The cardiovascular system

There is a significant fall in total peripheral resistance by 6 weeks gestation to a nadir of ~40% by mid-gestation, resulting in a fall in afterload. This is 'perceived' as circulatory underfilling, which activates the renin-angiotensin-aldosterone system and allows the necessary expansion of the plasma volume (PV; Fig. 2.1) [2,3]. By the late third trimester, the PV has increased from its baseline by about

Fig. 2.1 Flow chart of the probable sequence of initial cardiovascular activation. ALD, aldosterone; BP, systemic arterial blood pressure; CO, cardiac output; HR, heart rate; PROG, progesterone; PV, plasma volume; RAS, renin-angiotensin system; Symp NS, sympathetic nervous system; TPR, total peripheral resistance; UNa, urinary sodium excretion.

Fig. 2.1 Flow chart of the probable sequence of initial cardiovascular activation. ALD, aldosterone; BP, systemic arterial blood pressure; CO, cardiac output; HR, heart rate; PROG, progesterone; PV, plasma volume; RAS, renin-angiotensin system; Symp NS, sympathetic nervous system; TPR, total peripheral resistance; UNa, urinary sodium excretion.

50% in a first pregnancy and 60% in a second or subsequent pregnancy. The bigger the expansion is, the bigger, on average, the birthweight of the baby. The total extracellular fluid volume rises by about 16% by term, so the percentage rise in PV is disproportionate to the whole. The plasma osmolality falls by ~10 mOsm/kg as water is retained.

The heart rate rises synchronously, by 10-15 b.p.m., so the cardiac output begins to rise [4]. There is probably a fall in baroreflex sensitivity as pregnancy progresses and heart rate variability falls. Stroke volume rises a little later in the first trimester. These two factors push the cardiac output up by 35-40%in a first pregnancy, and ~50% in later pregnancies; it can rise by a further third in labour (Fig. 2.2). Table 2.1 summarizes the percentage changes in some cardiovascular variables during pregnancy.

Measuring systemic arterial blood pressure in pregnancy is notoriously difficult, but there is now broad consensus that Korotkoff 5 should be used with auscultatory techniques [5]. However measured, there is a small fall in systolic and a greater fall in diastolic blood pressure during the first half of pregnancy resulting in an increased pulse pressure. The blood pressure then rises steadily and, even in normotensive women, there is some late overshoot of non-pregnant values. Supine hypotension occurs in ~8% of women in late gestation.

The pressor response to angiotensin II (ANG II) is reduced in normal pregnancy but is unchanged to nora-drenaline. The reduced sensitivity to ANG II presumably protects against the potentially pressor levels of ANG II found in normal pregnancy and is associated with lower receptor density; plasma noradrenaline is not increased in normal pregnancy. Pregnancy does not alter the response of intramyometrial arteries to a variety of vasoconstrictors. Nitric oxide may modulate myogenic tone and flow-mediated responses in the resistance vasculature of the uterine circulation in normal pregnancy.

The venous pressure in the lower circulation rises for both mechanical and hydrodynamic reasons. The pulmonary circulation is able to absorb high rates of flow without an increase in pressure; so pressure in the right

90 r

Fig. 2.2 Major haemodynamic changes associated with normal human pregnancy. The marked augmentation of cardiac output results from asynchronous increases in both heart rate (HR) and stroke volume (SV). Despite the increases in cardiac output, blood pressure (BP) decreases for most of pregnancy. This implies a very substantial reduction in total peripheral vascular resistance (TPVR).

120 r ra

0 20

Weeks of pregnancy

0 20

Weeks of pregnancy

Fig. 2.2 Major haemodynamic changes associated with normal human pregnancy. The marked augmentation of cardiac output results from asynchronous increases in both heart rate (HR) and stroke volume (SV). Despite the increases in cardiac output, blood pressure (BP) decreases for most of pregnancy. This implies a very substantial reduction in total peripheral vascular resistance (TPVR).

120 r ra

1100 trvr'

Weeks of pregnancy

1400

1100 trvr'

Cardiac output

Weeks of pregnancy

Weeks of pregnancy

Table 2.1 Percentage change in some cardiovascular variables during pregnancy

First Second Third trimester trimester trimester

Table 2.1 Percentage change in some cardiovascular variables during pregnancy

First Second Third trimester trimester trimester

Heart rate (bpm)

+11

+13

+16

Stroke volume (ml)

+31

+29

+27

Cardiac output (l/min)

+45

+47

+48

Systolic BP (mmHg)

-1

+1

+6

Diastolic BP (mmHg)

-6

-3

+7

MPAP (mmHg)

+5

+5

+5

Total peripheral resistance

-27

-27

BP, systemic blook pressure; MPAP, mean pulmonary artery pressure. Data are derived from studies in which pre-conception values were determined. The mean values shown are those at the end of each trimester, and are thus not necessarily the maxima. Note that most changes are near maximal by the end of the first trimester. (Data from Robson etal, 1991.)

(resistance units)

BP, systemic blook pressure; MPAP, mean pulmonary artery pressure. Data are derived from studies in which pre-conception values were determined. The mean values shown are those at the end of each trimester, and are thus not necessarily the maxima. Note that most changes are near maximal by the end of the first trimester. (Data from Robson etal, 1991.)

ventricle and the pulmonary arteries and capillaries does not change. Pulmonary resistance falls in early pregnancy and does not change thereafter. There is progressive ven-odilatation and rises in venous distensibility and capacitance throughout a normal pregnancy, possibly because of increased local nitric oxide synthesis.

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Pregnancy Diet Plan

Pregnancy Diet Plan

The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.

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