Flow-by induced hypoventilation in high spinal cord lesions—report of two cases{dagger}

S. Kannan1, N. Sherwood*,2 and A. Armfield2

1 Department of Anaesthetics and 2 Department of Nursing, City Hospital, Dudley Road, Birmingham B18 7QH, UK*Corresponding author

{dagger} This article is accompanied by Editorial II.

Accepted for publication: March 28, 2002


    Abstract
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 Abstract
 Introduction
 Case reports
 Discussion
 References
 
Flow triggering in ventilators is an alternative to pressure triggering. Differences between these two trigger mechanisms may not be clinically significant in most patients. We report two patients with high spinal cord lesions in whom the use of flow triggering was unsuccessful. Severe muscle weakness in these patients made them sensitive to small changes in ventilator trigger characteristics.

Br J Anaesth 2002; 89: 512–14

Keywords: complications, quadriplegia; lung, tidal volume; ventilation, hypoventilation; ventilation, mechanical


    Introduction
 Top
 Abstract
 Introduction
 Case reports
 Discussion
 References
 
During mechanical ventilation, change from expiration to inspiration can be time cycled, or triggered by pressure or flow changes.1 Whereas time cycling is independent of patient activity, pressure and flow triggering require the patient to start the breath1. For a pressure-triggered breath, the ventilator detects the patient’s effort as a decrease in pressure. The decrease in pressure required to trigger the ventilator is set by the clinician.2 For a flow-triggered breath, the ventilator senses patient effort from changes in inspiratory flow caused by the patient.2 Flow-triggering and flow-by technology was introduced in the Puritan Bennett 7200 ventilators (Puritan Bennett Corporation, USA) to reduce the delay between the patient’s inspiratory effort and the supply of gas to the patient during assisted ventilation.3 The ventilator delivers a predetermined flow of gas (base flow) into the patient circuit before inspiration. The ventilator detects flow to the patient as the difference between the delivered flow and the flow leaving at the expiratory valve. The operator determines the inspiratory flow necessary to trigger a breath (flow sensitivity). When the difference between the delivered and the expiratory flow increases to this amount, then the ventilator triggers to inspiration. The default flow sensitivity setting in these ventilators is 3 litre min–1. In flow-by mode pressure triggering is thus replaced by flow triggering. We report two cases where using flow-by caused failure to trigger, which was reversed when the flow-by was stopped. There have been no similar reports in the literature.


    Case reports
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 Abstract
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 Case reports
 Discussion
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Case 1
A 66-yr-old man was being mechanically ventilated for respiratory failure after traumatic quadriplegia caused by fracture of the odontoid peg. The neck was stabilized by cervical traction by a Halo apparatus. The patient was conscious and no sedative agents were being given. There was loss of sensation to touch, pain, and temperature below C3 dermatome. Motor power was absent in all the limbs. Cranial nerves were intact.

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 min–1 and minute ventilation around 8.0 litre min–1. 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 min–1 and flow sensitivity of 3 litre min–1. The patient’s respiratory rate decreased to 14 min–1 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 min–1. The decrease in respiratory rate was considered to be caused by an inadequate base flow. Therefore, the base flow was increased to 10 litre min–1 and flow-by was re-started, but the same phenomenon occurred again, and persisted when the base flow was increased to 20 litre min–1. 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 C2–4 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 min–1, and minute ventilation was 9.4 litre min–1. When flow-by was started with a default base flow of 5 litre min–1 and flow sensitivity of 3 litre min–1, the tidal volume remained the same but the ventilatory frequency decreased to 8 min–1. On decreasing the flow sensitivity to 1 litre min–1, the ventilatory frequency increased to 15 min–1 and the tidal volumes were 0.49 litre. When the base flow was increased to 10 litre min–1 and flow sensitivity was increased to 5 litre min–1, 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.


    Discussion
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 Abstract
 Introduction
 Case reports
 Discussion
 References
 
Inspiratory efforts can be insufficient to trigger the ventilator in some patients.4 Factors which can affect this can be either ventilator related (high resistance, leaks, high base flow) or patient related (weakness, flail chest, intrinsic positive end expiratory pressure).5 The current ventilators are sensitive enough to allow a patient to trigger with minimum of effort. Studies comparing the efficacy of flow triggering vs pressure triggering have shown equivocal results.5 While some have shown flow triggering to be superior, others have found either minimal difference or pressure triggering to be better.1 2 6 7 The differences between pressure and flow triggering were not related to the trigger used but to the ventilatory support programming with each trigger.2 7 More recent ventilators have corrected this difference so that the inspiratory work of breathing is similar with the two modes of triggering.5 6 8

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 min–1 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 min–1 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 min–1. 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 min–1.

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.


    References
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 Abstract
 Introduction
 Case reports
 Discussion
 References
 
1 Sassoon CS, Gruer SE. Characteristics of the ventilator pressure- and flow-trigger variables. Intensive Care Med 1995; 21: 159–68[ISI][Medline]

2 Goulet R, Hess D, Kacmarek RM. Pressure vs flow triggering during pressure support ventilation. Chest 1997; 111: 1649–53[Abstract/Free Full Text]

3 Puritan Bennett Ventilator Manual. 22335 A 1–92 Option 50 (release 2.0), 1–8

4 Chao DC, Scheinhorn DJ, Stearn-Hassenpflug M. Patient-ventilator trigger asynchrony in prolonged mechanical ventilation. Chest 1997; 112: 1592–9[Abstract/Free Full Text]

5 Hill LL, Pearl RG. Flow triggering, pressure triggering, and autotriggering during mechanical ventilation. Crit Care Med 2000; 28: 579–81[ISI][Medline]

6 Aslanian P, El Atrous S, Isabey D, et al. Effects of flow triggering on breathing effort during partial ventilatory support. Am J Resp Crit Care Med 1998; 157: 135–43[Abstract/Free Full Text]

7 Polese G, Massara A, Poggi R, Brandolese R, Brandi G, Rossi A. Flow-triggering reduces inspiratory effort during weaning from mechanical ventilation. Intensive Care Med 1995; 21: 682–6[ISI][Medline]

8 Tutuncu AS, Cakar N, Camci E, Esen F, Telci L, Akpir K. Comparison of pressure- and flow-triggered pressure-support ventilation on weaning parameters in patients recovering from acute respiratory failure. Crit Care Med 1997; 25: 756–60[ISI][Medline]

9 Imanaka H, Nishimura M, Takeuchi M, et al. Autotriggering caused by cardiogenic oscillation during flow-triggered mechanical ventilation. Crit Care Med 2000; 28: 402–7[ISI][Medline]