Contribution of multiple inert gas elimination technique to pulmonary medicine--4. Gas exchange abnormalities in pulmonary vascular and cardiac disease

[Invasive Cardiopulmonary Exercise Testing: A Review]

Right heart catheterization (RHC) is the internationally standardized reference method for measuring pulmonary hemodynamics under resting conditions. In recent years, increasing efforts have been made to establish the reliable assessment of exercise hemodynamics as well, in order to obtain additional diagnostic and prognostic data. Furthermore, cardiopulmonary exercise testing (CPET), as the most comprehensive non-invasive exercise test, is increasingly performed in combination with RHC providing detailed pathophysiological insights into the exercise response, so-called invasive cardiopulmonary exercise testing (iCPET).In this review, the accumulated experience with iCPET is presented and methodological details are discussed. This complex examination is especially helpful in differentiating the underlying causes of unexplained dyspnea. In particular, early forms of cardiac or pulmonary vascular dysfunction can be detected by integrated analysis of hemodynamic as well as ventilatory and gas exchange data. It is expected that with increasing validation of iCPET parameters, a more reliable differentiation of normal from pathological stress reactions will be possible.

Disease-Targeted Treatment Improves Cognitive Function in Patients with Precapillary Pulmonary Hypertension

BACKGROUND

Patients with pulmonary hypertension (PH) may suffer from cognitive deficits that potentially relate to reduced oxygen delivery and cerebral tissue oxygenation (CTO).

OBJECTIVE

To evaluate the hypothesis that cognitive function improves with therapy, along with improved CTO.

METHODS

Twenty incident patients with arterial or chronic thromboembolic PH had CTO monitoring by near-infrared spectroscopy during diagnostic right heart catheterization. Cognitive tests [Trail Making Tests (TMTs), Victoria Stroop tests and the Five-Point Test (5PT)], the 6-min walk distance (6MWD) test, New York Heart Association (NYHA) class and health-related quality of life (HRQoL) were assessed and repeated after 3 months of disease-targeted medication.

RESULTS

At baseline, 45% of PH patients had cognitive deficits. At 3 months, the patients had improved on the TMT A and the Stroop 2 test [37 s (27; 55) versus 30 s (24; 42), p < 0.05, and 18 s (16; 22) versus 16 s (15; 20), p < 0.01], whereas CTO remained unchanged. Arterial oxygen saturation, NYHA class, 6MWD and HRQoL had also improved. Baseline CTO was the strongest predictor of cognitive function, even in multivariate analysis including age, 6MWD and HRQoL. Improvements in cognitive function were not associated with changes in CTO.

CONCLUSIONS

In patients with PH, 3 months of disease-targeted medication resulted in better cognitive function. Although CTO was the strongest predictor of cognitive function at baseline, it did not change during target therapy. The results of this pilot study should be confirmed in an adequately powered controlled trial.

Effects of exercise and vasodilators on cerebral tissue oxygenation in pulmonary hypertension

BACKGROUND

Arterial and thromboembolic pulmonary hypertension (PH) lead to arterial hypoxaemia.

OBJECTIVE

To investigate whether cerebral tissue oxygenation (CTO) in patients with PH is reduced and whether this is associated with reduced exercise tolerance.

METHODS

16 patients with PH (mean pulmonary arterial pressure ≥25 mmHg, 14 arterial, 2 chronic thromboembolic) and 15 controls underwent right heart catheterisation with monitoring of CTO at rest, during maximal bicycle exercise and during inhalation of oxygen and NO. The 6 min walk distance (6MWD) was measured.

RESULTS

Median CTO in PH-patients at rest was 62 % (quartiles 53; 71), during exercise 60 % (53; 65); corresponding values in controls were 65 % (73; 73) (P = NS) and 68 % (66; 70) (p = .013 vs. PH). Inhalation of NO and oxygen improved CTO in PH. In multivariate regression analysis CTO at maximal exercise predicted the work load achieved when controlled for age, pulmonary vascular resistance and mixed venous oxygen saturation (R (2) = .419, p < .000); in addition, the 6MWD was predicted by CTO (adjusted R (2) = .511, p < .000).

CONCLUSION

In PH-patients but not in controls CTO decreased during exercise. Since CTO was an independent predictor of the work load achieved and the 6MWD cerebral hypoxia may contribute to exercise limitation in PH. Clinicaltrials.gov: NCT01463514.

Severe hypoxaemia can predict unfavourable clinical outcomes in individuals with pulmonary embolism aged over 40 years

INTRODUCTION

Acute pulmonary embolism (APE) is an urgent clinical condition that can progress in a wide variety of ways. Therefore, we sought to develop an easy-to-apply algorithm, to be based on readily available clinical indicators, effective in predicting unfavourable outcomes.

METHODS

This was a retrospective cohort study based on systematically collected data in a database. The study included 102 patients with APE who were admitted to a tertiary care hospital. The following outcomes were defined as unfavourable: shock, the need for mechanical ventilation, the use of thrombolytics, and death. Logistic regression analysis was used to explore variables significantly associated with outcome and to calculate post-test probabilities.

RESULTS

The prevalence of unfavourable outcomes was 25.5% (26 of the 102 patients with APE). The risk of an unfavourable outcome was reduced to 7.0% for patients with APE who were aged ≤ 40 years. In patients with APE who were aged > 40 years, the presence of hypoxaemia (i.e. peripheral oxygen saturation < 90%) alone increased the risk of an unfavourable outcome to 57.0%. A recent history of trauma and the presence of pre-existing lung or heart disease were significantly associated with unfavourable outcomes. The inclusion of those variables in the logistic regression model increased the post-test risk of an unfavourable outcome to 65.0%-86.0%.

CONCLUSION

Advanced age (i.e. > 40 years), the presence of hypoxaemia, a recent history of trauma and the presence of pre-existing lung or heart disease are risk factors for unfavourable outcome in patients with APE.

Gas exchange and pulmonary hypertension following acute pulmonary thromboembolism: has the emperor got some new clothes yet?

Patients present with a wide range of hypoxemia after acute pulmonary thromboembolism (APTE). Recent studies using fluorescent microspheres demonstrated that the scattering of regional blood flows after APTE, created by the embolic obstruction unique in each patient, significantly worsened regional ventilation/perfusion (V/Q) heterogeneity and explained the variability in gas exchange. Furthermore, earlier investigators suggested the roles of released vasoactive mediators in affecting pulmonary hypertension after APTE, but their quantification remained challenging. The latest study reported that mechanical obstruction by clots accounted for most of the increase in pulmonary vascular resistance, but that endothelin-mediated vasoconstriction also persisted at significant level during the early phase.

Multiple inert gas elimination technique by micropore membrane inlet mass spectrometry--a comparison with reference gas chromatography

The mismatching of alveolar ventilation and perfusion (VA/Q) is the major determinant of impaired gas exchange. The gold standard for measuring VA/Q distributions is based on measurements of the elimination and retention of infused inert gases. Conventional multiple inert gas elimination technique (MIGET) uses gas chromatography (GC) to measure the inert gas partial pressures, which requires tonometry of blood samples with a gas that can then be injected into the chromatograph. The method is laborious and requires meticulous care. A new technique based on micropore membrane inlet mass spectrometry (MMIMS) facilitates the handling of blood and gas samples and provides nearly real-time analysis. In this study we compared MIGET by GC and MMIMS in 10 piglets: 1) 3 with healthy lungs; 2) 4 with oleic acid injury; and 3) 3 with isolated left lower lobe ventilation. The different protocols ensured a large range of normal and abnormal VA/Q distributions. Eight inert gases (SF6, krypton, ethane, cyclopropane, desflurane, enflurane, diethyl ether, and acetone) were infused; six of these gases were measured with MMIMS, and six were measured with GC. We found close agreement of retention and excretion of the gases and the constructed VA/Q distributions between GC and MMIMS, and predicted PaO2 from both methods compared well with measured PaO2. VA/Q by GC produced more widely dispersed modes than MMIMS, explained in part by differences in the algorithms used to calculate VA/Q distributions. In conclusion, MMIMS enables faster measurement of VA/Q, is less demanding than GC, and produces comparable results.

The Intelligent Ventilator (INVENT) project: the role of mathematical models in translating physiological knowledge into clinical practice

This dissertation has addressed the broad hypothesis as to whether building mathematical models is useful as a tool for translating physiological knowledge into clinical practice. In doing so it describes work on the INtelligent VENTilator project (INVENT), the goal of which is to build, evaluate and integrate into clinical practice, a model-based decision support system for control of mechanical ventilation. The dissertation describes the mathematical models included in INVENT, i.e. a model of pulmonary gas exchange focusing on oxygen transport, and a model of the acid-base status of blood, interstitial fluid and tissues. These models have been validated, and applied in two other systems: ALPE, a system for measuring pulmonary gas exchange and ARTY, a system for arterialisation of the acid-base and oxygen status of peripheral venous blood. The major contributions of this work are as follows. A mathematical model has been developed which can describe pulmonary gas exchange more accurately that current clinical techniques. This model is parsimonious in that it can describe pulmonary gas exchange from measurements easily available in the clinic, along with a readily automatable variation in F(I)O(2). This technique and model have been developed into a research and commercial tool (ALPE), and evaluated both in the clinical setting and when compared to the reference multiple inert gas elimination technique (MIGET). Mathematical models have been developed of the acid- base chemistry of blood, interstitial fluid and tissues, with these models formulated using a mass-action mass-balance approach. The model of blood has been validated against literature data describing the addition and removal of CO(2), strong acid or base, and haemoglobin; and the effects of oxygenation or deoxygenation. The model has also been validated in new studies, and shown to simulate accurately and precisely the mixing of blood samples at different PCO(2) and PO(2) levels. This model of acid-base chemistry of blood has been applied in the ARTY system. ARTY has been shown to accurately and precisely calculate arterial values of acid-base and oxygen status in patients residing in the ICU, and in those with chronic lung disease. The INtelligent VENTilator (INVENT) system has been developed for optimization of mechanical ventilator settings using physiological models and utility/penalty functions, separating physiological knowledge from clinical preference. The models can be tuned to the individual patient via parameter estimation, providing patient specific advice. The INVENT team has shown prospectively that the system provides advice on F(I)O(2) which is as good as clinical practice, and retrospectively that the system provides reasonable suggestions of tidal volume, respiratory frequency and F(I)O(2). In general, this dissertation has illustrated a further example of the role of modeling in describing and understanding complex systems. The dissertation has shown that when dealing with complexity the goal of the model must be in focus if a correct balance is to be maintained between system complexity and model parameterization. The original goal of the INVENT team, i.e. to build, evaluate and integrate a DSS for control of mechanical ventilation has not as yet been completed. However, the broader hypothesis that building models generates new and interesting questions has been successfully demonstrated. The ALPE model and system has been applied in intensive care, post operative care and cardiology and is currently being evaluated in new clinical domains. ARTY has been shown to have potential benefit in eliminating the need for painful arterial punctures, and may also be useful as a screening tool. These systems illustrate the benefits of investing in models as a mechanism for translating physiological knowledge to clinical practice.

Quantification of lung injury using ventilation and perfusion distributions obtained from gamma scintigraphy

This paper explores the potential of isotope V/Q lung scans to quantify lung disease. Areas of restricted perfusion in subjects with a pulmonary embolus (PE) were identified in 3D reconstructions of V/Q images achieved using anatomical data from the Visible Human Project. From these, the extent of lung damage was quantified. Significant differences in the values of both LogSD V and LogSD Q (p > 0.05) obtained from plots of V and Q against Log(V/Q) were found between normal subjects and subjects with a PE, but no correlation was found between either of these parameters and the degree of lung damage in subjects with a PE (p > 0.05). Whilst V/Q values were log normally distributed, the V/Q distributions from the subjects with a PE failed to show the bimodal distribution predicted from theoretical considerations and MIGET measurements previously reported. There was a statistically significant difference in the mean and standard deviation values of the V/Q distributions between normal subject and subjects with a PE (p < 0.05) but not in the median values (p > 0.05). There was no correlation between the mean, median and standard deviation of the distributions from the subjects with a PE and the percentage of damage present (p > 0.05).

A tidally breathing model of ventilation, perfusion and volume in normal and diseased lungs †

BACKGROUND

To simulate the short-term dynamics of soluble gas exchange (e.g. CO2 rebreathing), model structure, ventilation-perfusion (VA/Q) and ventilation-volume (VA/VA) parameters must be selected correctly. Some diseases affect mainly the VA/Q distribution while others affect both VA/Q and VA/VA distributions. Results from the multiple inert gas elimination technique (MIGET) and multiple breath nitrogen washout (MBNW) can be used to select VA/Q and VA/VA parameters, but no method exists for combining VA/Q and VA/VA parameters in a multicompartment lung model.

METHODS

We define a tidally breathing lung model containing shunt and up to eight alveolar compartments. Quantitative and qualitative understanding of the diseases is used to reduce the number of model compartments to achieve a unique solution. The reduced model is fitted simultaneously to inert gas retentions calculated from published VA/Q distributions and normalized MBNWs obtained from similar subjects. Normal lungs and representative cases of emphysema and embolism are studied.

RESULTS

The normal, emphysematous and embolism models simplify to one, three and two alveolar compartments, respectively.

CONCLUSIONS

The models reproduce their respective MIGET and MBNW patient results well, and predict disease-specific steady-state and dynamic soluble and insoluble gas responses.

Exhaled nitric oxide during exercise and dobutamine stress echocardiography in patients with mitral stenosis

BACKGROUND: Pulmonary hypertension (PH) is an important limiting factor of exercise tolerance in patients with mitral stenosis (MS). We wished to investigate the relationship between respiratory nitric oxide (NO), a potent vasodilator, and exercise tolerance in patients with moderate MS. In the same patients, we wondered whether acute change in pulmonary hemodynamics could affect respiratory NO. METHODS: Ten patients with moderate MS (valve area 1.4+/-0.2 cm(2)) were studied at rest, during incremental cycle ergometry exercise, and during dobutamine stress echocardiography (DSE). The concentration of NO in exhaled air (FE(NO)) and NO output (V'(NO)) were measured at baseline, at the end of exercise, and at the end of DSE. Eight healthy subjects served as normal controls for NO output during exercise. RESULTS: During exercise, FE(NO) decreased both in patients and in controls, while V'(NO) increased in both. At the end of exercise, both VO(2) max and V'(NO) were significantly higher in controls than in patients. The increase in V'(NO) during exercise was significantly correlated with VO(2) max, both in patients and in controls. During DSE, cardiac output (CO), pulmonary artery pressure (PAP), and mitral valve gradient increased. No changes in mean FE(NO), V'(NO), or ventilation were observed during DSE. There was a significant inverse correlation between FE(NO) and mitral valve gradient at the end of DSE. CONCLUSIONS: In patients with moderate MS, exercise performance is correlated with respiratory NO output. In the same patients, during DSE, the increase in CO, which is not accompanied by an increase in ventilation, is not associated with an increase in respiratory V'(NO).

Nasal oxygen effects on arterial carbon dioxide pressure and heart rate in chronic heart failure

Nasal oxygen applied by nasal prongs reduced tidal volume and increased carbon dioxide partial pressure in patients with chronic heart failure but not in comparable controls, whereas the patients showed a more pronounced decrease in heart rate with oxygen. These findings indicate that nasal oxygen has distinct effects on ventilation and heart rate in chronic heart failure.

Range of echocardiographic findings in term neonates with high oxygen requirements

AIMS

To examine the hypothesis that right to left shunting occurs mainly in the lungs rather than through the fetal channels in neonates.

METHODS

Thirty two term babies requiring over 70% oxygen had daily colour Doppler echocardiograms until recovery. Measurements included left ventricular fractional shortening, right and left ventricular outputs, colour and pulsed Doppler ductal and atrial shunting and systolic pulmonary artery pressure (SPAP) derived from ductal shunt or tricuspid incompetence velocities.

RESULTS

The babies were retrospectively classified into a respiratory group (n = 19) and a persistent pulmonary hypertension (PPHN) group (n = 13) on the basis of clinical history and radiology. At the initial echocardiogram, just 50% of babies had suprasystemic SPAP. Despite better oxygenation, more of the PPHN group had suprasystemic PAP (85% vs 26%). A correlation between SPAP and Oxygen index (OI) was present only in the respiratory group (r = 0.7). Low ventricular outputs (< 150 ml/kg/min) were common in both groups (53% and 79%). The respiratory group had more closed ducts (47% vs 0%) and those ducts which were patient were more constricted (1.75 mm vs 2.6 mm). Pure right to left ductal shunts were seen in just 15% and pure right to left atrial shunts in just 6% of all babies. The serial echocardiograms showed that SPAP fell and ducts closed well before oxygenation improved. Ventricular outputs increased with age in both groups.

CONCLUSIONS

Apart from early on in the sickest babies with a primarily respiratory diagnosis and the babies with primary PPHN, most right to left shunting occurred at an intrapulmonary level.