The effect of oxygen on sodium excretion in hypoxemic patients with chronic obstructive lung disease

Human hypoxia models in aerospace medicine: Potential applications for human pharmacological research

Aerospace medicine required controlled terrestrial models to investigate influences of altered atmosphere conditions, such as hypoxia, on human health and performance. These models could potentially be expanded to encompass disease conditions or treatment targets regulated through hypoxia or hypercapnia. Hypoxia, a condition in which the body is deprived of adequate oxygen supply, profoundly affects human physiology at multiple levels and contributes to the pathogenesis of various diseases. Experimental exposure to hypoxic conditions has gained recognition as a model for studying diseases such as pulmonary hypertension, chronic obstructive pulmonary disease, obstructive sleep apnoea, migraine and kidney disease. This approach may be particularly useful in mechanism-oriented early-stage clinical studies. This review discusses the ability of hypoxia models from space medicine research to mimic or induce these conditions in a controlled laboratory setting as a tool for testing the efficacy and safety of new pharmaceutical interventions.

Pulmonary heart disease: The heart-lung interaction and its impact on patient phenotypes

Pulmonary heart disease (PHD) refers to altered structure or function of the right ventricle occurring in association with abnormal respiratory function. Although nearly always associated with some degree of PH, the degree, nature, severity, and causality of PH in relation to the PHD is not necessarily linear and direct. Abnormal gas exchange is a fundamental underpinning of PHD, affecting pulmonary vascular, cardiac, renal, and neurohormonal systems. Direct and indirect effects of chronic respiratory disease can disrupt the right ventricular-pulmonary arterial (RV-PA) interaction and, likewise, factors such as sympathetic nervous system activation, altered blood viscosity, and salt and water retention can function in a feedback loop to further influence RV-PA function. Left heart function may also be affected, especially in those with pre-existing left heart disease. Thus, the physiologic interactions between abnormal respiratory and cardiovascular function are complex, with PHD representing a heterogeneous end organ effect of an integrated multisystem process. In this review, we propose to separate PHD into two distinct entities, “Type I” and “Type II” PHD. Type I PHD is most common, and refers to subjects with chronic respiratory disease (CRD) where the perturbations in respiratory function dominate over more mild cardiac and circulatory disruptions. In contrast, Type II PHD refers to the smaller subset of patients with more severe pulmonary vascular and right heart dysfunction, whom often present in a fashion similar to patients with PAH. Phenotypic differences are not made by PA pressure alone, but instead by differences in the overall physiology and clinical syndrome. Thus, key differences can be seen in symptomatology, physical signs, cardiac imaging, hemodynamics, and the cardiovascular and gas exchange responses to exercise. Such key baseline differences in the overall physiologic phenotype are likely critical to predicting response to PH specific therapy. Recognizing PHD as distinct phenotypes assists in the necessary distinction of these patients, and may also provide a key clinical and pathophysiologic framework for improved patient selection for future studies investigating the role of pulmonary hypertension-specific therapies in PHD.

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.

Pulmonary hypertension, right ventricular failure, and kidney: different from left ventricular failure?

In this article, the pathophysiology of left ventricular failure is reviewed. By contrast, the paucity of information about pulmonary arterial hypertension and right ventricular failure is acknowledged. The potential mechanisms whereby renal sodium and water retention in right ventricular failure secondary to pulmonary arterial hypertension can occur, despite normal left ventricular function, are discussed. With right ventricular failure as the primary cause of death in patients with pulmonary hypertension, more information about the mechanisms of renal sodium and water retention in these patients is direly needed. Specifically, studies to examine the activation of the neurohumoral axis at various stages of pulmonary arterial hypertension and right ventricular failure, including inhibition of mineralocorticoid and V2 vasopressin receptors, are indicated.

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.

Management of severe COPD

Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality worldwide, and the burden of the disorder will continue to increase over the next 20 years despite medical intervention. Apart from smoking cessation, no approach or agent affects the rate of decline in lung function and progression of the disease. Especially in the later phase, COPD is a multicomponent disorder, and various integrated intervention strategies are needed as part of the optimum management programme. This seminar describes largely non-pharmacological interventions aimed at improving health status and function of disabled patients. Exacerbations become progressively more troublesome as baseline lung function declines, commonly necessitating hospital admission and associated with the development of acute respiratory failure.

Renal function in hypoxaemic chronic obstructive pulmonary disease: effects of long-term oxygen treatment

Impaired renal function is an important cause for the oedema formation, which often occurs in severe chronic obstructive pulmonary disease (COPD). In the present study, the importance of nocturnal hypoxaemia (measured by a nocturnal pulse oximetry) for the renal function was determined in 19 COPD patients, with normal levels of serum creatinine. The effects on kidney function of alleviating the nocturnal hypoxaemia [using 6 months of long-term oxygen treatment (LTOT)], was assessed in 12 patients. Renal function was assessed by determining the clearances of intravenously administered inulin (C(In)) and para-amino-hippurate (C(PAH)) and orally supplemented lithium (C(Li)) and of circulating sodium (C(Na)). The 19 patients had a mean PaO2 of 7.63 +/- 1.08 kPa, a PaCO2 of 5.98 +/- 0.85 kPa, a mean nocturnal oxygen saturation (MnSaO2) of 87.7 +/- 2.8% and an FEV1 in %P of 25.6 +/- 14.6%. C(In) and C(PAH) were 35 and 45% lower than normal, respectively, whereas C(In)/C(PAH)=filtration fraction (FF) was 31% higher than normal. Six months of LTOT in 12 of the patients was not followed by any significant change in renal function in the entire study group. However, low pretreatment MnSaO2 correlated with reductions in post-treatment (FF) (r=0.73, P<0.05). Post-treatment PaCO2 did not change significantly in patients treated with oral diuretics, but increased (P<0.05) in patients without diuretics. C(Na) decreased after LTOT in six patients with an increase in PaCO2>6%, but C(Na) increased in four patients with unchanged or decreased PaCO2 following LTOT.

CONCLUSIONS

Renal function (including filtration fraction) is impaired in hypoxaemic COPD. Filtration fraction is decreased following 6 months of LTOT solely in patients with severe pretreatment hypoxaemia and sodium clearance seems to be increased if improved oxygenation is not accompanied by increased PaCO2.

Effects of hypoxia on renal hormonal balance in normal subjects and in patients with COPD

There is a complex interaction between pulmonary haemodynamics, hormonal, and salt and water balance in patients with chronic obstructive pulmonary disease (COPD) and in normal subjects exposed to hypoxia or high altitude. This study aims to investigate the effects of hypoxia on renal hormonal balance in normal subjects and patients with COPD, particularly the role of urinary dopamine and atrial natriuretic peptide (ANP). Urinary dopamine output, ANP, and plasma renin activity (PRA) were measured in 12 normal subjects exposed to hypoxia (12% O2) and hyperoxia (40% O2) for 1 h and in 15 patients with exacerbations of COPD while breathing air or O2. These measurements were repeated in six of the patients with exacerbations of COPD when they were clinically stable. Hypoxia caused an increase in ANP levels (49 +/- 6-62 +/- 6 pg ml-1, P < 0.05) and a fall in urinary dopamine output (277 +/- 39-205 +/- 33 ng h-1, P < 0.002) in normal subjects. Hyperoxia was associated with a return of plasma ANP to the baseline values. In patients with exacerbations of COPD plasma ANP levels were higher (181 +/- 36 pg ml-1) than in normal subjects (49.5 +/- 6.5 pg ml-1, P < 0.001). Urinary dopamine output breathing air (175 +/- 34 ng h-1) was similar to the levels when normal subjects were made hypoxaemic and PRA was elevated in comparison to normal values. There was no change in their levels following the acute administration of oxygen in patients presenting with exacerbations of COPD, but oxygen improved urinary sodium excretion (P < 0.05). In six patients re-studied when clinically stable there was a fall in urinary dopamine output, plasma ANP and PRA when breathing air in comparison to the acute stage of the disease (P < 0.05). These data suggest presence of renal hormonal imbalance including endogenous urinary dopamine output during hypoxic exacerbation of COPD and in normal subjects exposed to hypoxia.

The role of the renin-angiotensin and natriuretic peptide systems in the pulmonary vasculature

1. The role of vasoactive peptide systems in the pulmonary vasculature has been studied much less extensively than systemic vascular and endocrine effects. The current understanding of the role of the renin-angiotensin (RAS) and natriuretic peptide systems (NPS) in the pulmonary circulation is therefore reviewed. 2. Plasma concentrations of angiotensin II, the main vasoactive component of the RAS, are elevated in pulmonary hypertension and may interact with hypoxaemia to cause further pulmonary vasoconstriction. Pharmacological manipulation of angiotensin II can attenuate hypoxic pulmonary vasoconstriction but larger studies are needed to establish the efficacy of this therapeutic strategy in established pulmonary hypertension. 3. Although all the known natriuretic peptides, ANP, BNP and CNP are elevated in cor pulmonale, only ANP and BNP appear to have pulmonary vasorelaxant activity in humans. ANP and BNP can also attenuate hypoxic pulmonary vasoconstriction, suggesting a possible counter-regulatory role for these peptides. Inhibition of ANP/BNP metabolism by neutral endopeptidase has been shown to attenuate development of hypoxic pulmonary hypertension but this property has not been tested in humans. 4. It is also well established that there are potentially important endocrine and systemic circulatory interactions between the RAS and NPS. This also occurs in the pulmonary circulation and in humans, where at least BNP acts to attenuate angiotensin II induced pulmonary vasoconstriction. This interaction may be particularly relevant as a mechanism to counter-regulate overactivity of the RAS.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide in stable and decompensated chronic obstructive pulmonary disease

BACKGROUND

Plasma levels of atrial natriuretic peptide (ANP) are elevated in patients with chronic obstructive pulmonary disease (COPD) and may have a role in preventing oedema formation in these patients.

METHODS

Plasma ANP levels were measured in 60 patients with COPD and these measurements were related to pulmonary haemodynamics, response to treatment during exacerbations, and clinical patterns of the stable disease.

RESULTS

Plasma ANP levels did not correlate significantly with right atrial or pulmonary arterial pressures but did correlate significantly with both the right ventricular end diastolic volume and right ventricular wall volume measured by magnetic resonance imaging. Oxygen (2 1/min by nasal prongs for 30 minutes) did not change the mean pulmonary arterial pressure or the level of plasma ANP. In 20 patients with an acute exacerbation of COPD plasma ANP levels were higher in those with oedema (302 (185) pg/ml) than in those without oedema (87 (43) pg/ml). Oxygen given for one hour had no effect on plasma levels of ANP. However, plasma ANP levels fell over the first three days during treatment in those with oedema, the fall correlating with the change in body weight. In a further 20 stable patients with hypoxic COPD, those with hypercapnia and previous episodes of oedema had higher levels of plasma ANP (120 (50) pg/ml) than normocapnic patients with no previous oedema (54 (15) pg/ml).

CONCLUSIONS

The level of ANP is high in the plasma of patients with COPD, particularly during exacerbations in those with oedema. The association of a high plasma ANP level and volume overload is shown by the fall in ANP levels with treatment of the oedema, and the correlation between levels of ANP and right ventricular end diastolic or wall volumes.

Short term effect of oxygen on renal haemodynamics in patients with hypoxaemic chronic obstructive airways disease

BACKGROUND

Oxygen therapy is effective in the prevention and treatment of oedematous exacerbations of cor pulmonale. As renal blood flow is reduced in cor pulmonale a study was designed to investigate whether one of the beneficial effects of oxygen was to increase renal blood flow. The effect of oxygen therapy on renal haemodynamics measured noninvasively was examined in patients with chronic obstructive airways disease and previous episodes of oedema.

METHODS

Renal blood flow waveforms were recorded in a single vessel by colour flow Doppler ultrasound in nine hypoxaemic patients (PaO2) (arterial oxygen tension < 8 kPa while they were breathing air) with chronic obstructive airways disease and previous oedema and eight age matched normoxaemic volunteers (arterial oxygen saturation (SaO2) 97% or more when breathing air) while they were breathing air and oxygen. SaO2 and transcutaneous PaO2 (TcPO2) and PaCO2 (TcPCO2) were monitored. Five renal velocity profile recordings were made from the same segmental vessel with the patient breathing room air for one hour followed by oxygen titrated to achieve an oxygen saturation of 95% or more without a rise in TcPCO2 for 15 minutes. Control subjects breathed 35% oxygen.

RESULTS

No significant change in the pulsatility index (a measure of distal vascular resistance) or mean height of the waveform (Tamx, a measure of renal blood flow) occurred in the control subjects while they were breathing air or oxygen. The pulsatility index of the patients with chronic obstructive airways disease was significantly greater than that in the control subjects breathing air (1.44 (SD 0.28) v 1.03 (0.14). Breathing oxygen was associated with an increase in TcPO2 in the patients (from 6.9 (1.9) to 11.5 (0.7) kPa), a fall in pulsatility index (from 1.44 (0.28) to 1.26 (0.14) and an increase in Tamx (from 0.187 (0.055) to 0.234 (0.087) m/s).

CONCLUSIONS

The results suggest that renal vascular resistance is increased in patients with chronic obstructive airways disease and hypoxaemia and that short term oxygen therapy reduces renal vascular resistance and increases blood flow. Some of the benefits of oxygen therapy in cor pulmonale may be due to improvements in renal haemodynamics.

Oxygen may lower the O2 cost of ventilation in chronic obstructive lung disease

It is known that the O2 COV in COLD is high; O2 administration to these patients lowers airway resistance, a major determinant of the COV. Thus, O2 should lower the COV. We measured the COV in ten stable COLD patients and five normal control subjects breathing room air and 30 percent O2. Results indicate that the COV of our patients was elevated above that of control subjects, was related to disease severity, and was decreased with 30 percent O2. The COV of control subjects also was lowered by O2. At rest, O2 lowered VE, VEQ O2 and HR. During submaximal exercise O2 lowered VE, reduced VEQ O2 and extended total exercise time. An inverse correlation was noted between COV and maximal O2 uptake. Thus, in stable COLD, the COV is elevated in proportion to the degree of airway obstruction, inversely related to exercise capacity and lowered by O2 administration.

The effect of oxygen with exercise on atrial natriuretic peptide in chronic obstructive lung disease

A recent study on stable, hypoxemic, COLD patients in which ANP was stimulated by LBPP demonstrated that in these individuals elevation of ANP does not exert a "normal" suppressing effect on the PRA-PA axis. Accordingly, we exercised ten comparable COLD patients, another maneuver known to stimulate ANP and to elicit cardiorespiratory responses substantially different from those observed with LBPP. Patients were studied breathing room air and on 40 percent O2 to determine whether the level of oxygenation would modify ANP secretion. Basal levels of ANP on room air were markedly elevated above controls (269 +/- 65 SE vs 70 +/- 20 pg/ml, p less than 0.05); PRA (13.0 +/- 5.4 ng/ml/90 min) and PA (8.6 +/- 3.5 ng/100 ml) were elevated (greater than 2 SD over control levels of 8.1 +/- 1.3 and 2.6 +/- 0.7) in 6/10 and 2/10 patients, respectively. During exercise while breathing O2, only ANP increased; PRA and PA remained unchanged when breathing air and O2. Comprehensive statistical analyses failed to demonstrate a negative relationship between ANP and PRA or ANP and PA. We conclude that in patients with advanced COLD, ANP response to moderate exercise is significantly affected by correction of hypoxemia. This effect may be mediated through changes in airway resistance and consequently cardiac filling pressure.