Monday, September 13, 2010

Application of Mid-Frequency Ventilation


My facility is a Long-term Acute Care Hospital licensed for 42 beds, which specializes in ventilator weaning and wound care management. In regards to ventilator weaning our patients primarily arrive from outlying intensive care units and been on mechanical ventilation for greater than seven days.
Our allotted hospital length of stay for patients „coded. for “Ventilator weaning” is twenty-five days. During this time period patients are either weaned from mechanical ventilation or placed for extended long-term care. December 2009 ventilator days were a mean of 6.5 days per patient (7 samples/patients), and when calculated for outliers to 3 standard deviations the mean decreased to 5.4 days (5 samples).
There are respiratory therapy driven ventilator guidelines which include ventilator management and ventilator weaning. The only ventilator we utilize is the Respironics Esprit which has been fine in our patient population, however, it has been cumbersome in patients which require additional respiratory mechanic measurements e.g. static compliance & resistance (in the Esprit you can only obtain these measurements in VC-CMV). “I guess I’m spoiled not having to calculate these parameters with other machines”. However, it is nice to polish my calculation skills ever so often.
Our standard modalities for ventilator management are PC-CMV and PC-CSV.

History and Physical

Admitted to our hospital was a 52 year old male from an outlying ICU for ventilator weaning. Apparently this patient underwent right video-assisted thoracic surgery with partial thoracotomy and right lower lobectomy. The postoperative course was complicated by respiratory failure and the patient.s overall weakness in the upper and lower extremities.

Past Medical History:
1. Lumbar disk disease 2. Atrial fibrillation. 3. High blood pressure 4. Cachexia 5. Oropharyngeal dysphagia 6. Adenocarcinoma of the lungs 7. Anxieties 8. Delirium in ICU
Past Surgical History:
1. Thoracotomy with video-assisted device to resect cancer and perform right lower lobectomy. 2. Exploratory laparotomy was done in the 1980.s for stab wound.
Family History:
Mother had passed away from cancer unknown origin. Father died at the age of 71 from peripheral vascular disease.

Social History:

Used to be heavy smoker. Denied ETOH or illicit drug use.
Clinical Course

72 Hours Post Admission:

Ideal Body weight- 68 kg
Airway- # 8.0 cuff trach
Ventilator settings- PC-CMV, driving pressure 20 cmH20, PEEP + 8 cmH2O, frequency 12, FiO2 of 40%.
Arterial Blood gases on these settings- pH 7.43, PaCO2 56, PaO2 88, HCO3 35, BE +12, SpO2 97%
Pulmonary mechanics- exhaled tidal volume ~ 7.5 cc/kg/IDBW, calculated dynamic compliance of 24.

Chest X-Ray- Blunting interstitial alveolar opacities, consistent with cardiac decompensation.

Patient had large amounts of thick yellow sputum, which needed suctioning every 3-4 hours.
Five days post admission
Patient.s dynamic compliance decreases to 14 and has periods of de-saturations requiring FiO2 to be titrated up to 55%. Secretion management still remaining an issue with the patient.
Six days post admission

A.M. Shift:

Patient status continued to deteriorate FiO2 had to be titrated to 70% to maintain oxygenation in the low 90.s. Additionally, nurses reported ventricular dysrhythmias. Attending physician discussed with family the situation that the patient can be regarded as moribund. Patient code status was switched to “DNR” and Morphine drip was started for patient comfort.

P.M Shift:

Found patient on the following baseline settings:
PC-CMV, driving pressure 20, PEEP + 8, Frequency 16, FiO2 50%.
Measurements: exhaled tidal volume ~ 5cc/kg/IDBW.
2005: Patients SpO2 dropped into mid 80.s, patient looked lethargic & gasping with shallow Inspiratory efforts. Suctioned small amount of secretions, increased FiO2 to 55%, and increased set frequency to 25 decrease patient.s work of breathing.
2158: Patients Spo2 dropped to 84%, suctioned small amount of white sputum. Performed two recruitment maneuvers patients SpO2 increased to 98% and was able to titrate FiO2 back down to 45%. Patient still looked uncomfortable with a high spontaneous respiratory rate so I placed an EtCO2 monitor inline to evaluate ventilation.
Capnography showed square waveform with a plateau of 80 mmHg. I increased the frequency to 40 breaths per minute to off load work & to increase minute ventilation. I evaluated patient.s medical record to assess the most recent Arterial blood gas, & obtained equipment for arterial blood gas.
Arterial blood gases where obtained and sent off-site for processing, during the processing time I calculated the patient.s lung mechanics & predicted minute ventilation to decrease EtCO2 to an acceptable level. The ventilators driving pressure was already maxed out, creating plateau pressures equal to 30 cmH2O. I realized that I could only manipulate my respiratory rate to decrease the CO2.
The arterial results where the following:
pH 7.19, PaCO2 116, PaO2 70.3, HCO3 36, BE +15, SaO2 89%

I was concerned with this dramatic change in PaCo2 from three days ago & also concerned with this acidosis, since the patient experienced dysrhythmias earlier in the day. I notified the physician and stated that I could correct this acidosis.
I also realized that there was a huge difference between my measured EtCO2 and calculated PaCO2, I did not have my reference for calculating dead space so I just used the 0.46 used in the “Mid-Frequency Ventilation Simulator” (Cleveland Clinic © 2008 Cleveland, OH), and believe this should be sufficient.

Below are the following calculations from the simulator.

2300: After these calculations I set the ventilator frequency ~ to the optimum frequency (60 bpm), changed the duty cycle to 50%, assessed the patient for air-trapping by assessing the expiratory flow waveform & performing an expiratory hold maneuver, & auscultated breath sounds. Additionally, I watched for dysrhythmias & took frequent blood pressures.


The patient did experience a run of ventricular tachycardia two hours within the ventilator change without a compromise in blood pressure.

0430: Arterial Blood Gases-
pH 7.42, PaCO2 67, PaO2 60, HCO3 41, BE + 18, SaO2 92%
I was concerned with creating alkalemia & started to titrate the respiratory rate down, while reassessing Inspiratory time, expiratory flow, and hemodynamics. A target minute ventilation was determined & frequency was titrate to maintain this minute ventilation.

48 Hours post Mid-Frequency
Frequency was titrated down from 50 to 20 bpm,
ABG- pH 7.30, PACO2 99.5, PaO2 73.7HCO3 44, BE +20
10 days post Mid-Frequency
Patient remains stable on the following ventilator settings-
PC-CMV 18, PEEP 10, Frequency 12, FIO2 40-45%
However, secretion management & poor lung mechanics remain to be an issue with patient.s progress. Additionally, patient was diagnosed with ALS today.

Outcome & Prognosis
Mid-frequency ventilation was helpful in this situation where increasing tidal volume via increasing pressure would have led to dangerous plateau pressures. The technique was successful in decreasing PaCO2 and had no adverse effects on the patient.
Even though this strategy was successful, the patient.s prognosis for weaning off mechanical ventilation is poor due to muscle wasting, poor lung mechanics, and current diagnosis of ALS.


In retrospection I should have utilized our EtCO2 device & calculated the appropriate dead space & put this in the simulator for the correct calculation. That said I believe the simulator is a valuable tool for patients that are difficult to ventilate. It is nice to be assured that these high frequencies in the right patients will not create the suspected air-trapping. I also like that it shows the minute ventilation in comparison to the alveolar ventilation, for example I calculated a higher minute ventilation requirement for my patient. However, by increasing the minute ventilation past the optimum frequency I would have gained nothing in regards to alveolar ventilation.
References/Additional Reading
Mid-Frequency Ventilation in Acute Respiratory Distress Syndrome: The New Wave....Length.
Mid-Frequency Ventilation: Unconventional use of Conventional Mechanical Ventilation as a Lung-Protection Strategy.
Mid Frequency Ventilation: Optimum Settings for Neonates.