Physiological assessment of orthostatic intolerance in chronic fatigue syndrome

Orthostatic intolerance-OI is common in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome-ME/CFS. We used a 10-min passive vertical lean test as orthostatic challenge-OC and measured changes in vitals and end tidal CO₂ (eTCO2). An abnormal physiologic response to OC was identified in 60% of the 63 patients evaluated from one to three times over several years. Hypocapnia, either resting or induced by OC, was the most frequent abnormality, followed by postural orthostatic tachycardia.

Evaluate the physiologic response of patients with ME/CFS to a standardized OC.

Respiratory and heart rate, blood pressure and eTCO₂ were recorded twice at the end of 10-min supine rest and then every minute during the 10-min lean. Hypocapnia was eTCO₂ ≤ 32 mmHg. Orthostatic tachycardia was heart rate increase ≥ 30 beats per minute compared with resting or ≥ 120 BPM. Orthostatic hypotension was decreased systolic pressure ≥ 20 mmHg from baseline. Tachypnea was respiratory rate of  ≥ 20 breaths per minute-either supine or leaning. Questionnaire data on symptom severity, quality of life and mood were collected at visit #2.

63 consecutive patients fulfilling the 1994 case definition for CFS underwent lean testing at first visit and then annually at visit 2 (n = 48) and 3 (n = 29).

Supine hypocapnia; orthostatic tachycardia, hypocapnia or hypotension.

The majority of ME/CFS patients (60.3%, 38/63) had an abnormality detected during a lean test at any visit (51%, 50% and 45% at visits 1, 2 and 3, respectively). Hypocapnia at rest or induced by OC was more common and more likely to persist than postural orthostatic tachycardia. Anxiety scores did not differ between those with and without hypocapnia.

The 10-min lean test is useful in evaluation of OI in patients with ME/CFS. The most frequent abnormality, hypocapnia, would be missed without capnography.