1. Although it is well known that the microvessels of the skin constantly undergo spontaneous variations in volume, the significance of these rhythmic changes remains uncertain. 2. In 10 healthy males and in 15 patients in intensive care, we assessed the origin of the autonomic influences on spontaneous fluctuations in the microcirculation of the skin, obtained by an infra-red photoplethysmographic device; we used spectral analysis techniques to compare these fluctuations (which were recorded simultaneously in two sites) with those of blood pressure, in order to test the presence of autonomic control of any synchronous fluctuations in these different measurements from the cardiovascular system. In order to minimize mechanical fluctuations caused by occasional slow breaths, rather than nervously mediated fluctuations in skin blood flow, respiration was controlled at 15 breaths/min (0.25 Hz). 3. Spontaneous infra-red photoplethysmographic fluctuations were observed in different body areas (left index finger and left ear lobe, right and left index finger), and all were evident at 0.1 Hz, as well as respiration-related components at 0.25 Hz. Active standing increased the power of the 0.1 Hz fluctuations (sympathetic activity) in both blood pressure (from 62.7 +/- 7.1 to 79.2 +/- 3.7 normalized units, P < 0.05) and IRP (finger: from 68.5 +/- 6.4 to 86.9 +/- 3.4 normalized units, P < 0.05; ear: from 59.0 +/- 5.9 to 88.1 +/- 2.0, P < 0.01). There was a high (> 0.5) coherence between the fluctuations obtained in blood pressure, in IRP signals obtained simultaneously at the finger and at the ear, and in R-R interval. This synchronization between the oscillations in all these signals, which were unrelated to the respiratory frequency or to the pulse rate, suggests a common neural, non-local origin. The phase between IRP and blood pressure was positive in the 0.1 Hz region (+1.65 +/- 0.41 radians, i.e. IRP was leading blood pressure, showing that 0.1 Hz fluctuations were not passively transmitted to the skin microvessels from large arteries) and negative in the 0.25 Hz region (-0.74 +/- 0.19 radians, P < 0.01 compared with phase in the 0.1 Hz region, i.e. IRP was lagging behind blood pressure, suggesting possible passive transmission to the skin microvessels of blood pressure fluctuations caused by respiration). Fluctuations at lower frequency were observed in all IRP recordings, suggesting a local origin for these. Intra-arterial and IRP fluctuations were compared in the 15 intensive care patients and gave similar results. 4. The skin microcirculation is thus not only under local control, but also reflects changes in sympathetic activity; the effect of these changes on the skin microcirculation can be easily evaluated by the spectral analysis of the IRP signal obtained simultaneously in multiple areas, in conjunction with the spectra of R-R interval and blood pressure.

Three patients presenting for elective aortic surgery were studied before operation and for up to 3 days after operation with ambulatory electrocardiographic monitoring and nocturnal pulse oximetry. Supplementary oxygen was administered for the first 2 days after operation and withdrawn on the 3rd morning after operation. Heart rate remained increased for the first 3 days after operation. Two patients developed episodes of myocardial ischaemia during the first 48 h after operation while their arterial oxygen saturation remained greater than 90%. On the third day, and during the third night after operation, both the severity and duration of myocardial ischaemia increased markedly, associated with arterial hypoxaemia. A temporal relationship between decreases in oxygen saturation and fluctuations in ST segment level were observed in the three patients.

We have studied the severity and duration of nocturnal hypoxaemia before operation and for the first five nights after operation after elective major abdominal vascular surgery. Oxygen supplementation was almost 100% effective in keeping oxygen saturation greater than 90% during the early postoperative period; however, 50% of patients spent prolonged periods with an SpO2 less than 85% during at least one night after operation. The risk of severe hypoxaemia persists well beyond the current prescription of supplementary oxygen in these high risk patients. A significant association exists between the mean preoperative overnight saturation value and the nocturnal saturation observed subsequently in the later postoperative period.

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SummaryTwenty-four patients (23 male) who presented for aortic reconstructive surgery were studied with pulse oximetry on a pre-operative night and during the first five postoperative nights. Patients with five or more dips in oxygen saturation of greater than 4% (with a prompt recovery back toward baseline of 3% or more) per hour of monitoring were classified as having a significant abnormality of respiration. Pre-operatively, four of 24 patients ( I 7%) demonstrated such an abnormality. Postoperatively. 12patients (50%) met these criteria on at least one of the first five postoperative nights and six of these had two or more nights with severe episodic hypoxaemia. Frequent severe episodic dips in arterial oxygen saturation (to less than 85% saturation) occurred in the late postoperative period at a time when oxygen therapy would usually have been discontinued. Pre-opera five overnight pulse oximetry studies fail to predict the development of abnormal respiratory patterns in the postoperative period in the majority of patients. Key words Surgery; abdominal.Complications; postoperative hypoxaemia. Measurement techniques; pulse oximetry.Postoperative hypoxaemia may be persistent or episodic. Persistent hypoxaemia is usually the result of intrapulmonary complications which affect ventilation/perfusion relationships in the lung. Episodic hypoxaemia is usually the result of alterations in central neural respiratory drive to the respiratory and accessory respiratory muscles, which give rise to upper airway obstruction or periodic breathing patterns with central apnoea. These episodic phenomena occur almost exclusively during sleep.Several studies [ 1-31 have shown that abnormal patterns of respiration occur commonly in the early postoperative period. Catley et al. [ I ] in 1985 observed frequent episodic dips in oxygen saturation in patients who received an intravenous infusion of morphine during the first 16 h after surgery. These episodes, which occurred only while patients were asleep, were associated with both central and obstructive apnoeas. A recent study in patients undergoing major abdominal surgery [2] has shown that episodic dips in arterial oxygen saturation occur during the second as well as the first night after surgery. These studies reflect respiratory patterns in the relatively early postoperative period. However, little is known regarding the incidence, duration and severity of episodic hypoxaemia during the late postoperative period when supplementary oxygen has normally been discontinued. Furthermore, the clinical significance of such episodes in the generation of morbidity in the postoperative period, particularly in patients at risk from ischaemic heart disease, is unexplored.The aim of the present study was to document, in patients undergoing major abdominal vascular surgery, the pre-operative incidence of nocturnal episodic dips in arterial oxygen saturation, and to compare this with the incidence and severity of episodic nocturnal hypoxaemia occurring during the first five postoperative nigh...

Repetitive nocturnal cyclic arterial desaturation and cyclic increases in heart rate are associated with nocturnal myocardial ischemia in individuals with clinical risk factors for ischemic heart disease. Further investigation in a large patient sample utilizing non-invasive monitoring of saturation, heart rate, and blood pressure may provide definitive evidence regarding causation of some of the nocturnal myocardial ischemia occurring in older individuals with vascular disease.

We have studied before operation 156 patients aged more than 40 yr presenting for elective vascular or non-vascular surgery, using ambulatory ECG monitoring to detect silent myocardial ischaemia (SMI). The prevalence of SMI was 18.2% in the vascular group (n = 102) and 7.6% in the non-vascular group (n = 54). A history of ischaemic heart disease, or an abnormal ECG suggestive of a previous myocardial infarction, predicted a high risk of SMI (28% compared with 9% in the absence of these variables). However, a significant amount of SMI (36% of the total) occurred in patients without one of the defined risk factors. In addition, 24 of the patients with abdominal aortic disease underwent cardiac gated blood pool (MUGA) scans. Abnormal ventricular wall function was observed in 62.5% of the patients. Twenty-nine percent of the patients studied with MUGA scans had SMI and 21% had ejection fractions less than 40%. A significant association (P less than 0.05) existed between the presence of SMI and a ventricular ejection fraction of less than 40%.

Summary Key wordsComplications; sleep apnoea. Ventilation; continuous positive pressure.Frequent episodic reductions in arterial oxygen saturation (Sao,) have been observed in normal patients with intravenous opioid infusions during the first 16 hours after surgery, and are related to complete or partial respiratory obstruction.' Similar episodes have been reported during the first 48 hours following surgery., We report a case of previously undiagnosed obstructive sleep apnoea, in which severe episodic respiratory obstructions occurred both early and late in the postoperative period. The haemodynamic changes accompanying early episodes of respiratory obstruction and the effect of nasal continuous positive airway pressure (CPAP) are reported. MethodsThe patient was studied in an investigation of pre-and postoperative levels of arterial oxygen saturation in patients undergoing elective major abdominal vascular surgery. Data for Sao, and heart rate were sampled once each second from the digital output (RS-232) of an Ohmeda 3700 pulse oximeter, using a computer program designed for respiratory sleep studies. Data were redisplayed and plotted graphically against time. On the first postoperative night, blood pressure was recorded noninvasively from a Finapres 2300 (Ohmeda). The digital output of the Finapres (systolic, diastolic mean BP and heart rate) was sampled each beat by a microcomputer simultaneously with Sao, data from the oximeter.On the pre-operative night oxygen saturation and heart rate alone were recorded. On the first postoperative night, the patient's respiratory effort was assessed by the pressure changes occurring in an intra-oesophageal balloon, and aidow was detected using nasal and oral thermistors. The signals from these were displayed by a Kontron Colormon ITU monitor and then recorded directly onto VHS video tape. Detailed analysis of airflow and respiratory effort was made by replaying the tape. Case historyA 74-year-old man, weight 93 kg and height 174 cm, was admitted to hospital bleeding from an aorto-duodenal fistula. He had previously undergone an aortic tube graft for an inflammatory aortic aneurysm. Although overweight he had no history of respiratory or cardiovascular disease. However, on later questioning, a history of heavy snoring was obtained from the patient and confirmed by his wife. Pre-operative overnight pulse oximetryThere were 295 episodes of desaturation greater than 4% during the pre-operative night (36 events per hour). Many