1 Department of Anaesthetics and 2 Department of Nursing, City Hospital, Dudley Road, Birmingham B18 7QH, UK*Corresponding author
This article is accompanied by Editorial II.
Accepted for publication: March 28, 2002
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Abstract |
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Br J Anaesth 2002; 89: 51214
Keywords: complications, quadriplegia; lung, tidal volume; ventilation, hypoventilation; ventilation, mechanical
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Introduction |
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Case reports |
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The ventilator (Puritan Bennett 7200) settings were as follows: continuous positive airway pressure mode with pressure support 16 cm H2O, pressure sensitivity 0.5 cm H2O, positive end expiratory pressure 7.6 cm H2O, and fraction of inspired oxygen concentration 0.35. On these settings, the tidal volume was about 0.32 litre, ventilatory frequency 25 min1 and minute ventilation around 8.0 litre min1. The patient was unable to trigger the ventilator if the pressure sensitivity was set to 1 cm H2O. To assess the ability of the patient to trigger the ventilator with flow triggering, flow-by was started with the default base flow of 5 litre min1 and flow sensitivity of 3 litre min1. The patients respiratory rate decreased to 14 min1 while the tidal volume, oxygen saturation, and cardiovascular measurements remained the same. The patient did not appear to be uncomfortable and there were no accessory muscle movements to suggest that the patient was unsuccessfully attempting to trigger the ventilator. An error in the flow-by settings was suspected, and the ventilator was set to the original settings. The respiratory rate returned to 25 min1. The decrease in respiratory rate was considered to be caused by an inadequate base flow. Therefore, the base flow was increased to 10 litre min1 and flow-by was re-started, but the same phenomenon occurred again, and persisted when the base flow was increased to 20 litre min1. The flow sensitivity was not altered during these changes in base flow settings. The ventilator was then returned to the original settings.
Case 2
A 34-yr-old man mechanically ventilated for respiratory failure caused by pneumonia. In the past, he had undergone total pancreatectomy for chronic pancreatitis. He was an insulin-dependant diabetic and had a history of recurrent multiple abscesses in various parts of his body. Ten days later, when he was being weaned from the ventilator, it was noticed that the patient was unable to move his limbs. He was conscious and not sedated. A computerized tomographic scan of the neck showed an epidural abscess at the C24 level. Clinically, there was no sensation to touch, pain, or temperature below C4 dermatome. There was no movement in his limbs. Cranial nerves were not involved. Before admission to the intensive care unit, there had been no clinical evidence of this epidural abscess.
The ventilator settings were continuous positive airway pressure mode with pressure support of 18 cm H2O, pressure sensitivity of 0.5 cm H2O, positive end expiratory pressure of 5 cm H2O and fraction of inspired oxygen concentration of 0.35. With these settings, the tidal volume was around 0.48 litre, ventilatory frequency 18 min1, and minute ventilation was 9.4 litre min1. When flow-by was started with a default base flow of 5 litre min1 and flow sensitivity of 3 litre min1, the tidal volume remained the same but the ventilatory frequency decreased to 8 min1. On decreasing the flow sensitivity to 1 litre min1, the ventilatory frequency increased to 15 min1 and the tidal volumes were 0.49 litre. When the base flow was increased to 10 litre min1 and flow sensitivity was increased to 5 litre min1, the patient could not trigger the ventilator but weak respiratory efforts were seen. The ventilator was re-set to pressure sensitivity settings and flow-by was abandoned.
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Discussion |
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It is likely that the high spinal lesion in these patients caused some diaphragmatic weakness. Indeed, the second patient had decreased diaphragmatic movements on ultrasonography. Therefore, these patients were extremely sensitive to small changes in trigger settings. Using flow sensitivity of 3 litre min1 made it more difficult for the patient to trigger the ventilator. Trigger pressure (the maximum pressure decrease at the beginning of an inspiratory effort) and pressure time product on flow sensitivity settings of 2 and 3 litre min1 lies between pressure sensitivity settings of 0.5 and 1 cm H2O.2 The second patient could trigger the ventilator on a flow sensitivity setting of 1 litre min1. It is likely that the first patient could have responded in the same way. Although in many patients the minor differences between pressure and flow triggering and the various settings of flow triggering may not be clinically significant,6 this may not be so in patients with quadriplegia who rely on their feeble muscle strength to trigger the ventilator.
One of the risks of very high sensitivity settings is triggering by movements caused by cardiac activity.9 Although this did not happen here, it should be kept in mind especially if patients have a hyperdynamic circulation. A further hazard with flow-by settings needs to be considered. In Puritan Bennett 7200 ventilators, flow-by is automatically inactivated during nebulization and needs to be manually switched on again if the mode is to continue. For this reason, flow-by was abandoned in the second patient even though he could trigger the ventilator with a flow sensitivity of 1 litre min1.
To summarize, we describe two patients with quadriplegia from high spinal cord lesions where the flow-by mode led to hypoventilation. In patients with extensive muscle weakness, the ability to trigger the ventilator can be affected by small changes in trigger sensitivity. The default flow sensitivity settings in flow-by mode may not be sufficient for such patients to successfully trigger the ventilator.
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References |
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3 Puritan Bennett Ventilator Manual. 22335 A 192 Option 50 (release 2.0), 18
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