Reduction of pulse pressure following sublingual glyceryl trinitrate is not explained by physiological changes in aortic windkessel properties: a modelling study

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    Abstract

    Objective. Glyceryl trinitrate (GTN) reduces pulse pressure, with greater reductions seen centrally than peripherally. The conventional mechanism is attributed principally to reduced intensity of peripheral wave reflection. However, recent studies reviving windkessel theory propose that the dominant mechanism for reduced aortic pulse pressure (aPP) is alteration of the storage capacity of the aorta, with wave reflection playing a minor role. This study aims to assess the magnitude of change in physiological parameters required to explain the effects of sublingual GTN on aPP solely in terms of changes in aortic compliance (windkessel). Methods. A 3 element windkessel model was used with characteristic impedance (Zo), aortic compliance (C) and peripheral resistance (R). Nominal values of model parameters were determined from normal physiological values of mean arterial pressure, cardiac output and heart rate. Data of effect of GTN on aPP were taken from previous experiments in human subjects (Kelly et al. Eur Heart J, 1990;11:138-144). The model was used to simulate the effects of GTN on mean pressure (MP) and aPP using a constant Zo (0.25 mmHg/ml/sec) and stroke volume (SV, 100ml). A 10% reduction in MP was simulated by either a 10% reduction in (SV) or in R. For each case the change in C was calculated to achieve a reduction of aPP of 30% as seen in the experiments. Results. Relations for C and aPP for (i) constant SV and change in R: log(C) = 6.0 - 3.92log(aPP); and (ii) constant R and change in SV: log(C) = 5.62 – 3.73log(aPP). Therefore, a 30% reduction in aPP would require a 3.9Â$\pm$1.1 fold increase in C over a starting pulse pressure range of 45 to 72 mmHg. For a simple uniform tube representation of the aorta of length L, radius r, Elastic modulus E and wall thickness h, the equivalent compliance C = 3π.r2L/2E(h/r). Hence for a constant h/r, the 3.9 fold change in C would entail non-physiological changes in r (2.0 fold increase) and E (3.9 fold decrease). Conclusions. Calculations for a 3 element windkessel model indicate a strong non-linear relation between C and aPP. Model simulation suggests the effects of GTN on aPP would require changes in aortic storage capacity of non-physiological magnitudes. Hence the change in aortic windkessel effect, and so windkessel pressure, is not the dominant mechanism to explain the reduction of aPP obtained with sublingual GTN.
    Original languageEnglish
    Article numberPP.10.401
    Pages (from-to)e170
    Number of pages1
    JournalJournal of Hypertension
    Volume28
    Issue numbere-Supplement A
    DOIs
    Publication statusPublished - Jun 2010
    Event20th Annual Meeting of the European Society of Hypertension - Oslo, Norway
    Duration: 18 Jun 201021 Jun 2010

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