The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts

A CASE STUDY: VENTILATOR GRAPHICS USED TO IDENTIFY CIRCUIT RESISTANCE THAT CONTRIBUTED TO AN ABNORMAL HEMODYNAMIC RESPONSE IN A NEUROSURGICAL PATIENT

Raymond Wolff Jr., RRT, Mario Gonzales MD FACP FCCP, Michael Oyedeji CRT, Ken Hargett BS RRT The Methodist Hospital, Houston, TX

Introduction: The practice of nebulizing medications in the ventilator circuit may result in deposition of the medication in the expiratory filter creating expiratory resistance. While Respiratory Care departments have processes in place to protect the expiratory system, patients on prolonged mechanical ventilation may be affected as expiratory resistance increases over time. Most often high expiratory resistance may affect the patient by reducing cardiac output causing hypotension.

Case Summary:
Patient was a 71 year old black male admitted to the neuro-intensive care unit (NICU). He suffered from left hemiparesis as a result of a sagittal tumor that produced an 8cm raised section of the skull. In addition, he presented with a history of mild hypertension. His plan of care included cerebral angioplasty with staged resection followed by cranioplasty. Patient was ventilated with the Puritan Bennett 840 ventilator. Ventilator settings were are as follows, SIMV rate 6, Vt .700L, PS 12, PEEP 5, FiO2 .40, Inspiratory time 0.9sec., Expiratory sensitivity 25% with volume targeted (VC+) breath type. Patient was receiving nebulized therapy with 2.5mg Albuterol, 0.5mg Atrovent, and 2cc 10% Mucomyst ever 4 hours for wheezing and thick secretions that were difficult to mobilize. Due to the invasive neurosurgical procedure the patient required mechanical ventilation for an extended period. On day 10 of mechanical ventilation the patient began to develop hypertension that was refractory to titration of vasodilators to maximum dose. Observation of the ventilator graphics demonstrated that in the pressure/time wave form there was a delayed return to the PEEP baseline after inspiration was terminated. The flow/time wave form indicated that expiratory flow was diminished and expiration terminated by the following breath cycle, suggesting incomplete exhalation and the presence of AutoPEEP. The simultaneous presentation of these wave forms indicated there was a possibility of increased expiratory resistance, either within the ventilator or the patient. We elected to start with a simple replacement of the expiratory filter. The filter was replaced and the graphic wave forms returned to a more normal display. The pressure returned to the PEEP baseline without delay and the flow/time wave form indicated that expiratory flow reached zero before the commencement of the next breath. As shown in the table below, there was an immediate resolution of the patient's hypertension after the expiratory filter change. The patient no longer required frequent titration of vasodilators and sedation was weaned to a steady drip rate. The patient was eventually liberated from the ventilator and placed on trach collar.

  8hr Average Pre-filter change 8hr Average Post-filter change
Systolic 167mmHg 121mmHg
Diastolic 90mmHg 67mmHg
Mean 117mmHg 84mmHg

Discussion: Mechanical ventilation may adversely affect a patient's hemodynamic stability. It is generally understood that positive pressure may result in impaired venous return and reduced cardiac out and overall hypotension. This case illustrates an unusual response to positive pressure in that due to the high expiratory resistance and resulting AutoPEEP this patient developed hypertension. The clinical application and proper interpretation of ventilator graphics assisted in recognizing the increased resistance and interventions that facilitated the patient's management. We have found that routine review of ventilator graphics combined with a complete patient assessment improves patient outcomes for mechanically ventilated patients.

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