The Usefulness of Submaximal Exercise Gas Exchange in Pulmonary Arterial Hypertension: A Case Series

Introduction

Although the 6 minute walk test is simple and widely employed to assess submaximal exercise capacity and response to therapy in pulmonary arterial hypertension (PAH), it provides limited information. ¹ Maximal cardiopulmonary exercise testing (CPET) can provide more comprehensive physiologic information, but is less widely available, is subject to varying interpretation across centers, and may be inconvenient for patients to perform. Submaximal exercise gas exchange measurement, using simplified technology (ie, ‘plug and play’ systems), ² may be a simple alternative to formal CPET that could provide information beyond that available from a 6 minute walk.

Methods

Three patients diagnosed with idiopathic pulmonary arterial hypertension were included in the present report. Each patient was seen twice in the PH clinic, with the visits separated by approximately 3-5 months. As part of their assessments standard clinical tests were performed, which included a transthoracic echo and 6 minute walk test, at both visits. In addition, submaximal exercise step tests were performed in conjunction with clinical tests, which consisted of 2 minutes rest, 3 minutes exercise and 1 minute recovery. Gas exchange was measured throughout using a SHAPE gas analysis system (SHAPE Medical Systems Inc, St. Paul, MN). The SHAPE system is a newly designed, small and portable gas analysis system that is self calibrating, containing built-in automated temperature, barometric pressure and humidity corrections. The interface is simplified and the software designed so that extensive understanding of the methodology and equipment is not required (Fig. 1). ²

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Figure 1.

A SHAPE system which consists of a gas analyser, laptop, mouthpiece and pneumotach appliance and finger pulse oximeter.

Case Descriptions

Results

Traditional clinical tests

Echocardiographic measurements and 6-minute walk distance are shown in Table 1.

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Table 1.

Routine clinical measures used to assess pulmonary hypertension severity.

	Patient A		Patient B		Patient C
	Pre	Post	Pre	Post	Pre	Post
Echocardiography
RVSP (mmHg)	49	43	65	69	66	68
TAPSE (mm)	17	19	na	12	21	31
Lateral annulus systolic	0.12	0.11	0.11	0.08	0.14	0.14
velocity (m/sec)
Cardiac index (l/min/m2)	2.46	2.66	3.04	3	2.48	3.15
6 minute walk
Distance (m)	356	370	449.9	436.2	660.3	594.8
% predicted	50	52	87	84	116	104

Abbreviations: RVSP, estimated right ventricle systolic pressure; TAPSE, tricuspid annular plane systolic excursion by M-mode.

Patient A was free of recurrent syncope and was functional class III at follow up. The 6 minute walk increased from 357 m to 371 m after 3 months. Echocardiographic estimates of pulmonary artery pressure suggested improvement but were felt to be potentially unreliable due to minimal tricuspid regurgitation. There did appear to be improvement in right heart function as the tricuspid annular plane systolic excursion by M-mode [TAPSE] improved from 17 to 19 mm at 3 months. Right ventricular index of myocardial performance (RIMP) improved from 0.88 to 0.54 at 3 months, while there was no significant change in tricuspid lateral annulus systolic velocity (0.12 vs 0.11 m/sec).

Patient B remained functional class II-III. The 6-minute walk distance decreased slightly from 450 m to 436 m. Estimates of right ventricular pressure by echocardiography were felt to be potentially spuriously low due to trivial tricuspid regurgitation but was estimated at 65 and then 69 mmHg. Echocardiographic assessments of right ventricular function were equivocal, with tricuspid lateral annulus systolic velocity changing from 0.11 m/sec to 0.08 m/sec, RIMP 0.55 to 0.50, and TAPSE 10 mm to 12 mm. N terminal BNP decreased from 792 pg/ml to 547 pg/ml.

Patient C symptomatically felt improved but had very little change in RVSP or lateral annulus systolic velocity, which was within the normal range at baseline and follow up. There was however a large change in TAPSE, which increased from 21 to a supranormal value of 31 mm following 4 months of increased sildenafil treatment. Six-minute walk distance was decreased by ∼60 metres however despite this decrease the patient still achieved over 100% of age predicted distance (595 m). NBNP level was not available at the baseline assessment but was 100 pg/ml at time of followup, compatible with preserved right ventricular function.

Submaximal Exercise Gas Exchange

Patient A had an improved end-tidal CO₂ (P_ETCO₂) and breathing efficiency (V_E/VCO₂ slope/ratio), in accordance with improved heart function and walk distance, after 3-months treatment (Fig. 2). End-tidal CO₂ was higher at rest and throughout exercise following treatment (Fig. 2A). A similar pattern was evident for breathing efficiency where the V_E/VCO₂ ratio was lower at rest and all points during exercise (Fig. 2B). In addition, the V_E/VCO₂ slope decreased favorably from 49, at baseline, to 43 (Fig. 2C).

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Figure 2.

Patient A: Gas exchange variables at rest and during exercise at baseline (circle) and 3-months following treatment (square). Data is presented for P_ETCO₂ (A), V_E/VCO₂ ratio (B) and V_E/VCO₂ slope (C).

Patient B had a reduced P_ETCO₂ and breathing efficiency at the time of 5 month follow up. End-tidal CO₂ was lower at rest and during exercise (Fig. 3A) and V_E/VCO₂ ratio higher at all points. V_E/VCO₂ slope was therefore elevated from 37 at baseline to 45 (Fig. 3C).

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Figure 3.

Patient B: Gas exchange variables at rest and during exercise at baseline (circle) and 5-months following treatment (square). Data is presented for P_ETCO₂ (A), V_E/VCO₂ ratio (B) and V_E/VCO₂ slope (C).

Patient C demonstrated an improved P_ETCO₂ and breathing efficiency during exercise. This was despite an actual decrease in walk distance which may reflect the nonspecific nature of moderate changes in walk distance within this well preserved range. End tidal CO₂ was similar at rest between the two conditions but differed during exercise (Fig. 4A). Whilst P_ETCO₂ remained constant at ∼28 mmHg during exercise at baseline, after 4 months of treatment P_ETCO₂ increased to 34 mmHg. V_E/VCO₂ ratio was lower at rest and throughout exercise (Fig. 4B) after treatment, as was V_E/VCO₂ slope (Fig. 4C), which decreased from 39 to 30.

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Figure 4.

Patient C: Gas exchange variables at rest and during exercise at baseline (circle) and 4-months following treatment (square). Data is presented for P_ETCO₂ (A), V_E/VCO₂ ratio (B) and V_E/VCO₂ slope (C).

Discussion

Multiple parameters are currently utilized in an effort to track progress of patients with pulmonary arterial hypertension, and each of them has their strengths and weaknesses. Following a major shift in therapeutic approach such as occurred in patient A, we saw substantial improvement in multiple parameters including gas exchange variables. Patient B, with no fundamental change in treatment plan, had multiple parameters that were persistently abnormal but without consistent or major changes in direction. Worsening of submaximal gas exchange variables, P_ETCO₂ and V_E/VCO₂, was of concern but the significance of such changes needs to be further elucidated by serial follow up. Patient C was in a category of quite well preserved 6 minute walk distance and RV function where sensitivity of such methods for improvement in condition is limited. In fact he had little change in his right heart function/pressure by echo estimates aside from the increase in TAPSE, and a decrease in 6 minute walk distance. Gas exchange variables suggested there was an improved function following 4 months of therapy and appeared consistent with his impression of clinical improvement. It is possible that gas exchange variables, such as V_E/VCO₂, are more sensitive measures than other clinical measures and are able to better detect smaller changes earlier following treatment. Gas exchange data during submaximal exercise could, therefore, be a useful tool to track disease severity in PAH patients. Reduced pulmonary perfusion and V/Q (ventilatory/ perfusion) mismatch is thought to be the most likely cause of alterations in breathing efficiency and end-tidal CO₂ levels in PAH.

Traditional clinical measures used to determine PAH severity and track patient responses to treatment are expensive, time consuming and require technical expertise (echocardiography) or lack sensitivity and ignore important aspects of disease (6 minute walk test). ¹ There is currently a need for more sophisticated endpoints that can be simply and inexpensively obtained. Recently it has been suggested that measuring gas exchange during exercise may be the answer, considering that one of the main symptoms of PAH is dyspnea. Submaximal CPET may be a more meaningful and specific endpoint measure that allows for the measurement of key variables (ie, physiological dead space, breathing efficiency and end-tidal CO₂) that are likely to be affected by PAH. Recent research has described characteristic gas exchange abnormalities in PAH patients, including a high V_E/VCO₂ slope/ratio, ³ a low resting P_ETCO₂ and decreased P_ETCO₂ during exercise. ⁴ It has been suggested that these gas exchange variables track physiological changes in response to therapy. ⁵ There is potential for developing algorithms, using multiple gas exchange variables (ie, V_E/VCO₂, P_ETCO₂ and other parameters), that may allow a more sophisticated and sensitive, yet simple, way to track disease status. More work is clearly required in this area to determine which parameters should be used and also to standardize testing so that there is uniformity in clinical practice.

Conclusion

These cases demonstrate the potential utility of measuring submaximal exercise gas exchange to track responses to therapies in PAH patients. This will need to be evaluated and validated in a study of multiple PAH patients before its potential role in a clinical setting and as an endpoint measure can be fully understood.

Abbreviations

Disclosure

This manuscript has been read and approved by all authors. This paper is unique and is not under consideration by any other publication and has not been published elsewhere. The authors and peer reviewers of this paper report no conflicts of interest. The authors confirm that they have permission to reproduce any copyrighted material. Written consent was obtained from the patients for publication of this study.