Natural Frequency Response of Intraaortic Balloons

Abstract

An Intraaortic Balloon (IAB) is a cardiac assist device that consists of a catheter (lumen) and a balloon at the catheter’s distal end. A balloon pump, or IABP, and a transducer are connected to the hub of the catheter that remains outside of the body. Connecting the inner lumen to the balloon pump through the hub of the catheter is typically used to monitor pressures within the human body and can be characterized as a catheter-transducer system. The three main components of this system and their functions are as follows: the IAB gives access to the blood pressure system through the inner lumen; a transducer converts a mechanical impulse of a pressure wave into an electrical signal; the IABP records the electrical signals for the pump operator’s and pump’s interpretation. The pump analyzes the incoming electrical signal and, in turn, triggers the appropriate IAB inflate and deflate times. Catheter-transducer systems can be modeled as underdamped, second-order dynamic systems. Each system has both a characteristic frequency and a damping coefficient, where damping refers to the accuracy of the monitoring set-up to reproduce pressure waveform characteristics. To achieve an accurate pressure reading, optimal damping must be achieved. If the system is underdamped, very little damping in the system, the pressure readings tend to overestimate the systolic blood pressure and underestimate the diastolic blood pressure, and vice versa. The ability for the pressure monitoring system to detect changes quickly is called the frequency response. Catheter-transducer systems are characterized as underdamped because the addition of damping causes the response of the system to decrease. In order to counteract this, the system must have a high natural or undamped frequency — the frequency that would occur in the absence of frictional forces or damping. A high natural frequency will reduce the effects of damping such that underdamped or overdamped waveforms are unlikely to appear. This study compares the natural frequency and damping coefficients of four different IABs: Datascope Fidelity, Arrow Nitinol, Arrow Stainless Steel and Arrow Polyimide. Each of these IABs had a natural frequency greater than the minimum specified value of 20 Hz. The Arrow Polymide inner lumen had the lowest natural frequency response. This is due to the flexibility of the Polyimide, which absorbs some of the pressure wave’s energy, and thus distorts the pressure wave output. Likewise, the Arrow Nitinol, with the stiffest inner lumen, had the highest natural frequency. The damping coefficients for these IABs varied from 0.16 to 0.38. None of the four IABs achieved an average damping coefficient near the optimal range of 0.60 to 0.75 and thus all of them can be characterized as underdamped systems.