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Why the s wave is negative in V1 chest lead when it views the right side of the heart and the vector of s wave is rightward and upwards

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electrocardiography AND precordial leads

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ECG precordial lead V1 QRS vector S wave negative explanation

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V1 is placed on the right anterior chest, but it does not record “the right ventricle only.” It records the net electrical vector projected toward or away from its positive electrode.
  • A depolarization vector moving toward V1 produces an upward deflection.
  • A vector moving away from V1 produces a downward deflection.
In normal ventricular activation:
  1. Septum depolarizes left-to-right and anteriorly. This points somewhat toward V1, so V1 usually shows a small initial r wave.
  2. Next, both ventricles depolarize, but the much larger left ventricular muscle mass dominates the net vector. The mean vector points leftward and posteriorly. That is away from the right-anterior V1 electrode, so V1 records a large S wave.
So the usual V1 pattern is rS: small positive r, then deep negative S. Harrison’s describes the major ventricular vector as leftward and posterior because of LV electrical predominance, producing this pattern in V1. Harrison’s Principles of Internal Medicine, 22e, p. 1913.
The key correction is this:
The large normal S wave in V1 is not primarily due to a rightward, upward vector. It reflects the dominant ventricular depolarization vector traveling leftward and posteriorly, away from V1.
A final small basal activation vector can be directed superiorly and posteriorly, but the label “S wave” is simply any negative QRS deflection after an R wave. It is not necessarily a separate wave that uniquely represents that final vector.
Think in the horizontal plane:
V1 electrode: right anterior chest

             anterior
                V1
                 |
right  <---------+--------->  left
                 |
              posterior

Normal dominant LV vector: leftward + posterior
Therefore: away from V1 -> negative S wave
As you move from V1 toward the left chest leads V4 to V6, the same dominant leftward vector increasingly points toward the electrodes. Thus, the S wave shrinks and the R wave becomes larger: normal R-wave progression.

I am talking about S wave not R. S wave represents basal depolarisation the vector of which is right and upwards

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ECG terminal QRS basal ventricular depolarization vector rightward superior posterior V1 S wave

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electrocardiography AND ventricular activation

You are right to separate the terminal basal vector from the large mid-QRS vector. My earlier answer blurred them.
The important point is that the S wave in V1 is not a pure record of basal depolarization. It is the entire negative deflection after the initial r wave, and most of its depth is generated earlier by the dominant LV free-wall vector.

Sequence in V1

  1. Septal activation: left-to-right and anterior
    • Toward V1
    • Produces small initial r
  2. Main ventricular free-wall activation: dominated by LV mass, directed leftward and posteriorly
    • Away from V1
    • Produces the deep descending S wave
  3. Late basal activation: often described as rightward and superior, but also importantly has a posterior component in the horizontal plane
    • Its projection in V1 may be small, and can be toward V1 or less negative than the preceding vector.
    • Therefore it commonly produces the upsloping terminal part of the S wave rather than a separate positive wave.
So, even if the final basal vector has a rightward component, the tracing may still be below the baseline because it starts from the deep S produced by the preceding LV-dominant vector:
V1 QRS:

 small r          deep S caused chiefly by LV activation
   /\                  |
__/  \_________________|____
                       \  /
                        \/     <- terminal basal vector may make
                               the S wave rise toward baseline
If that terminal basal/rightward component is sufficiently large and anteriorly directed, it can cross the baseline and appear as a small terminal r′ in V1. If it does not cross the baseline, it is still called part of the S wave, because ECG wave names are based on the trace's position relative to the isoelectric line, not on one unique anatomical activation event.
Also, “upward” refers to the frontal plane, whereas V1 mainly examines the horizontal plane. For V1 positivity, the decisive direction is anterior versus posterior:
  • anterior toward V1 -> positive
  • posterior away from V1 -> negative
A review describing the three-vector model makes this distinction: the terminal basal vector can be rightward and superior, produces a small terminal s in V5-V6, and contributes to the ascending limb of the S wave in V1-V2. Review of rSr′ patterns
So the concise answer is:
The basal vector can make the terminal S in V1 become less negative or rise upward, but the deep S in V1 mainly reflects the preceding leftward-posterior LV-dominant activation. The terminal vector usually does not create a positive deflection large enough to cross baseline.
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