Breathe easier

An intrusive method for monitoring the ventilation of patients in intensive care units could soon be replaced by a non-invasive technique being developed by Oxford University scientists.

The new equipment would measure the blood flow out of the heart and the lung volume by monitoring the properties of the gases being sent in and out of the lung by the ventilator.

At present an ultrasound emitter and receiver probe is inserted into the oesophagus to measure the speed of blood flow using the Doppler principle.

Although an intensive care patient may need artificial ventilation, this unnaturally increases the pressure in the airwaves and can damage the lung by causing areas of it to collapse. The same can happen to the air sacs (alveoli) in the lung.

Dr Andrew Farmery, a principal investigator on the project, said: ‘If you have someone supine, the recoil forces of the chest that would normally keep the lung open are no longer there, so the lung flops shut. So some of the air sacs, usually at the bottom of the lung, are squashed shut. When you put air in with the ventilator, the ones just a bit up from the bottom will spend their lives snapping open and shut. That is called cyclical atelectasis.

‘We think the act of repetitively and cyclically opening and closing induces powerful shear stresses in the alveolar walls and that damages them.’

During normal breathing conditions, the alveoli never snap shut — they are small when you exhale and get bigger when inhaling.

To combat these problems, doctors can apply a positive end expiratory pressure (PEEP) to the ventilator to control the amount of air ‘breathed’ out. The idea is to apply a variable impedance so the pressure in the airway is never allowed to drop to atmospheric.

PEEP is useful but difficult for doctors to regulate. The new technology overcomes this by sending sine wave signals of gas into the lung, comparing the amplitude and the phase shift of the signals entering and exiting the lung. This enables the doctors to calculate the cardiac output, the lung volume and the dead space volume — part of the lung that does not take part in the gas exchange. The fraction is larger in a ventilated person, and enables the efficiency of the ventilation to be monitored.

‘When you ventilate the patient, each breath will have a slightly different oxygen concentration and, if you plotted those out on graph paper, they would describe a sine wave,’ said Farmery.

‘Rather than having a fixed inspired oxygen concentration, we are going to set the mean at 40 per cent but vary it sinusoidally — so it might go from 44 to 36 per cent peak to trough.’

When a patient breathes out, there is a sinusoidal signal in the concentration at the end of each breath, which can be compared with the signal in the inspired gas to indicate the lung volume.

Farmery explained: ‘The AC signal is diminished because when you feed this concentration signal into a big volume, you effectively dilute the signal and the signal in the expired gas is small. If, however, the lung volume is very small, then the signal on the expired gas will be almost as big as the signal that went into it because it has not been attenuated.’

Nitrus oxide would also be fed in sinusoidally, and be attenuated in the same way as oxygen. Measuring these signals would allow doctors to monitor blood flow through the lung (which is the same as the cardiac output) and control the amount of PEEP being applied. Farmery said: ‘The difference is that nitrus oxide is slightly soluble in blood, so the amplitude will diminish more than oxygen. The greater the blood flow, the greater the nitrus oxide is taken out of the lung and into the blood stream.’

The specific gas mixture will be administered by a mass flow controller (MFC), commonly used in the semiconductor industry to control the atmosphere required for the manufacturing of semiconductors.

‘Finding MFCs that are sufficiently dynamic and whose response time is sufficiently fast is a key problem — but commercial companies are getting better. At the moment it is a standalone device that squirts tracer gas into the pipe coming from the ventilator towards the patient. Eventually we are going to build it into the ventilator,’ said Farmery.