It’s normal for blood pressure to drop when breathing in.

When you breathe in, the negative intrathoracic pressure increases. That basically means the extra force of all the air you’re leeching from the atmosphere pushes on the insides of the body, squishing the important bits of the cardiovascular system. So, the blood flow gushing out to the rest of the body is reduced a bit, referring to the cardiac output emitting from the left side of the heart. Note that venous return to the right side of the heart increases with inspiration.

A drop in SBP<10 is normal during inspiration. When it’s greater than this, it’s pulsus paradoxus. That can happen in things like:

• croup
  • inflammatory reaction to parainfluenza virus
  • results in airway obstruction, mainly in children because they have smaller airways
  • airway obstruction means even bigger negative intrathoracic pressures
  • this increases left ventricular afterload, which is the force the heart has to push blood against
• cardiac tamponade
  • heart is compressed
  • leads to reduced cardiac output

References

1. Olfa Hamzaoui, Xavier Monnet, Jean-Louis Teboul, Pulsus paradoxus, European Respiratory Journal Dec 2013, 42 (6) 1696-1705; DOI: 10.1183/09031936.00138912
2. Van Dam MN, Fitzgerald BM. Pulsus Paradoxus. [Updated 2019 Jun 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482292/