In patients with neuromuscular disorders that are characterized by respiratory failure resulting from restrictive pulmonary diseases caused by respiratory muscle weakness, assessing the partial pressure of blood carbon dioxide is crucial to evaluate ventilatory condition.1 Arterial blood gas analysis (ABGA) is known to indicate the partial pressures of blood oxygen and carbon dioxide accurately.1,2 On the other hand, ABGA values are indicated only at the time point of measurement, and further to this, patients feel pain due to the invasiveness of the method, which may lead to hyperventilation or apnea. In such cases, results may not reflect patients' usual ventilatory condition and it is also significant that sleep hypoventilation is a common symptom in patients with neuromuscular disorders.3-5 Thus normal values from ABGA do not necessarily indicate a normal ventilatory condition at all times. As the case stands, there is a need to analyze ventilatory conditions seriately by use of transcutaneous blood gas analysis.6 In the department of rehabilitation medicine at Gangnam Severance hospital, serial measurements were made of the ventilatory conditions of patients with suspected respiratory failure or with underlying diseases, by use of ABGA and a transcutaneous blood gas analysis device, the SenTec Digital Monitor System with V-Sign™ Sensor (Sentec AG, Therwil, Switzerland), on the premise that ABGA does not sufficiently reflect usual ventilatory conditions. The aim of this study is to assess the importance of serial analysis of ventilatory conditions in decision-making about artificial ventilatory support in patients with neuromuscular disorders who have undergone ABGA and serial transcutaneous blood gas monitoring in their sleep before ventilator application.

Aggravation of pulmonary insufficiency causes a decrease in the total time of spontaneous ventilation without respiratory muscle fatigue in daily living. Likewise, a delay in treatment of pulmonary insufficiency may increase the burden of cardiovascular work, which can increase cardiovascular complications.12,13 In early-stage respiratory failure, mechanical ventilator support can resolve the fatigue of respiratory muscles. Therefore, early detection of respiratory failure and early application of mechanical ventilatory support is important.14 In particular, it is crucial to determine when ventilatory support should be given, because the lifespan of patients with restrictive pulmonary diseases derived from neuromuscular disorder can be lengthened by ventilatory support.14 In the case of such patients, hypercapnia occurs due to alveolar hypoventilation. The problem is that early-stage hypercapnia may occur without subjective symptoms.14 Accordingly, regular evaluation of hypercapnia and the initiation of ventilatory support in case of need is critical.

ABGA is known to be highly effective for measuring the partial pressures of oxygen and carbon dioxide and for evaluate ventilatory conditions.15 However, it is an invasive method that causes pain and thus may lead to hyperventilation, and moreover, it cannot be performed on sleeping patients. Furthermore, the data are indicated only at the time point of measurement. Altogether, it is not a sufficient tool for accurate analysis of respiratory failure.14,16-18 In particular, analysis of respiratory failure by ABGA is not suitable for patients with neuromuscular diseases accompanied by respiratory muscle weakness because in the daytime such patients show a low level of carbon dioxide due to hyperventilation, but overnight, their supine posture decreases lung capacity and weakens spontaneous respiration. Therefore, continuous monitoring of ventilation during sleep is crucial.

The aim of this study was to research the utility of continuous overnight transcutaneous blood gas monitoring, which was conducted in 36 patients whose hypercapnia was not observed on ABGA but was observed on the transcutaneous blood gas monitoring, from a total group of 89 patients with chronic respiratory failure caused by respiratory muscle weakness, such as hypercapnia, who needed ventilators. These 36 patients' differing results might have been because the tests were affected by postures, sleep states, moods and mental states.4

In this study, patients whose hypercapnia was observed only on overnight transcutaneous blood gas monitoring comprised 40.45% of the total subjects. The times at which hypercapnia was observed occupied over 3 hours of the total sleeping time. This result suggests that 4 out of 10 patients who undergo ABGA may miss an opportunity to receive timely treatment. Considering that early-stage hypercapnia may be observed only during sleep, ventilation during sleep needs to be monitored and assessment should consider many factors that could influence ventilation, in order that respiratory failure may be detected early and treatment may be performed in time.

In the study of Storre et al., it was found that the carbon dioxide partial pressure, measured by overnight transcutaneous blood gas monitoring, was significantly related to that of ABGA6 and that the error between ABGA and a transcutaneous blood gas analyzer was 1.0±5.0 mmHg.2,6,19-24 This error is different from that of this study.23 The reason for this might be that values were measured at different times. The difference was not significant, but it is nevertheless very suggestive that hypercapnia lasted (198.97 out of 357.28±150.12 minutes; 55.69%) to the extent that a ventilator needed to be applied.

It is most effective to make a serial analysis of arterial blood gas by puncturing the artery, but this is highly invasive and thus may cause complications such as infection and hemorrhage. This method is hardly used, except in cases for which vital signs should be monitored. Instead, ETCO₂ analysis and transcutaneous blood gas monitoring are widely used as measuring instruments.2,6,20 In the case of ETCO₂ analysis, air may leak and the reliability may differ according to underlying diseases, leading to the wider usage of transcutaneous methods. Hinkelbein et al. and Bolliger et al. reported that transcutaneous methods were superior to the ETCO₂ analysis in terms of reliability.2,19,20 In this study, a transcutaneous blood gas analyzer was used on the authority of the report of Storre et al.6,7,24,25

The limitation of this study is that carbon dioxide partial pressure was analyzed by a retrograde method using medical records, and thus respiratory rates could not be reflected. The respiratory rate is not displayed on the transcutaneous blood gas analyzer, and it was also not recorded during ABGA. As a result, the occurrence of hyperventilation or apnea could not be monitored. Nevertheless, this study confirmed that serial overnight transcutaneous blood gas monitoring could compensate for the shortcomings of ABGA. For the present, however, it was impossible to clarify the reason for which ABGA and overnight transcutaneous blood gas monitoring showed different results. Another problem is that this study was conducted without a control group from the normal population. Changes in body temperature were also not reflected, though the measuring instrument was sensitive to temperature. Mahutte et al. reported that the test became less reliable and that patients might be scalded if it was performed for over 4 hours,26 but other researchers have reported that measuring hours did not affect results significantly.7,21,27

The exclusive application of ABGA is insufficient to decide whether to apply mechanical ventilatory support. Considering that respiratory insufficiency may occur in recumbent patients with neuromuscular diseases, serial overnight transcutaneous blood gas monitoring is appropriate to evaluate their ventilatory conditions. Particularly in the case of patients whose diseases are more likely to cause respiratory muscle fatigue, carbon dioxide partial pressure should be periodically monitored in order that respiratory failure can be detected early. A further large-scale study should be conducted with various relevant indexes, including respiratory rates as well as the use of a normal control group.