 0.8 mm Hg to 3.8 +/- 0.7, 4.3 +/- 0.7 and 4.5 +/- 0.6 mm Hg (p less than 0.001) after 10, 20 and 30 min, respectively. Systolic, diastolic and mean arterial pressure and heart rate did not change significantly during NPB. Diuresis, natriuresis, kaliuresis, osmotic excretion and clearance were slightly increased during the recovery hour after NPB compared to prestudy, while urine osmolality decreased during NPB (n = 6). However, none of these changes were significant. There was no significant correlation between CVP-P and pAVP. In conclusion, -10 mm Hg NPB for 30 min in upright sitting subjects did not change pAVP consistently, while CVP-P was significantly increased and HR and arterial pressures were unchanged. This lends support to the concept that arterial baroreceptors and not cardiopulmonary mechanoreceptors are of importance in regulating AVP secretion in man.

Cardiac afferents and the renal response to positive pressure ventilation in the dog

The effects of cardiac denervation on renal function during spontaneous breathing (SB) and controlled mechanical ventilation (CMV) were investigated in six mongrel dogs. Selective and reversible blockade of cardiac afferents was achieved by instillation of procaine 2% into the pericardium. Application of procaine 2% into the pericardium during SB caused a statistically significant depression of urine flow (-55%), of sodium (-64%) and potassium excretion (-42%), and of inulin (-21%) and PAH-clearance (-30%). After institution of CMV with a positive end-expiratory pressure (PEEP) of 10 cm H2O a further, statistically significant decrease in urine flow (-42%) and sodium excretion (-70%) and of the inulin (-15%) and PAH-clearance (-38%) was observed. Global hemodynamics, mean arterial pressure (MAP), central venous pressure (CVP), mean pulmonary artery pressure (MPAP) and cardiac index (CI) did not change significantly after installing procaine 2% into the pericardium during SB. After institution of CMV an increase in CVP and MPAP occurred whereas MAP and CI remained unchanged. During the following periods of spontaneous breathing first with blockade of cardiac afferents and later after washing out the procaine with NaCl 0.9% all parameters of renal function approached control levels as measured in the first period of spontaneous breathing without cardiac denervation.

A cholinergic link in the reflex release of vasopressin by hypotension in the rat

Inhalation of amyl nitrite in the water-loaded rat under ethanol anaesthesia produced a brief fall of blood pressure followed by a prolonged antidiuretic response. The antidiuretic response to amyl nitrite was accompanied by increased urinary excretion of vasopressin, it was blocked by a specific vasopressin antagonist and by a barbiturate and it was absent in the Brattleboro rat with congenital diabetes insipidus. These results show that the antidiuretic response to the hypotension induced by amyl nitrite is due to the release of vasopressin and that this release is mediated by a neuroendocrine reflex acting through the brain stem. Carbachol and nicotine produced an antidiuretic response on injection into a lateral cerebral ventricle (i. vent.). Carbachol was almost ineffective, but nicotine much more effective, when injected into the cisterna magna (i.cist.) from which in the rat there is no access to the ventricles. Carbachol therefore acts at a site reached from the ventricles, possibly the paraventricular nucleus. Nicotine acts at a more distal site reached from the subarachnoid space. This site may correspond with the nicotine-sensitive area on the ventral surface of the brain stem which has been described in the cat. Atropine blocked the antidiuretic response to carbachol but not that to amyl nitrite. Hexamethonium blocked the antidiuretic response to amyl nitrite as well as that to nicotine and was more effective on i.cist. than i.vent. injection. These results reveal a cholinergic link with a nicotinic but not a muscarinic receptor in the neural pathways controlling the release of vasopressin in response to hypotension. A hypothetical model is presented in which the release of vasopressin is stimulated by a pathway arising from chemoreceptors and inhibited by a second pathway arising from stretch- and baroreceptors. Hypotension acts by suppressing the normally predominant inhibitory pathway and stimulating the excitatory pathway. Hexamethonium is presumed to block transmission at a synapse in the excitatory pathway at the ventral surface or, less probably, at the paraventricular and supraoptic nuclei.

Facilitation of renal function by intermittent mandatory ventilation

The effects of intermittent mandatory ventilation (IMV) and controlled mechanical ventilation (CMV) on excretory function and the hemodynamics of the kidneys were studied in two groups of anaesthetized dogs during periods of 3 and 4 h. IMV was associated with statistically significant improved urinary output and renal plasma flow of approximately 50 and 35%, respectively. Graphical and statistical analysis revealed certain cross-over effects indicating that the beneficial effect of IMV was more pronounced if it was used following CMV. The improvements in renal function were interpreted as consequences of decreased mean intrathoracic pressures during IMV as compared to CMV. A correlation to global hemodynamic changes could not be established. IMV does facilitate kidney function and hence may successfully counteract the retention of water and salt which occurs during prolonged mechanical ventilation.

Cardiac and sinoaortic reflexes during aortic constriction in awake calves

The hemodynamic responses to increased afterload of the left ventricle were studied in conscious calves before and during cooling of the cervical vagus nerves bilaterally. The calves were chronically instrumented to measure (or derive) heart rate, stroke volume, cardiac output, iliac, superior mesenteric and renal flows and resistances, mean aortic, right atrial, pulmonary artery, and left atrial pressures, the systemic and pulmonary pressure gradients, and total systemic and total pulmonary resistances. The calves were also instrumented to produce reversible partial constriction of the ascending aorta and for cooling of the cervical vagus nerves. The hemodynamic responses to increased afterload were characterized before and during bilateral cervical vagus nerve cooling to 6-7 degrees C. Aortic constriction causes stroke volume, cardiac output and aortic pulse pressure to decrease. Left atrial pressure and total peripheral resistance increase. Mean aortic pressure is constant during aortic constriction alone, despite a continued decrease in pulse pressure, due to a balance between sinoaortic reflexes which attempt to increase arterial pressure in response to the decreased pulse pressure and cardiopulmonary reflexes which attempt to decrease arterial pressure in response to the increased left atrial and cardiopulmonary pressures. Vagal cooling removes cardiopulmonary reflex modulation of the sinoaortic reflexes. During aortic constriction and vagal cooling, the carotid sinus reflex, acting alone, causes large increases in renal and total peripheral resistance and mean aortic pressure.

Pathogenesis of sodium and water retention in edematous disorders

Edema is a collection of fluid within the body's interstitial space which occurs when there is an alteration of the Starling forces which control transfer of fluid from the vascular compartment to surrounding tissue spaces. Generalized edema results when altered Starling forces affect all capillary beds, such as occurs in cardiac failure, cirrhosis, and nephrotic syndrome. Common to these conditions is the development of increased total body sodium and water content. The kidneys play an essential role in the retention of this sodium and water. In this article we shall discuss the signals the kidneys receive for sodium and water retention in these edematous disorders (afferent mechanisms). We shall also examine the means by which the kidney responds to these signals and retains sodium and water (efferent mechanisms). As shall become apparent these edematous states may share many of the same afferent and efferent mechanisms for sodium and water retention.

Enhanced renal function associated with intermittent mandatory ventilation in acute respiratory failure

In ten patients suffering from acute respiratory failure (ARF) renal function was evaluated during 2-h periods of intermittent mandatory ventilation (IMV) or controlled mechanical ventilation (CMV). Urine flow, osmolal and creatinine clearances were significantly lower during CMV in comparison to both IMV phases and the free water clearance was less negative. Potassium excretion declined with CMV but remained reduced during the second IMV phase. There was no change in sodium excretion. This study suggests that in order to maintain renal function and prevent water retention the use of IMV should be considered whenever a sufficient mechanical reserve for partial spontaneous ventilation is present.

Antidiuretic hormone excess

Known physiologic mechanisms explain the elevated blood ADH levels observed in most patients with the syndrome of inappropriate ADH. Therefore the word "inappropriate" is a misnomer. It implies that the mechanisms that regulate ADH release are not functioning normally--which is not true. The term misleads the physician who, ideally, should determine why a patient has an excessive blood ADH level and initiate appropriate treatment. Patients with ectopic production of ADH and hyponatremia should be so labeled: "Hyponatremia due to ectopic ADH production." The term SIADH, if used at all, should be reserved for the rare patient with CNS injury or disease that causes increased ADH release and in which the hypothalamic center does not respond normally to afferent peripheral stimuli.

The contribution of atrial stretch receptors to salt and water homeostasis in the human

Studies carried out in both the human and experimental animals provide reasonably strong evidence that receptors contained within the atria are involved in the control of body water. When atrial pressure is increased (and the atria distended) atrial receptor discharge is increased leading to an increase in urine flow and, depending upon the experimental condition, a less consistent increase in sodium excretion. At least two mechanisms appear to contribute to the renal response; inhibition of the secretion of antidiuretic hormone and inhibition of renal nerve discharge. None of the factors presently known to alter sodium excretion have been shown to account completely for the increase in sodium excretion. The failure of patients with chronically distended atria to experience a chronic diuresis appears to be partly the result of a resetting of atrial receptor sensitivity. Recent evidence suggests that the sensitivity of atrial volume receptors has a species variation.