Statistical analysis Unit

HEALTH STATISTICS

Statisticalanalysis

Unit

Abstract

Background:Transcranial Doppler (TCD) is one of common tests used in diagnosisof vasospasm by measuring the velocity of blood flow in the brain’smain blood vessels.

Method:Four stroke patients (n-4) were assessed for right MCA velocity andleft MCA velocity over seven days using TCD. Results were quantifiedand compared between the RMCA and LMCA.

Results:The observed difference between the sample means forRMCA and LMCA (78.1 71.7) is convincing enough to claim that theaverage velocity between RMCA and LMCA differ significantly.

Conclusion:TCD alone is not enough to assess vasospasm.

Vasospasmis one of the main causes of mortality and morbidity among patientswith aneurysm subarachnoid hemorrhage (SAH). Cerebral vasospasm isdefined as delayed narrowing of the large cerebral arteries inresponse to blood clots left in the subarachnoid space from any causesuch as the aneurismal SAH. Early diagnosis of vasospasm is criticalin treatment of the condition. Transcranial Doppler (TCD) is one ofcommon tests used in diagnosis of vasospasm by measuring the velocityof blood flow in the brain’s main blood vessels. The middlecerebral artery (MCA) is one of the largest of three arteries thatsupply blood to the cerebrum whose velocity is indicative vasospasm.It comprises of a set of paired arteries in the brain, right and leftthat supply blood to the lateral surfaces of the frontal, temporaland parietal lobes and also the inferior surface portions of thefrontal and temporal lobes (Medscape2014).

Introducedto our facility was TCD to help predict vasospasm in patients withintracranial bleed from a ruptured aneurysm. A total of 247 strokepatients were tested in the month of august 4 patients presented withintracranial bleed and TCD over different time frames were performed.

Discussionof expectations

Studieshave shown that MCA velocity can fluctuate or progress graduallydepending on a number of factors. In this case, the TCD readings wereexpected to display a large deviation and spread. Gutman (2008)velocity is expected to increase with age while it is also influencedby history of active cigarette smoking and history of preexistinghypertension either in the patient of from the family.

Descriptionof data and descriptive statisticsOfthe four patients tested, three were female and only one male. Theoldest was aged 70, the youngest 31 and two were aged 48 with the onemale being among them. Data on the velocity in the right MCA and leftMCA was measured over a seven day period and recorded as shown below.As expected, there were fluctuations and gradual changes in both setsof MCA velocity. In one patient, the RMCA velocity recorded in thefirst day was 62 before shooting to 153 in the next day and topping148 the 6thday. The male patient exhibited the most consistent RMCA and LMCAcompared to the rest. The oldest patient exhibited significantfluctuation with the lowest RMCA velocity recorded being 37 and thehighest being 84 while her LMCA velocity recorded a high of 92 and alow of 36.

GraphicalPresentation

Table1. 7-day MCA velocities

AGE

GENDER

VELOCITIES OVER 7 DAYS PERIOD OF RMCA

average

70

F

70

42

66

37

84

78

62

62.7

48

F

62

153

113

131

132

148

104

120.4

31

F

90

71

76

73

56

65

86

73.9

48

M

55

49

57

59

50

62

57

55.6

AGE

GENDER

VELOCITIES OVER 7 DAY PERIOD OF LMCA

70

F

36

64

76

56

54

92

39

59.6

48

F

97

103

97

82

82

113

81

93.6

k31

F

101

78

62

79

63

82

88

79

48

M

48

58

51

49

41

59

76

54.6

Allparticipants except one recorded an average higher RMCA than LMCA.The variations in both RMCA and LMCA are large over the seven dayswith one participant showing a huge variation from a low RMCAvelocity of 62 to 153 following day. The variations in MCA velocityacross the gender divide is huge with a female aged 48 recording anaverage RMCA of 120.4 compared to male of similar age who recorded55.6. The scatter diagram for the two sets of data as below by fig. 1and 2.

Figure1 RMCA scatter diagram

Figure2 LMCA scatter diagram

Thefrequencies in the velocities are as shown in the Fig. 2 below. Itshows that the most frequent RMCA velocity over the seven days in allparticipants was 46-86.

Figure3. RMCA velocity frequencies

Fig.4 shows the frequencies on left MCA over the seven day period. Thehighest frequency was recorded in 46-86 range same as in the rightMCA but slightly lower.

Figure4. LMCA velocity frequencies

Discussionof inferential analysis and results

Studieshave revealed that frontal subcortical microangiopathy and cognitivedysfunction is associated with aging and is linked highly to reducedblood flow to the cerebrum (Sorond et al. 2008). The velocity alsovaries with systolic and diastolic blood pressure changes which arealso affected by a number of factors such as diet and activity. MCAvelocity is closely controlled during strenuous activity to avoiddamage to organs though blood flow increases gradually (Ogoh &ampAinslie, 2009). In some studies, the LMCA has recorded highervelocity than the RMCA though the findings are not consistent butvary with sample selection. To check for any significant differencein the average RMCA velocity and the LMCA velocity, a t-test analysisassuming unequal variances was employed. As below

t-Test: Two-Sample Assuming Unequal Variances

&nbsp

Variable 1

Variable 2

Mean

78.14287

71.67857

Variance

851.3205

324.0459

Observations

4

4

Hypothesized Mean Difference

0

Df

5

t Stat

0.377107

P(T&lt=t) one-tail

0.360785

t Critical one-tail

2.015048

P(T&lt=t) two-tail

0.72157

t Critical two-tail

2.570582

&nbsp

Forthe t-test results AssumingUnequal Variances,the null hypothesis is that the two population means are the same butthe two population variances may differ.

H0=there is no significance difference

Alternatehypothesis

Ha≠there is no significance difference

t-stat=0.377107

tCritical two-tail =2.570582

Toreject the null hypothesis, t Stat &gt t Criticaltwo-tail. However, this is not the case with 0.377107&lt2.570582meaning that the null hypothesis is not rejected. Theobserved difference between the sample means for RMCA and LMCA (78.171.7) is convincing enough to claim that the average velocity betweenRMCA and LMCA differ significantly. The TDA test is thus not enoughto capture vasospasm in this particular sample.

Conclusionsand summary

TDAis not overly efficient in detecting vasospasm. It is not efficientenough to capture minute variations in MCA velocity. Transcranialcolor Doppler sonography (TCCD) should be used as other studies haveestablished the test is more efficient. To assess the efficacy ofTCD, it should be compared to results obtained using TCCD.

References

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