AIMS AND METHODS
2.5 Methods
2.5.3 Echocardiography
All participants had a clinical assessment and examination by the author to rule out pre-existing heart disease. Enrolled women rested in the left lateral position with the assistance of a wedge for 10 minutes. The echocardiographic study was then
performed over a period of 12-18 minutes on the Ultramark 9-HDI imaging system with a 2.5 MHz transducer (Scientific Medical System). Two-dimensional
echocardiography facilitated accurate M-Mode recordings and colour flow mapping facilitated Doppler measurements according to standard criteria (Devereux, 1997).
The procedure was carried out as follows:
An initial two-dimensional study in the standard parasternal long axis and short axis planes followed by the apical 4 chamber plane view was performed to evaluate cardiac structure and obtain a visual assessment of left ventricle contractile function (Fig 1). Two-dimensional imaging directed M-Mode studies at the level of aorta, left atrium, left ventricle and mid-position between the tips of the mitral valve leaflets and papillary muscle. Frozen M-Mode images on screen were used to measure chamber size and ventricular wall thickness (Fig 4). Pulsed Doppler flow across the mitral
valve was recorded just beyond tips of the mitral valve leaflets to obtain LV diastolic filling pattern.
For the cardiac output study, 3 estimates of the aortic annulus diameter were made in the parasternal long axis plane (Fig 2). The Doppler study was then performed using the apical 4 and 5 chamber views (Fig 3). Doppler measurements were then
performed at the aortic annulus to obtain the best velocity-time profile from which screen measurements of velocity time integral were made.
The formulae for calculating maternal cardiac structural and functional and
hemodynamic variables are listed in appendix 1. The following echo parameters were measured:
i) Structure and function of the left heart was studied in standard accepted views. Two-dimensional directed M-Mode views were used to measure left atrial and aortic root diameters and the ratio calculated,
ii) Two-dimensional directed M-Mode recordings of the left ventricle were made in the short axis view at a level just beyond the tips of the mitral valve leaflets. On screen measurements of the M-mode tracing was used to measure left ventricular (LV) cavity dimensions in systole and diastole, from which indices of LV contractile function (fractional shortening;
ejection fraction; velocity of circumferential fibre shortening and LV end systolic wall stress) was computed. At least 2 M-mode tracings of the left ventricle was performed to ensure that cavity dimensions were within 2mm and of ventricular wall thickness was within 1mm of each other. A third trace was performed if the first two M-mode traces did not provide
satisfactory recordings and representative values of the left ventricle M- mode trace was obtained by consensus with a cardiology colleague, iii) The LV wall thickness of the septum and posterior wall together with LV
cavity size in diastole was used to calculate LV mass (grams) according to the formula of Devereux (1997). Left ventricular mass (g) was corrected for maternal size by dividing LV mass by body surface area (m ) to obtain LV mass index (g/m2). The LV mass was also corrected for height to obtain LV mass/height (g/m). The relative wall thickness ratio, a
measurement to describe LV geometry and LV hypertrophy was derived from measurements of LV posterior wall thickness and LV internal diameter.
iv) LV diastolic filling velocities across the mitral valve were obtained by pulsed Doppler in the apical four chamber view; recordings were made at a position just distal to the mitral valve leaflets (fig 5). The results were recorded as early filling (E) velocity (m/sec), late filling velocity (A) and the E/A diastolic filling ratio was calculated.
v) LV Doppler cardiac output was obtained by standard accepted method as described by Ihlen et al, (1984) as follows: The cross sectional area was measured from the maximum systolic diameter at the level of the aortic annulus using 2-Dimensional echocardiography. Three (3) measurements were made and the average was accepted as representative cross sectional area at the aortic annulus. The velocity time integral measurement was performed in the apical - 5- chamber view at the level of the aortic valve
using pulsed Doppler. The best of 3 velocity time integral profile measurements were selected and measured; these were measured in the resting state with quiet breathing and after visualisation of at least 8-10 profiles on the screen to obtain a steady state.
LV Doppler-derived cardiac output was determined from the product of stroke volume (aortic cross sectional area X velocity time integral) and the average heart rate during the echo study. Systemic vascular resistance was derived from the cardiac output and mean BP and the systemic vascular resistance index obtained by correcting for body surface area. The LV ejection time, measured from the aortic velocity time integral profile was used to calculate velocity of circumferential fibre shortening, an index of LV contractile function.
vi) Heart rate and blood pressure (5-7 recordings) were measured by an automated blood pressure measuring device (Critikon) at intervals of 3 minutes during the echo study. Blood pressure was measured in the dependent arm and the average of 5-7 measurements of heart rate and blood pressure made during the Doppler echocardiographic study at 3 minute intervals was accepted as the representative measurements. These were used with echocardiographic Doppler determined stroke volume to derive cardiac output and systemic vascular resistance.