Effect of altered intrathoracic pressure on renal hemodynamics, electrolyte excretion and water clearance

Respiratory parameters and acute kidney injury in acute respiratory distress syndrome: a causal inference study

Background

Assess the respiratory-related parameters associated with subsequent severe acute kidney injury in mechanically ventilated patients with acute respiratory distress syndrome (ARDS).

Methods

Retrospective cohort, analyzing a large public database-Multiparameter Intelligent Monitoring in Intensive Care-III. Adult patients with at least 48 h of mechanical ventilation (MV), under volume controlled ventilation and an oxygenation index less than 300 mmHg were included.

Results

A total of 1,142 patients had complete data and were included in the final analyses. According to a causal directed acyclic graph (DAG) that included respiratory system compliance (C_rs), tidal volume (Vt), driving pressure (ΔP), plateau pressure (P_Plat), PEEP, PaO₂ and PaCO₂ as possible exposures related to severe AKI, only C_rs and PEEP levels had significant causal association with severe acute kidney injury (AKI) (OR 0.90, 95% CI: 0.84-0.94 for each 5-mL/cmH₂O reduction in C_rs; OR, 1.05 95% CI: 1.03-1.10 for each 1-cmH₂O increase of PEEP). Using mediation analysis, we examined whether any mechanical ventilation, blood gas or hemodynamic parameters could explain the effects of C_sr on AKI. Only PEEP mediated the significant but small effect (less than 5%) of C_sr on severe AKI. The effects of PEEP, in turn, were not mediated by any other evaluated parameter. Several sensitivity analyses with (I) need of renal replacement therapy (RRT) as an alternative outcome and (II) only patients with Vt <8 mL/kg, confirmed our main findings. In trying to validate our DAG assumptions, we confirmed that only ΔP was associated with mortality but not with severe AKI.

Conclusions

C_rs and PEEP are the only respiratory-related variables with a direct causal association in severe AKI. No mechanical ventilator or blood gas parameter mediated the effects of C_rs. Approaches reducing Vt and/or ΔP in ARDS can have limited effect on renal protection.

Effects of adjunct treatments on end-organ damage and histological injury severity in acute respiratory distress syndrome and multiorgan failure caused by smoke inhalation injury and burns

BACKGROUND

We investigated effects of mesenchymal stem cells (MSC) or low-flow extracorporeal life support (ECLS) as adjunctive treatments for acute respiratory distress syndrome (ARDS) due to inhalation injury and burns. We hypothesized that these interventions decrease histological end-organ damage.

METHODS

Anesthetized female swine underwent smoke inhalation injury and 40% TBSA burns, then critical care for 72h. The following groups were studied: CTR (no injury, n = 4), ICTR (injured untreated, n = 10), Allo (injured treated with allogenic MSC, n = 10), Auto (injured treated with autologous MSC, n = 10), Hemo (injured and treated with the Hemolung low flow ECLS system, n = 9), and Nova (injured and treated with the NovaLung low flow ECLS system, n = 8). Histology scores from lung, kidneys, liver, and jejunum were calculated. Data are presented as means±SEM.

RESULTS

Survival at 72h was 100% in CTR; 40% in ICTR; 50% in Allo; 90% in Auto; 33% in Hemo; 63% in Nova. ARDS developed in 0/10 CTR; 10/10 ICTR; 8/9 Hemo; 5/8 Nova; 9/10 Allo; 6/10 Auto. Diffuse alveolar damage (DAD) was present in all injured groups. MSC groups had significantly lower DAD scores than ICTR animals (Allo 26.6 ± 3.4 and Auto 18.9 ± 1.5 vs. ICTR 46.8 ± 2.1, p < 0.001). MSC groups also had lower DAD scores than ECLS animals (Allo vs. Nova, p < 0.05, Allo vs. Hemo p < 0.001, Auto vs. Nova p < 0.001, Auto vs. Hemo, p < 0.001). Kidney injury ICTR (p < 0.05) and Hemo (p < 0.01) were higher than in CTR. By logistic regression, a PaO₂-to-FiO₂ ratio (PFR) < 300 was a function of the DAD score: logit (PFR < 300) = 0.84 + 0.072*DAD Score, odds ratio 1.074 (1.007, 1.147, p < 0.05) with a ROC AUC of 0.76, p < 0.001.

CONCLUSION

Treatment with Auto MSC followed by Allo and then Nova were most effective in mitigating ARDS and MOF severity in this model. Further studies will elucidate the role of combination therapies of MSC and ECLS as comprehensive treatments for ARDS and MOF.

Partial extracorporeal carbon dioxide removal using a standard continuous renal replacement therapy device: a preliminary study

To test the feasibility, safety, and efficacy of partial extracorporeal CO2 removal (PECCO2R) using a standard continuous renal replacement (CRRT) device with a pediatric oxygenation membrane introduced into the circuit in a serial manner. In this retrospective single-center study, we have studied mechanically ventilated patients with persistent significant respiratory acidosis and acute renal failure requiring ongoing CRRT. Sixteen patients were treated with our PECCO2R device. PaCO2 and arterial pH were measured before as well as at 6 and 12 hours after PECCO2R implementation. Hemodynamic parameters were continuously monitored. Our PECCO2R system was efficient to significantly reduce PaCO2 and increase arterial pH. The median PaCO2 before treatment was 77 mm Hg (59-112) with a median reduction of 24 mm Hg after 6 hours and 30 mm Hg after 12 hours (31% and 39%, respectively). The median pH increase was 0.16 at 6 hours and 0.23 at 12 hours. Partial extracorporeal CO2 removal treatment had no effect on oxygenation. No complication was observed. Our PECCO2R approach based on the simple introduction of a pediatric extracorporeal membrane oxygenation membrane into the circuit of a standard CRRT device is easy to implement, safe, and efficient to improve respiratory acidosis.

Acute respiratory distress syndrome and risk of AKI among critically ill patients

BACKGROUND AND OBJECTIVES

Increasing experimental evidence suggests that acute respiratory distress syndrome (ARDS) may promote AKI. The primary objective of this study was to assess ARDS as a risk factor for AKI in critically ill patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This was an observational study on a prospective database fed by 18 intensive care units (ICUs). Patients with ICU stays >24 hours were enrolled over a 14-year period. ARDS was defined using the Berlin criteria and AKI was defined using the Risk, Injury, Failure, Loss of kidney function, and End-stage kidney disease criteria. Patients with AKI before ARDS onset were excluded.

RESULTS

This study enrolled 8029 patients, including 1879 patients with ARDS. AKI occurred in 31.3% of patients and was more common in patients with ARDS (44.3% versus 27.4% in patients without ARDS; P<0.001). After adjustment for confounders, both mechanical ventilation without ARDS (odds ratio [OR], 4.34; 95% confidence interval [95% CI], 3.71 to 5.10) and ARDS (OR, 11.01; 95% CI, 6.83 to 17.73) were independently associated with AKI. Hospital mortality was 14.2% (n=1140) and was higher in patients with ARDS (27.9% versus 10.0% in patients without ARDS; P<0.001) and in patients with AKI (27.6% versus 8.1% in those without AKI; P<0.001). AKI was associated with higher mortality in patients with ARDS (42.3% versus 20.2%; P<0.001).

CONCLUSIONS

ARDS was independently associated with AKI. This study suggests that ARDS should be considered as a risk factor for AKI in critically ill patients.

Neonatal non-invasive respiratory support: physiological implications

The introduction of assisted ventilation for neonatal pulmonary insufficiency has resulted in the successful treatment of many previously fatal diseases. During the past three decades, refinement of invasive mechanical ventilation techniques has dramatically improved survival of many high-risk neonates. However, as with many advances in medicine, while mortality has been reduced, morbidity has increased in the surviving high-risk neonate. In this regard, introduction of assisted ventilation has been associated with chronic lung injury, also known as bronchopulmonary dysplasia. This disease, unknown prior to the appearance of mechanical ventilation, has produced a population of patients characterized by ventilator or oxygen dependence with serious accompanying pulmonary and neurodevelopmental morbidity. The purpose of this article is to review non-invasive respiratory support methodologies to address the physiologic mechanisms by which these methods may prevent the pathophysiologic effects of invasive mechanical ventilation.

Differential effects of kidney-lung cross-talk during acute kidney injury and bacterial pneumonia

Acute injuries of the kidney or lung each represent serious, complex clinical problems, and their combination drastically decreases patient survival. However, detailed understanding of interactions between these two organs is scarce. To evaluate this further, we used the folic acid (FA) and myohemoglobinuria models of acute kidney injury (AKI) together with Pseudomonas aeruginosa inhalation to study kidney-lung cross-talk in mice during acute kidney and lung injury. Subgroups of mice received antineutrophil antibody or platelet-depleting serum to assess the role of neutrophil and platelets, respectively. AKI by itself did not cause clinically relevant acute lung injury. Pneumonia was neutrophil dependent, whereas pneumonia-induced AKI was platelet dependent. AKI attenuated pulmonary neutrophil recruitment and worsened pneumonia. Mice with AKI had lower oxygen saturations and greater bacterial load than mice without. Neutrophils isolated from mice with FA-induced AKI also had impaired transmigration and F-actin polymerization in vitro. Thus, during acute kidney and pneumonia-induced lung injury, clinically relevant kidney-lung interactions are both neutrophil and platelet dependent.

Kidney-lung crosstalk in the critically ill patient

Despite advances in renal replacement therapy, the mortality of acute kidney injury (AKI) has remained high, especially when associated with distant organ dysfunction such as acute lung injury (ALI). Mortality rates for combined AKI/ALI reach 80% in critically ill patients. While the clinical presentation of AKI-associated ALI is characterized by increased pulmonary edema, a defining feature of the syndrome, the AKI-induced lung effects extend beyond simple volume overload. Furthermore, ALI and associated mechanical ventilation frequently lead to a decline in renal hemodynamics, structure and function. New experimental data have emerged in recent years focusing on the interactive effects of kidney and lung dysfunction, and these studies have highlighted the pathophysiological importance of proinflammatory and proapoptotic pathways as well as the complex nature of interorgan crosstalk. This review will examine our current understanding of the deleterious kidney-lung crosstalk in the critically ill.

Impact of mild hypoxemia on renal function and renal resistive index during mechanical ventilation

RATIONALE

Short-term hypoxemia affects diuresis and natriuresis in healthy individuals. No data are available on the impact of the mild hypoxemia levels usually tolerated in critically ill patients receiving mechanical ventilation.

OBJECTIVES

To assess the renal effects of mild hypoxemia during mechanical ventilation for acute lung injury (ALI).

METHODS

Prospective, physiological study in 12 mechanically ventilated patients with ALI. Patients were studied at baseline with an arterial saturation (SaO(2)) of 96% [94-98] then a comparison was performed between SaO(2) values of 88-90% (mild hypoxemia) and 98-99% (high oxygenation).

MAIN RESULTS

FiO(2) was set at 0.25 [0.23-0.32] and 0.7 [0.63-0.8], respectively, to obtain SaO(2) of 89 [89-90] and 99% [98-99]. Hemodynamic or respiratory parameters were not significantly affected by FiO(2) levels. Compared with high oxygenation level, mild hypoxemia using low FiO(2) was associated with increase in diuresis (median [interquartile range], 67 [55-105] vs. 55 [45-60] ml/h; P = 0.003) and in doppler-based renal resistive index (RI) (0.78 [0.66-0.85] vs. 0.72 [0.60-0.78]; P = 0.003). The 2-h calculated creatinine clearance also increased (63 [46-103] vs. 35 [30-85] ml/min; P = 0.005) without change in urinary creatinine (P = 0.13). No significant change in natriuresis was observed. Half of the patients were under norepinephrine infusion and the renal response did not differ according to the presence of vasopressors.

CONCLUSION

In patients with ALI, mild hypoxemia related to short-term low FiO(2) induce increases in diuresis and in renal RI. This latter point suggests intra-renal mechanisms that need to be further investigated.

Mechanical ventilation and acute renal failure

OBJECTIVE

To review the current literature on possible mechanisms by which mechanical ventilation may initiate or aggravate acute renal failure.

DATA SOURCE

A Medline database and references from identified articles were used to perform a literature search relating to mechanical ventilation and acute renal failure.

DATA SYNTHESIS

Acute renal failure may be initiated or aggravated by mechanical ventilation through three different mechanisms. First, strategies such as permissive hypercapnia or permissive hypoxemia may compromise renal blood flow. Second, through effects on cardiac output, mechanical ventilation affects systemic and renal hemodynamics. Third, mechanical ventilation may cause biotrauma-a pulmonary inflammatory reaction that may generate systemic release of inflammatory mediators. The harmful effects of mechanical ventilation may become more significant when a comorbidity is present. In these situations, it is more difficult to maintain normal gas exchange, and moderate arterial hypoxemia and hypercapnia are often accepted. Renal blood flow is compromised due to a decreased cardiac output as a consequence of high intrathoracic pressures. Furthermore, the effects of biotrauma are not limited to the lungs but may lead to a systemic inflammatory reaction.

CONCLUSIONS

The development of acute renal failure during mechanical ventilation likely represents a multifactorial process that may become more important in the presence of comorbidities. Development of optimal interventional strategies requires an understanding of physiologic principles and greater insight into the precise molecular and cellular mechanisms that may also play a role.

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.

Newer experience with CPAP

Progress in neonatal intensive care is closely linked to improvements in the management of respiratory failure in small infants. Current modalities of ventilatory assistance range from more benign continuous positive airway pressure (CPAP) to various modes of mechanical ventilation (including high frequency ventilation). The advent of less invasive methods of delivering CPAP has permitted earlier treatment of infants with respiratory distress syndrome and avoided the need for mechanical ventilation. Children's Hospital of New York (Columbia University) places all spontaneously breathing infants on nasal prong CPAP as the first mode of respiratory support. The early initiation of nasal prong CPAP in combination with a tolerance to elevated PCO(2) levels has reduced the incidence of chronic lung disease to <5% in infants weighing less than 1500g. This report will present an historical review and summarize the experience with CPAP at Columbia and other centres. In addition, it reviews the clinical applications and physiological effects of CPAP in preterm infants with respiratory distress syndrome.

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.

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.

Low blood flow extracorporeal carbon dioxide removal (ECCO2R): a review of the concept and a case report

Despite advances in respiratory and critical care medicine, the mortality from ARDS remains unchanged. Recent research suggests current ventilatory therapy may produce additional lung injury, retarding the recovery process of the lung. Alternative supportive therapies, such as ECMO and ECCO2R, ultimately may result in less ventilator induced lung injury. Due to the invasiveness of ECMO/ECCO2R, these modalities are initiated reluctantly and commonly not until patients suffer from terminal or near-terminal respiratory failure. Low flow ECCO2R may offer advantages of less invasiveness and be suitable for early institution before ARDS becomes irreversible. We describe a patient with ARDS and severe macroscopic barotrauma supported with low flow ECCO2R resulting in significant CO2 clearance, reduction of peak, mean airway pressures and minute 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.

Hyponatremia in spinal cord injury

Hypoosmolar hyponatremia (serum Na+ less than 130 mmol/L) has proven a common and incompletely explained phenomenon in the spinal cord injured patient. When present, it has generally been preceded by excessive fluid intake and environmental/dietary factors which reversibly restrict free water excretion. We have attempted to more fully characterize the determinants of SCI-associated hyponatremia by retrospectively analyzing its features and treatment response in a series of 14 hyponatremic SCI patients. In most instances, hyponatremia could be attributed to uncontrolled fluid intake in the presence of an acute or semiacute illness and thus stimuli for non-osmotic releases of arginine vasopressin. Treatment measures generally included administration of 3% saline, with all patients recovering uneventfully from their episode of hyponatremia.

Physiologic implications of mechanical ventilation on pharmacokinetics

Numerous factors present in the critically ill patient decrease drug clearance. The contribution of one factor, mechanical ventilation, to this decrease is largely unknown and unquantified. This article attempts to review the physiologic effects of mechanical ventilation and to propose theoretical changes in the pharmacokinetics of concomitantly administered drugs. Mechanical ventilation with or without positive end-expiratory pressure is a well-documented cause of decreases in cardiac output, hepatic and renal blood flow, glomerular filtration rate, and urine flow. The mean airway pressure delivered, the pathophysiologic state of the patient, and coexisting therapeutic interventions affect the degree of hemodynamic alteration. Theoretically, these hemodynamic changes can decrease the clearance of several drugs frequently administered to critically ill patients. Decreased hepatic blood flow decreases the clearance of nonrestrictively cleared drugs. The pharmacokinetics of drugs predominantly renally cleared, by either glomerular filtration or tubular secretion, are affected by a decrease in renal blood flow or glomerular filtration rate. Also, the clearance of agents for which tubular reabsorption is important may decrease because the reduction in urine flow resulting from mechanical ventilation allows increased time for drug reabsorption. Interventions that minimize the decrease in cardiac output and organ blood flow and, theoretically, the risk of the adverse drug reactions from decreased drug clearance include expansion of intravascular volume, administering positive inotropic agents, and decreasing mean airway pressure. Monitoring serum concentration of critical and toxic agents suspected to have altered clearance in patients receiving mechanical ventilation is recommended. We hope that our article will stimulate future research in this area to give clinicians guidelines for drug dosing in patients receiving mechanical ventilation.

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.

Sympathetic nerve activity and central haemodynamics during mechanical ventilation with positive end-expiratory pressure in rats

The aim of this study was to examine the effects of mechanical ventilation with increasing levels of positive end-expiratory pressure (PEEP) on sympathetic nerve activity (SNA), cardiac output (CO), stroke volume (SV), heart rate (HR), central blood volume (CBV), total peripheral resistance (TPR), mean arterial pressure (MAP), pulse pressure (PP) and right and left atrial transmural pressure in chloralose anaesthetized rats before and after vagotomy. Changing ventilatory pattern from spontaneous breathing (SB) to artificial ventilation with 10 cm H2O PEEP in intact animals caused a significant fall in CO, SV and CBV (42, 48 and 17%, respectively) and an increase in SNA, HR and TPR (90, 13 and 83%, respectively). The MAP increased slightly but significantly from 103 +/- 4 to 107 +/- 4 mmHg while PP decreased from 48 +/- 2 to 37 +/- 3, from spontaneous breathing (SB) to 10 cm H2O PEEP. Transmural left atrial pressure decreased significantly from 4.5 +/- 0.3 to 3.0 +/- 0.4 mmHg. After vagotomy, MAP and CO were significantly lower at 10 cm H2O PEEP and PP and SV were significantly lower at all levels of positive end-expiratory pressure than the corresponding prevagotomy values. In spite of a greater fall in MAP and PP during PEEP after vagotomy, the absolute and relative increase of SNA was significantly lower compared to corresponding prevagotomy values. We conclude that reflex cardiovascular adjustments elicited by ventilation with PEEP are not solely due to unloading of arterial baroreceptors as has been claimed by others. Unloading of cardiac receptors with tonically active inhibitory afferents in the vagi is probably also of great importance for the excitation of the sympathetic nervous system during mechanical ventilation with PEEP.

Alternative modes of ventilation. Part I. Disadvantages of controlled mechanical ventilation: intermittent mandatory ventilation

Controlled mechanical ventilation is an accepted therapy for acute respiratory failure but by virtue of the increase in intrathoracic pressure has a large number of disadvantages. It is to overcome these disadvantages that alternative modes of ventilation have been introduced. These aim to reduce the effects of abnormally high airway pressure on the lung whilst recruiting solid alveoli and at the same time maintaining effective blood volume. Intermittent mandatory ventilation is a mode of ventilation first introduced to aid weaning which may reduce the need for sedation, permit better tolerance of high levels of PEEP and maintain urine osmolar output. High frequency ventilation utilising low airway pressures can maintain pulmonary gas exchange whilst reducing the effects of stretch on the lung. Its major role would seem to be in cases of bronchopleural fistula and necrotising pneumonia where a low mean airway pressure is essential. Low frequency positive pressure ventilation with extra corporeal CO2 removal, whilst a very labour intensive technique, has produced a favourable outcome in patients with terminal respiratory failure. Use of PEEP is associated with further deleterious haemodynamic effects which are largely overcome with use of continuous positive airway pressure during spontaneous respiration. PEEP is widely used. Its effect on pulmonary compliance, dead space and oxygen delivery are unpredictable making haemodynamic monitoring mandatory. Inversed ratio ventilation requires further evaluation whereas differential lung ventilation is logical, complicated but very valuable where the time constants for each lung are significantly different.

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.

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.

Effects of positive end-expiratory pressure on renal function

The effects were studied positive end-expiratory pressure (PEEP) on renal function in eight patients with acute respiratory failure, requiring mechanical ventilation. On application of PEEP + 10 cm H2O, central venous pressure increased, systolic blood pressure decreased, urine flow and PAH-clearance were reduced, while inulin clearance remained stable. There was a marked increase in fractional sodium reabsorption and a concurrent decrease in fractional osmolal excretion. Fractional free-water clearance and the ratio UOsm/POsm did change.

Sleep-induced apnea. 1. A respiratory and autonomic dysfunction syndrome following bilateral percutaneous cervical cordotomy

✓ Data are reported on 10 patients who developed a syndrome of sleepinduced apnea preceded by lethargy and asthenia following bilateral percutaneous cervical cordotomy. Respiratory dysfunction occurred within 24 to 48 hours in most cases but appeared as early as 1 hour and as late as 6 days. One type of respiratory dysfunction was characterized by an attenuated CO2 response with a normal vital capacity; in a second type an attenuated CO2 response and a decreased vital capacity were both present. A variety of other autonomic dysfunctions were present in some of the patients; these included hypotension, hyponatremia, inappropriate antidiuretic hormone secretion, and difficulties in micturition. The syndrome lasted from 3 to 32 days in surviving patients. Five patients required endotracheal intubation. Three deaths were attributable to this syndrome; two occurred in patients who were not intubated and died in their sleep. The incidence of apnea during sleep, its reversal by arousal, and the absence of significant motor changes strongly suggest that the ascending reticular fibers in the ventrolateral segment of the spinal cord have been damaged.