Duration of the project Four years (1999-2002)

Abstract

The aim of the proposed project is to develop a clinical device which can continuously non-invasively measures the degree of bronchial obstruction and the related work of breathing. The device is based on the variation in time in which a blood pulse propagates from heart to fingertip. This so-called Pulse Transit Time (PTT) is closely correlated to the degree of bronchial obstruction.

Introduction

At inhalation the pressure in the thorax of a human becomes negative compared to the ambient atmospheric pressure so air will flow in the lungs. At expiration there is a small positive pressure. Normally a small thoracic pressure swing is enough to obtain an adequate tidal volume. When the bronchial tree becomes obstructed, for instance due to diseases, the thoracic pressure swing must be increased to obtain the same tidal volume. Also in-elastic (low-compliant) lungs which is a common disease for premature neonates, high pressure swings are mandatory. The thoracic pressure swing is a quantity for the work of breathing especially when measuring respiration under normal (rest) conditions.

The heart pumps blood through vessels to peripheral extremities like fingers and toes. Depending on contraction force, blood volume, elasticity of the large and small vessels the wavepulse of the blood will take some time to reach the peripheral extremities. The time between hart contraction and arrival of the bloodpulse at the finger is called the Pulse Transit Time (PTT). The time of which the heart contracts can be characterised with the R-top of an electrocardiogram (ECG) and can non-invasivley be measured by attaching three elctrodes to a patient. The blood pulse will take a longer time to reach the fingertip during inhalation compared to breath holding or exhalation since the negative thoracic pressure will expand the blood vessels in the thorax. When there is no respiratory obstruction the thoracic pressure swing will be very small so the PTT-swing will also be very small. Wit a severe respiratory obstruction the large thoracic pressure swing will result in a large PTT swing. In this way the measurement of the swing in PTT yields indirectly a quantity which is related to the degree of bronchial obstruction.

Not only the PTT will change due to respiration, but also the blood pressure swing at the finger tip itself. The phenomenon is known as the pulsus paradoxus which is defeined as the difference between maxiume systolic blood pressure (during expiration) and minimum systolic blood pressure (during inspiraton). Nowadays blood-pressure can be continuously non-invasively be measured at the finger tip with a portable device. In this research both systems are used to see which gives the best correlation with the degree of bronchial obstruction / work of breathing. It might be possible that both systems are needed to measure an accurate work of breathing. Both systems might give inaccurate results due to complicating factors such as absolute blood pressure of the body. If possible the final device will contain only system.

In the initial phase, the thoracic pressure swing must be measured to find the exact correlation with PTT-swing and blood pressure swing. Normally, thoracic pressures are measured by an oesophagus pressure measurement, since the assumption is that the oesophagus pressure equals the thoracic pressure. Thoracic pressure measurements itself are very invasive, since they require operation of the patient and are therefore never performed. Oesophagus pressure measurements are slightly invasive since the patient has to swallow a catheter a which tip a pressure sensor is connected.

A basic device is currently under construction in the Academic Medical Centre under supervision of drs. R.W. Griffioen and dr.ir. F.H.C. de Jongh. The device consist of a rack with hardware which is connected to a personal computer (pentium class) via an analog-digital interface card. The rack contains several modules: 1) A bridge amplifier at which a microtip-pressure catheter is connected to measure the oeosophagus pressure of the patient 2) An ECG module to measure the ECG of the patient 3) A finger plethysmograph to measure (in combination with the ECG) the PTT of the patient 4) Four analog input channels of free choice at which for instance the non invasive continous blood pressure device (port-a press, TNO-BMI) can be connected 5) If necessary a hardware version of an R-top detector, which detects the peaks of the ECG signal and which can be used as trigger signal. Otherwise this R-top detection is implemented by software.

The approach taken is to develop the device while simultanously the processes are modelled and the models numerically verified. With the model one can obtain qualitative and quantitative insight in the transport of blood from heart to fingertip or toe. With the variation of this time the

1. the geometric system of the blood vessel system, its surroundings and its mechanical characteristics, where the latter may also depend on various factors like filling degree with blood of the vessel system. 2. the flow of blood during spontaneous breathing or breathing through an obstruction 3. the

The utilisation lies in the clinical application, for:

1. continuous non-invasive registration of the degree of bronchial obstruction

2. objective registration how medication may alter the severity of the bronchial obstruction

3. studying diseases in which increased work of breathing plays a major role, such as:
    - asthma, bronchitis, bronchopulmonary dysplasia, emphysema
    - cardiovascular diseases with hypercirculation, hypertension, atherosclerosis
    - diseases with edema (cardiac disease, renal diseases)
    - thoracic disorders (skeletal diseases, scoliosis, kyphosis)
    - muscular diseases (dysptrophia, wasting) with asymmetrical growth disturbances
    - patients on the ventilator