NEONATAL VENTILATION TECHNIQUES

Neil Farringion is the National Maternity Hospital’s first Biomedical Engineer. He has developed an expertise in the area of neonatal care.

In the not too distant past premature babies have been treated with less than a scientific approach. In the 1950’s and 1960’s it was postulated that premature infants felt no pain and thus many invasive procedures were performed without anaesthesia of any kind. In the late 60’s early 70’s the medical approach to the premature baby was beginning to change. Clinicians began to realise that many of the assumptions made when treating these patients were in fact not true and treatment programs began to change. Ventilators were introduced in the 1970’s with the first devices adopted from adult units. It was found that using the adopted adult system did not provide any real advantage and in the late 1970’s the first neonatal ventilators were introduced. The original systems were somewhat crude devices comprising pressure limited operation if you were lucky enough to have pressure limiting, with fixed time cycled systems. Despite the fact that such devices were being used survival rates of premature infants were on the increase. Drugs were now emerging onto the market making it possible in conjunction with ventilator technology to treat more premature patients. Infants of 28 weeks plus gestation with lung difficulties were now surviving and as a consequence presenting a whole new set of treatment complications.

Pressure cycled time limited ventilators were now common place in the NICU with further and further technological advances being made in ventilator technology as a consequence of increased user expeijence. IMV Ventilation (Intermittent Mandatory Ventilation) was used throughout the 70’s and 80’s and indeed today but systems were needed which were more responsive and less traumatic to the patient. In the mid 1980’s SIMV (Synchronised Intermittent Mandatory Ventilation) was developed. This allowed the patient to receive a mandatory number of ventilator generated breaths (in synchrony with the patient) while allowing free gas flow in between from which the patient could breathe. All this technology was making its mark on the survival rates of the patient and with the introduction of Surractant in the late 80’s ever more premature patients with undeveloped lungs were now surviving.

High Frequency Oscillation.

In the early 1980’s It was suggested that if a patient’s lung could be inflated above the pressure at which the lung begins to open, and that pressure maintained while oscillating gas at high frequency i.e. up to 40Hz, an improvement in oxygenation would result. This principal is now utilised today in high frequency oscillation. Conventional ventilation fails in many cases of patients with severe respiratory failure or RDS and suggestions are that HFO(High Frequency Oscillation) may provide an alternative to IMV or SIMV. Other suggested uses may be in neonatal patients with adult RDS.

Basic Principals Of Operation

HFO works by increasing the lung volume / Kg by a factor of three or above ( this is frequency dependent) e.g. a typical neonatal tidal volume is 5-8 ml/Kg on standard ventilation while on HFO tidal volumes of 15-25 ml/Kg are achievable.

Mean airway pressure is established on the oscillator to provide the lung with opening pressure and the gas flow is oscillated to achieve a saw tooth pressure waveform from the base-line pressure at approximately 10 Hz ( patient condition dependent) with an amplitude which is established from patient to patient. Various models allow conventional ventilation with HFO on the complete respiration cycle or on just the inspiratory or expiratory phase or indeed as just a pure oscillation. Typical advantages of HFO are:

• Improved Oxygenation
• Improved C02 elimination
• Improved blood pH
• Reduced Barotrauma
• Improved Lung mechanics

Standard HFO set-up

• The patients mean airway pressure is established from conventional ventilation eg. 22 CmH2O.
• The Mean airway pressure is now set on the oscillator with 5 - 10 CmH2O added.
• The frequency is set at 10 Hz and the amplitude adjusted until the chest wall is seen to "bounce".
• Various respiratory conditions require variations on the basic set-up and should be considered on an individual patient set-up.

Typical HFO Applications

Uses of HFO vary. Various clinically suggested uses are;

1. RDS - Respiratory distress syndrome
2. ARDS - Adult respiratory distress syndrome
3. SRF - Severe respiratory failure
4. Sepsis
5. PPHN - Persistent pulmonary hypertension
6. MAS - Meconium aspiration syndrome

The set-up of the HFO varies for each clinical condition. For correct set-up the operators manual should be consulted.

Conclusions

HFO is a new technology and as a new technology much research in its application is under way. New applications are suggested on a regular basis as its use becomes more wide spread. While HFO provides the Clinician with an alternative ventilation treatment to IMV or SIMV for the patient with respiratory difficulty great care in its set-up and routine operation is required to assure safe operation. Changes to Nursing practice to the patient on HFO are required as disconnection from the oscillator source for even a short time i.e. suctioning, causes clinical complications.

Suggested Reading

Many papers and publications are produced on HFO by various sources. Prof. Ann Greenough’s paper A Manual Of High Frequency Oscillation is a good starting place. Other authors include;

HIFI Study group (1989) Conventional Ventilation compared with HFO.

Ogawa, Y. Miyyasaka (1993) Multicenter randomised trial of HFO in preterm infants.

For further references please contact the author